Hello Guys,
Here's part of a discussion we were having on the NAWS forum related to grounding. Below are some comments that I cut and paste from NAWS member BB (Bill).
<start of Bill's comments>
Ok, I am going into deep water here... And take what I say with a grain of salt. boB or Ryan would probably a much better source of information regarding this than I.
As I understand people's solutions to the NEC requirement for ground fault protection for DC solar arrays, it really does violate the very definition of safety ground.
Pretty much all DC GF systems put a 1-5 amp fuse/breaker/ or in the case of the Classic a 1 amp PTC Resistor (basically a resistor that when it passes too much current gets hot and dramatically increases its resistance) between DC ground and safety ground (add missing text).
At this point, the popping of a fuse/opening a breaker will open the connection between the + solar array and the DC charge controller (or GT inverter) (the 5 amp breaker trips, then this trips the 80 amp breaker too with the handle bar/rod).
In the case of the Classic and a few others, the "high voltage" across the open fuse or hot PTC signals the charge controller (or GT inverter) to stop conversion.
Sort of sounds OK--but, from my humble point of view, all they have done is stopped the power transfer into/through the DC input of the device.
The have not limited to less than 5 milliamps (~limit that can cause heart failure), and they have now potentially energized the DC grounded section of the circuitry by "floating" the ground (open fuse, open breaker, hot PTC)...
In the "olden days", when something was ground referenced, it was done with at least a 6 gauge piece of wire that could handle >600 amps of fusing current and would guarantee to trip a circuit breaker/fuse in the + or hot leads from the energy source (DC or AC systems).
Now, your DC "ground" is a 1-5 amp fuse, that when popped, it has now un-referenced your ground system. And, depending on what happens, it has de-referenced your otherwise operating battery bank--while just stopping the solar panels from charging the batteries (hope you notice in time the warning LED, beeper, etc.).
This is typically considered very bad form (again from my humble opinion) to have the ground automatically lift... Heck, when I designed/installed AC equipment we had to use a separate ground stud so that when a person was servicing the device they could not unhook the green wire by accident (or double nut the safety ground/other ground connections).
<end of Bill's comments>
We were just wondering what you Midnite guys thought about the safest way to ground DC equipment (traditional method of grounding negative battery connection versus newer GFP method).
Thanks,
Edward
I may have to read BBs post again, but the intent for PV GFP is not the same as for bathroom AC GFCI protection.
5 milliAmp protection for GFCI is to reduce electical shock and PV GFP is to reduce electrical fires. This is why
the current is different. Not worried about voltage, just current through a less than optimal connection....
i.e. a bad connection with high resistance. A nice solid ground fault will usually be fine because there
would be little or no heat buildup.
boB
I will just simply say "That is the way the NEC says to do it" do I agree ??? Seriously as a manufacturer we have to follow the code by the letter and could not possibly advocate not using this wonderful device ::)
Hey guys,definitely not an expert or otherwise NEC savvy, so please explain...
From a non understander of the NEC's point, these seem to be in opposition.
<start of Bill's comments>
-but, from my humble point of view, all they have done is stopped the power transfer into/through the DC input of the device.
The have not limited to less than 5 milliamps (~limit that can cause heart failure), and they have now potentially energized the DC grounded section of the circuitry by "floating" the ground (open fuse, open breaker, hot PTC)...
boB said:
5 milliAmp protection for GFCI is to reduce electrical shock and PV GFP is to reduce electrical fires. This is why the current is different. Not worried about voltage, just current through a less than optimal connection....
If I have it right, based on bills comment, there is now another path for the current to follow once the PVGFP is triggered, which would be handling all the PV output.... ??? ???
Eric
Quote from: Westbranch on March 17, 2011, 08:13:35 PM
Hey guys,definitely not an expert or otherwise NEC savvy, so please explain...
From a non understander of the NEC's point, these seem to be in opposition.
If I have it right, based on bills comment, there is now another path for the current to follow once the PVGFP is triggered, which would be handling all the PV output.... ??? ???
Eric
Yeah, you probably are right... A better way to take care of this is probably to short the entire array at the array.
We built a PV GFP in the 1990s at Trace that did that. It was very expensive. Opening up most of the path is cheaper.
Not everything the NEC or UL says to do is perfect. And usually does not take care of a double fault.
However, ground faults are very rare and GFPs usually just show the installer that it was mis-wired in the first place.
Usually, that is.
boB
Hi, it is me, Bill (BB.) from the Wind Sun forum. ;D
I will give a couple of examples why, I believe, that the NEC GFP type protection is actually making things less safe rather than safer...
First example, typical charge controller setup with "GFP" supplied by 1-5 amp fuse/breaker between safety ground and negative DC ground (does not really matter if Classic or any other brand--details may matter, but I am not the one to answer the specific issue and how a Classic would behave)... Assume charge controller has solid connecting from PV Input (-) lead to Battery (-) lead (no switching, no fuses/breakers).
Setup system per NEC design. GFP 1-5 amp fuse/breaker from (-) controller ground to safety ground.
1. Short Array (+) line to metal frame/solar array mounts.
2. Current flows from + array to safety ground, return path from safety ground to - array, pops fuse/breaker, controller stops + current flow to + battery lead (or trips double pole breaker which opens + array lead to charge controller/battery bank).
3. Now, + array lead is "hard grounded" to safety ground.
- array is now at Vmp/Voc below ground (~17 volts to 150 VDC below safety/earth ground).
4. Ground reference is now carried through negative PV to negative Battery connection to, now floating, negative battery lead at ~17 to 150 VDC (or more) below ground.
5. Your battery bank - bus is now held to 17-150 VDC below ground with XX amps from the solar array worth of current behind it.
6. The + battery bus is now +Vbatt above the 17-150 VDC below ground reference.
7. If you have a car radio (am/fm or CB, Ham, etc.) that uses the case as a ground/safety ground reference... And have an antenna cable with braid to case and the braid tied to earth ground for lightning protection, you have a hard return path through the braid from - Varray to safety ground, through short to + array and can be carrying 10's of amps (or more) of Isc from the array.
So, in the above, your - bus which as always been assumed to be at or very near earth ground / safety ground potential is now at Varray--which is both a voltage hazard and a current hazard.
And, if you have system where you have DC ground loops (not a good idea, but not always obvious that such a ground loop exists), you are now driving Isc current through those incidental ground loops.
There is also are failures which are a fire hazard...
First failure--assume that you have a second fault (or incidental ground loop) with a + Varray to earth ground fault (the only fault that the DC GFP system "protects against). If you trace the entire path, you will find that there is no over current fault protection from Power of the array to the current flow in the negative bus (say you have a small LED circuit with a 2 amp fuse on the + lead, there is no 2 amp fuse in the - lead)... You will have Isc Array pumping full current through the small gauge return lead that has (for this example) a negative ground to safety ground short...
The above failure would only be protected if A) all ground returns are sized for Isc of the array (even ridiculously small LED fixtures, radios, etc.) or B) if all negative return leads are fused/breaker-ed just like positive power leads are (central fuse/breaker panel for negative bus).
And if you follow NEC requirements (for example) 120/240 VAC split phase circuits--Such circuits are protected by two pole breakers--If one breaker overloads, then the second breaker is automatically switched off because it is ganged with the first breaker.
That would mean that, to be compliant with NEC code, all +/- leads would need to be protected against shorts using ganged two pole breakers just like a 120/240 vac split phase system.
Another thought experiment... Imagine I wanted the same GFP system for my grid tied home.
I would put a 1 amp breaker between safety ground and neutral bond. And if that breaker tripped, then it would turn off my Grid Tied Inverter but leave the Utility power connected and operating normally. And now my neutral was floating with respect to safety ground... Would never be allowed in a million years by NEC (as I understand).
I can create more examples--but this is what I was trying to say that NEC DC GFP is actually a huge safety problem and should never have seen the light of day. By its vary action (tripping to open the positive lead to the charge controller/gt inverter) now creates a huge safety concern with the "negative" side of the system.
It takes a minor problem (short from + Varray to safety ground short) which, in itself is not dangerous (remember, all array wiring is either rated to carry Isc*1.25, or in the case of paralleled array strings, each string is protected to Iseries max rating)--And now creates a huge fire hazard with driving current into an unprotected ground/return wire power distribution system.
Again, I have no knowledge of how a Classic (or any other) charge controller/GT inverter operates/connects from Varray to Vbattery--This is my assumption of how, in general, the system operates and the issues that this NEC DC GFP affects the generic DC system.
In decades past (when I designed larger -48 VDC powered computer systems), NEC had some very bad assumptions about DC power in code at that time too.
-Bill
By the way, here is the original thread from the Wind-Sun forum:
http://www.wind-sun.com/ForumVB/showthread.php?t=10650 (http://www.wind-sun.com/ForumVB/showthread.php?t=10650)
The ground fault protection specified in NEC 690.5 is intended to reduce fire hazard not to protect personal. Evan at that purpose, it's hardly perfect. And it certainly creates other risks.
When GFP is is implemented with a fuse or breaker, a negative to ground fault is very likely to be undetected. That leaves the system unprotected against positive to ground faults. Even if the GFP fuse of breaker operates, the fault current will not be interrupted.
When the GFP is activated, the normally grounded PV wire may be operating at -17 to -600 volts. That's why 690.5(C) requires a warning label at the GFP device that states that when then GFP is activated there is an electrical shock hazard - normally grounded conductors may be ungrounded and energized. A person that isn't familiar with these risks, shouldn't be trying to find a ground fault in a PV array!
Bill, your concern about the the negative battery bus being operating below ground potential is incorrect. When the GFP is activated, the battery is still grounded to earth through a high impedance. Voltage measurements to ground would be completely normal.
Quote from: Kent0 on March 18, 2011, 02:06:07 PM
The ground fault protection specified in NEC 690.5 is intended to reduce fire hazard not to protect personal. Evan at that purpose, it's hardly perfect. And it certainly creates other risks.
It provides only a very small realm of protection (the + wire from the array combiner box to the charge controller--and possibly from the + battery bus--to safety ground.
It does not protect against + to - faults (main PV breaker will probably never trip as Isc << Ibreaker by design).
And, if you have a + to earth ground short, the only breaker/fuse that trips is the 1-5 amp safety to - bus connection--Rest of system is still energized, and if there is a second - return lead to safety/earth ground short, the battery bank could very easily cause an over current/fire hazard in the -/return wiring paths.
QuoteWhen GFP is is implemented with a fuse or breaker, a negative to ground fault is very likely to be undetected. That leaves the system unprotected against positive to ground faults. Even if the GFP fuse of breaker operates, the fault current will not be interrupted.
Yep... A failure elsewhere (such as a -/return to earth fault) is not detected and can bypass the DC GFP protection. That sort of protective circuit design would never be allowed in any standard AC wiring system.
QuoteWhen the GFP is activated, the normally grounded PV wire may be operating at -17 to -600 volts. That's why 690.5(C) requires a warning label at the GFP device that states that when then GFP is activated there is an electrical shock hazard - normally grounded conductors may be ungrounded and energized. A person that isn't familiar with these risks, shouldn't be trying to find a ground fault in a PV array!
So, with or without DC GFP, we have the same problem and a $1 label can fix it...
QuoteBill, your concern about the the negative battery bus being operating below ground potential is incorrect. When the GFP is activated, the battery is still grounded to earth through a high impedance. Voltage measurements to ground would be completely normal.
No, in general, there is no intentional connection between return and safety ground... In the case of the Classic, there is a high impedance connection through the PTC--But it is by definition unable to "hard ground" the Isc/Imp energy from the Solar panel.
And depending on where the + Battery bus to safety ground fault occurs (on the + bus at the battery bank with no fuses, or the + lead of a 2 amp LED lamp), the -/return bus is now energized, potentially with more current available than the "DC return Branch Wiring" is capable of carrying.
Again, there are some assumptions in my argument about the negative/return connection from the Varray to Vbattery through the charge controller--And NEC is not designed for protection against double faults (i.e., you do not assume both a circuit breaker fails shorted and a charge controller fails shorted at the same time)--However, any single and subsequent faults should fail in a safe manner. Energyizing a current path (- return branch circuitry) with more current than it is capable of handling (when properly installed per code) is (in my humble opinion) a violation of the single and subsequent faults requirement.
In this case, DC GFP requirement, fails the design guidelines (as I understand them) for the NEC.
-Bill
Thanks for discussing this guys. I have so little background in this area, that it is very educational for me to be reading this.
Edward
Hi bill is it possible for you to post a quick sketch of example #1.
I am not conversant with the difference between 'Safety' and 'Earth' Ground as well as several other terms you have used. have to do some reading ...
thanks
Eric
For my discussion--Earth=Safety Ground... Basically, I am typing about +Varray, +Vbattery, Safety/Earth Ground, and assuming that -Varray=-Vbattery.
And the functional difference between a "Normal" system ground (-Varray/-Vbattery tied to Earth/Safety Ground in one location) vs the DC GFP which puts a 1-5 amp fuse between -Vbatt/-Varray and Earth/Safety ground.
The second difference is there a switch/breaker OR an Inhibit (DC charge Controller/GT Inverter) that prevents +Varray current flowing to +Vbattery. I submit that this second difference (inhibiting current flow through the "converter") does not increase safety--but instead, is simply the equivalent of a flashing light/buzzer that you cannot ignore if you have a "Ground Fault" between V+ (array or battery?) and safety ground (cannot ignore because the "converter" turns off).
The Switch / Inhibit does not prevent excessive (i.e., hazardous) current flow UNLESS the system is improperly designed / installed in the first place (i.e., no fuses/breakers in battery bus, too small gauge of wiring, etc.). And in many cases, an improperly designed/installed system would have had problems anyways (too small of breaker would trip, too small of wiring would overheat with Imp anyway, etc.).
A Ground Fault will not cause "excess current" to flow anywhere in a well designed/installed system. About the only thing a DC GFP will reduce the hazard of is the point of the short circuit and limiting current to 1-5 amps for the initial fault (i.e., short may not get as hot as it pop ground fuse/breaker).
But we could just as easy have a +/- fault (wiring failure from rats chewing, or a broken panel with internal shorts, shorted diodes, etc.) and the DC GFP will have no chance of detecting/interrupting those types of failures. Those are still addressed by proper design/wire size/combiner box protection.
I will see what I can do about sketching something up (my hand drawing is terrible--an't no artist--And all my professional drawing stuff is back on my old jobs).
-Bill
One other quick way to look at the DC GFP setup... Imagine you designed a Negative Ground system and put fuses/breakers on all positive wiring. No fuses/breakers on negative wiring, and one positive solid connection between the negative bus and earth ground (including grounding to water pipe, fixtures, wiring boxes, conduit).
If there is a fault from anywhere in the Positive circuit path, positive wire will be protected by an appropriate fuse/breaker which will be tripped by the excessive current flow.
If there is any negative/return wire to earth ground, virtually nothing will happen as both are very near the same voltage. (it is possible with ground loops to overheat return wiring, but for now, NEC does not address over current from ground loops other than recommending/requiring single point grounding).
Now, you replace that one Earth Ground with a 1-5 amp fuse. And now earth ground, randomly, anywhere in the Positive Wiring Path (PV wiring or Battery wiring). Pops the 1-5 amp fuse, and stops power flowing from solar panels to/through charge controller... Now you have a positive earth grounded system with fuses in the earthed positive paths.
No current limits on Negative Grounds. Items that were assumed to be "touch safe" (like lamp sockets) are now powered by battery or PV Array voltage/power. And the current limit available anywhere on the negative/return wiring is dependent on where, specifically, the Positive to Earth Fault was made.
-Bill
Quote from: BB. on March 18, 2011, 10:13:35 PMand one positive connection between the negative bus and earth ground
Bill, I may be having a brain fart here, but I think I'm reading that part wrong ??
boB
boB,
I intended Positive Connection as meaning bolted up / solid connection (on purpose) as opposed to faulted connections later in the discussions...
Probably too many uses of the word "positive" in one paragraph/post on my part. :-[
-Bill
Thanks Bill ! I DID try reading it that way once.
I'm not positive which ones' negative.
OK, now to go read it again...
boB
Quote from: BB. on March 19, 2011, 01:40:09 AM
I intended Positive Connection as meaning bolted up / solid connection (on purpose) as opposed to faulted connections later in the discussions...
I fairly positive that my brother-the-electrical-contractor uses the terminology "permanently bonded".
Keith
Quote from: BB. on March 18, 2011, 12:53:14 PM
Hi, it is me, Bill (BB.) from the Wind Sun forum. ;D
7. If you have a car radio (am/fm or CB, Ham, etc.) that uses the case as a ground/safety ground reference... And have an antenna cable with braid to case and the braid tied to earth ground for lightning protection, you have a hard return path through the braid from - Varray to safety ground, through short to + array and can be carrying 10's of amps (or more) of Isc from the array.
So, in the above, your - bus which as always been assumed to be at or very near earth ground / safety ground potential is now at Varray--which is both a voltage hazard and a current hazard.
Bill in this example the GFP would not trip because the bond in the radio would bypass the GFP sensing circuit.
Bill,
On further review, I agree with you about the battery negative bus being at a hazardous voltage under ground fault conditions. The diagram below explains how this occurs. In terms of safety, it is a compromise. The immediate hazard of the ground fault arc is removed and a potential fire is hopefully averted. I say hopefully because, as mentioned before, its not perfect. For example, if someone installs a piece of automotive radio gear that has the chassis connected to battery negative and the antenna connected to a properly installed ground, the GFP system will be rendered ineffective and PV fault currents may flow on the antenna cable shield.
It seems like the Classic that has the responsibility for the dc system bonding jumper should check on startup, and daily thereafter, to confirm that there is only one dc system bond. When multiple dc system bonds exist, the class should remain inactive (which provides no protection, but at least requires attention) and display a warning that the GFP system has been compromised.
This diagram shows the internal GFP in the Classic charge controller. The hazardous voltage possible when a ground fault exists is the same for the dc GFP used by other manufacturers.
[attachment deleted by admin]
That's why the OLD way of shorting out the whole array was better. Just way more expensive
Thank you Kent0 for the drawing... Yes, that is one of the faults conditions I am concerned about.
-Bill
After a PM to/back from boB--I will post this statement. This is my personal opinion and has not be blessed by Midnite Solar, this forum, or any others. This is just my opinion/interpretation of the various regulatory codes as they exist today (as I understand them). (boB is not making add this--It is my honest opinion :)).
The NEC DC GFP system has little to no safety improvement and in fact violates several NRTL Recognition/Listing requirements for multiple countries.
And because of the fundamental flaws in the NEC DC GFP requirements/system, it should immediately be disabled and permanent DC Return/Earth Grounds be used in place of the sensing fuse in DC / Off Grid Systems as a safety/fire hazard.
This includes Hybrid inverter systems that use DC Solar Charge Controllers as part of their DC power design.I am not too keen on having NEC DC GFP on Grid Tied (Utility Interactive) Inverter systems (I don't think it adds very much in the way of safety) and it does create a nightmare in terms of a ambiguously grounded solar array. And it creates potential confusion of what is or is not grounded in the DC PV Array side of the circuitry--which is always a potentially dangerous situation. Also, I do not know how the NEC DC GFP system affects the new non-galvanically isolated GT inverters. I don't like NEC DC GFP and believe it makes things less safe for a service person--but I don't know any method of bypassing the NEC DC GFP circuitry--and in any case, it may be different by vendor/model number anyway.
Now the details why:
Per Kent0's nice drawing--Yes, that is one of the two (as far as I can tell) failure modes, that DC GFP, is designed to detect/protect against, it actually makes the system very unsafe and an actual safety/fire hazard if it operates as designed (per NEC requirements--Again, not picking on Midnite--They are giving their customers the option of NEC DC GFP grounding or the traditional hard ground of the return bus/earth ground that does not "meet" current NEC).
If there is a fault between +Vpanel (assuming negative ground system) and frame/earth ground, this causes the whole DC battery wiring to no longer meet ELV ratings (more or less, no longer "touch safe") because of the opening of the Ground Fault Sense Fuse/Breaker. Now all down stream 12-48 VDC components/wiring/devices/etc. have to rated for 150+ VDC and Highpot tested to a minimum of 750 VDC. (It has been decades since I have been at this level of design and test for safety/code issues--so I have probably messed some of the details up--but the basics, I believe, are still correct).
And this causes a whole set of nightmares for device manufacturers. For example, the fuse/breaker/PTC needs now to be rated to 150 VDC (or higher for high voltage array product). And if there are any capacitors from -DC to Frame ground, those now need >150 VDC rating. Also, need to check barrier requirements between -DC (and +DC) and Frame ground now that you can have >150 VDC between DC power and frame ground.
Now this means that the DC side of a PV system cannot be considered to be SELV (Safety Extra Low Voltage).
Assuming Wiki is more current than my knowledge:
http://en.wikipedia.org/wiki/Extra-low_voltage
QuoteSeparated or safety extra-low voltage (SELV)
IEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".
This outright affects and violates UL/NRTL registration/listings...
The only way that a NEC DC GFP system would not violate the SELV clause is if Voc-cold is guaranteed to never exceed 120 VDC. I am not sure how solar panels are rated to ensure that this requirement could be satisfied, especially now that Listed Solar panels are rated to 600-1,000 volt (600 VAC / 1,000 VDC?).
And even if the the SELV rating of 120 VDC is not exceeded, I still believe the inversion of the safety ground reference is unsafe because the normal single pole DC breaker/fuses are now on the return lines when the ground reference is reversed (and the Ground Fault detection fuse/breaker is opened). This is also a violation of multiple safety requirements and create both a serious shock hazard and fire hazard.
Personally, I would recommend that people never install (or configure) a NEC type DC GFP system. It is not safe.
-Bill
QuoteThe NEC DC GFP system has little to no safety improvement and in fact violates several NRTL Recognition/Listing requirements for multiple countries.
I have to vigorously disagree. The dc GFP does provide significant reduction in fire risk from PV arrays. At present the dc GFP is the only device that provides any protection from arcing faults between positive and ground. Regardless of your opinion of it, the NEC is the letter of the law in the US, Canada, and many other countries and no NRTL listing is possible unless it complies with the NEC. That's not to say I think the dc GFP scheme is perfect, there is certainly room to improve it.
Some ways to do this would be:
1) At system startup, and daily thereafter, check the uniqueness of the dc system ground connection. Any inverter or charge controller responsible for establishing a dc system bond should temporarily disconnect it and confirm that there are no other system bonds. If necessary, the system should remain deactivated until corrections are made. This is especially important for charge controllers because flaws in premises wiring can effect the GFP as seriously as system installation errors.
2) Charge controllers should be required to open the PV array dc grounded conductor as allowed in NEC 690.5(A); not just open the dc system bond. This would remove the high voltage risk on all the user equipment. There would still be a high voltage risk on normally grounded wires in the PV array but there is no reason that risk should carry onto any dc wiring on the premises. Unfortunately, this requires the addition of a switch in the charge controller between PV- and Batt- and it requires the PV array negative to be wired directly to the charge controller. I think this was actually the intention of 690.5(A) and the NRTL that has listed the system used by all charge controller manufacturers has been negligent in allowing the dc system bond to be opened rather than the grounded conductor. The system bond is not a grounded conductor. NEC 690.5(A) only refers to opening grounded conductors not the system bond.
Good Afternoon Kent,
Quote from: Kent0 on March 20, 2011, 02:31:22 PM
I have to vigorously disagree. The dc GFP does provide significant reduction in fire risk from PV arrays. At present the dc GFP is the only device that provides any protection from arcing faults between positive and ground. Regardless of your opinion of it, the NEC is the letter of the law in the US, Canada, and many other countries and no NRTL listing is possible unless it complies with the NEC. That's not to say I think the dc GFP scheme is perfect, there is certainly room to improve it.
I respectfully disagree.
The ONLY point at which the NEC DC GFP supplies arc fault protection is for (assuming positive ground) is for +Vpanel to Earth Ground faults that are over ~0.5 to 5 amps. It does not protect against + to - arc faults (which I would guess are the majority of the arc fault failures). And it is not really arc fault sensitive--it is just earth fault current sensitive). Also, to stop arc faults to earth, the Fuse/PTC/Circuit Breaker needs to be rated for the maximum interrupt current of the system... That means > 150 VDC and ~80+ amps (for the larger DC controllers) of the solar panels. And if you go >250 VDC, the typical breakers/fuses/PTC are not rated for near those high of DC voltages.
And if there is a +Vbattery to Earth Fault, the interrupting device (ground fault detector/fuse/breaker/ptc) must be rated to the Isc of the battery bank--which for a 80 amp charge controller and a 5% rate of charge battery bank at 12-48 volts is probably well over >> 5,000 amps of DC current--Also an almost impossible job for a small/cheap ground fault detector fuse/breaker/PTC.
Then we get into the part of of the UL/IEC documents which specifically state:
QuoteIEC defines a SELV system as "an electrical system in which the voltage cannot exceed ELV under normal conditions, and under single-fault conditions, including earth faults in other circuits".
There have been lots of times my design job would have been much easier if I did not have to consider the wide range of power systems/installations I generically had to address, both to meet code/safety requirements/and marketing requirements.
I do not see any DC power device that goes through same safety requirements that a simple 120/240 VAC device has to go through (including double insulation, or earth grounding, or leakage current tests).
And, given now UL/NRTL requires you to evaluate the "earth faults in other cirucits" requirement--and NEC DC GFP is currently a requirement--I don't see any easy way of passing this short of a fully double insulated DC to DC conversion section (meeting the same requirements as a typical 120-240 VAC power supply).
QuoteSome ways to do this would be:
1) At system startup, and daily thereafter, check the uniqueness of the dc system ground connection. Any inverter or charge controller responsible for establishing a dc system bond should temporarily disconnect it and confirm that there are no other system bonds. If necessary, the system should remain deactivated until corrections are made. This is especially important for charge controllers because flaws in premises wiring can effect the GFP as seriously as system installation errors.
Obviously more expensive and, again appears to violate the SELV requirements unless there is a hard return to earth bond in the system--exactly what you are trying to detect and fault the system over.
Quote2) Charge controllers should be required to open the PV array dc grounded conductor as allowed in NEC 690.5(A); not just open the dc system bond. This would remove the high voltage risk on all the user equipment. There would still be a high voltage risk on normally grounded wires in the PV array but there is no reason that risk should carry onto any dc wiring on the premises. Unfortunately, this requires the addition of a switch in the charge controller between PV- and Batt- and it requires the PV array negative to be wired directly to the charge controller. I think this was actually the intention of 690.5(A) and the NRTL that has listed the system used by all charge controller manufacturers has been negligent in allowing the dc system bond to be opened rather than the grounded conductor. The system bond is not a grounded conductor. NEC 690.5(A) only refers to opening grounded conductors not the system bond.
I do not have a copy of NEC (too many years since I had a need for one).
But, I would find it hard to believe that NEC would allow a switchable Return / Earth Ground bond--given all the trouble I had to go into designing a "fool proof" Return to Earth Bond in my systems.
If there was a return path switch that opened the "negative" current path in the controller and there was still a "negative" to earth bond on the battery side of the system--I could see that requirement working. Although, it would:
A) require some sort of active circuitry to figure out the difference between Earth Current Flow and Return Current flow... This is easily and cheaply done with 120 VAC Ground Fault Detectors (current transformer). DC, not so easy. And in any case, GFI's are only required for branch circuits to prevent shock where water is expected (sinks, pools, out doors, etc.). It is not required on equipment that uses a permanent "safety to earth" bond.
B) or require a double insulated, isolated, DC to DC switcher for solar charge controllers--a complete different architecture than is normally done today in Solar (although very common in my industry).
And this does not even address the fact we have a fault, for which NEC DC GFP is designed to "protect against" causing a reversal of "DC Neutral" and "DC Hot"...
This is the equivalent of placing a rinky-dink fuse in the Earth to Neutral bond of a split phase 120/240 VAC home main panel.
Try looking at a Line A to Earth fault in that case... You will now see that the neutral has no over current protective devices inline... And the wire may run from #4 to #14--any random neutral/earth fault can cause excessive current flow in the neutral if the fault is at a 14 awg branch circuit (where it is, statistically, more likely to happen).
And to protect against ARC Faults, that fuse would need to be rated at 10,000 AIC (amps interrupt current) like all AC Mains Breakers/Fuses are required (maximum current supplied by a pole mounted transformer)--A simple ACG fuse or equivalent does not meet this limit.
Very Respectfully,
-Bill
And to be clear, I intend this to be a friendly discussion... I enjoy debating (and learning) technical interchanges. Doing this purely by written communications prevents the normal voice inflections and body language that comes through with face to face talks. :)
If anything here appears to be overbearing on my part--It is simply my failure to communicate my intentions clearly...
Although, I still think (from day one) what NEC did with DC GFP is dangerous. >:(
Just use a DC GFP using circuit breakers that has the 2 higher current disconnects ganged with the 1/2 Amp fault breaker Like the OBDC-GFP/2 and use one high current breaker for positive and the other for PV negative.
That's not going to easily be put into a charge controller though.
boB
Just an FYI to everyone...
With the help from the folks at Midnite solar, I have just submitted a 26 page document to John Wiles asking for him (and NEC) to review the entire NEC DC Ground Fault Protection requirement in face of the serious issues of safety that affect Solar PV Power Systems when installed per NEC Code Requirements.
If anything further happens, I will let you guys know.
Thank you all for the ongoing discussions that we have had about safety. I have documented many of those in the submission.
Cross posted to:
http://www.wind-sun.com/ForumVB/showthread.php?t=10650
Sincerely,
-Bill B.
Here is a link to Bills paper http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf (http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf)
Good job Bill (and the rest of you guys that helped with this)!
Looks good.
Edward
Quote from: Halfcrazy on March 30, 2011, 04:06:08 PM
Here is a link to Bills paper http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf (http://www.midnitesolar.com/pdfs/DC-GFP-Draft3-5.pdf)
I don't know what kind of system BB is thinking of -- in my system, the negative bus is locked behind a battery box and runs in conduit and wiring boxes -- there is no way to come in contact with it. The way I understand it from Kent's drawing, the equipment ground would still not have a high voltage under ground fault condition, even when using the DC-GFP required by NEC.
So what is all this fuss about? Is BB using an RV, where it *would* matter a great deal if the frame was energized?
laszlo,
I am sorry, I have not be monitoring this thread/forum, hence the reason for the late answer.
A very simple reason that the 1 amp fuse/breaker in the "neutral" to Earth Ground connection is dangerous (and explained in the too long paper) is the fact that a "hard" earth grounded neutral insures that "neutral" (negative battery bus in the case of negative ground system) never exceeds zero volts with respect to ground.
Since the "neutral" never exceeds ground potential, there is never any reason to place a fuse/breaker on the neutral/return wiring.
I can have a 4/0 battery ground cable go to the negative (neutral) bus and run anything from 4/0 cable to 18 awg wiring anywhere in the system without any fuse/breaker on the neutral wiring.
If, as has been shown here, you get a + or "hot" cable fault to earth (or some other battery/solar/load + or "hot" to earth fault), this simply pops the 1 amp neutral to earth fault detection fuse/breaker. And, depending on exact location of fault and protective devices installed) can now earth reference the 4/0 wiring behind a 300 amp fuse (if anyone installed the fuse/breaker on the battery bus) driven by a 12-48 volt battery bank.
And, now the neutral (formerly earth referenced negative battery/solar bus) is energized around -12 to -48 volts and 300 amps (just an example) of battery +bus current.
And that un-fused distribution circuit with a 14 awg neutral/-bus wire going to your sump pump, radio, fan, led lighting has 300 amps of battery bus voltage/current available.
At that point, the 14 awg wiring is your fuse (rated ~166 amps) and will fail... How it will fail and what it will take down with it--all depends on your configuration and the specifics of the fault.
There are lots of other safety/regulatory issues too (UL/NRTL requirements for grounding of conductors, isolation between "low voltage" and "high voltage" circuits, that floating/both hot circuits need multiple pole/gang trip protective devices, issues with ground references of communications circuits like RS-232 & RS-485 etc....).
Imagine if your your very nicely negative grounded car battery system all of a sudden could flip to positive ground with a simple + wiring short... What are all of the issues that you can think of (car/ham radio antenna grounding, unfused negative wiring, cigarette lighter grounded outlet now hot, what happens to all of the various computers, sensors, servos in your car, you now have to disconnect the positive side of the battery first to avoid grounding your metal wrench instead of the negative side first).
From a systems point of fuse, placing a 1 amp fuse between "neutral/-battery" and "earth/frame" ground is ugly with its multitude of safety/functionality implications.
-Bill
PS July 2, 2018... The link to the my white paper is available through:
https://us.v-cdn.net/6024911/uploads/attachments/512/1965.pdf
-Blll
Hi Bill!
I have been busy with the job so I didn't read this until now but I took the time to read and digest what you are saying, and I 'get it'.
I thought the only issue was a human coming in contact with the DC negative bus in the case of a ground fault, but as you meticulously describe, in the case of a GF the neutral/negative now has -negative potential compared to the ground and there is no overcurrent protection so it will fault uncheched across the negative/neutral in the same way or worse than it would if you touched the positive/hot. Yikes!
Actually, I did have this big question mark in my head when I took the big fat bonding jumper off in the Magnum in order to enable the GFP in the Classic -- is the #6 that I have running to the Classic ground terminal going to as good as that 2/0 or higher bonding jumper? Luckily I saved the jumper so I can put it back.
So what is Wiles saying? Are they going to revise the standard to address this concern?
Laszlo,
Obviously, I can not give you safety advise over the Internet... But, you can see I would normally be much happier with a solid earth to negative ground than a 1 amp fuse between them.
The grounding/bonding wire really depends on what it is you are trying to do/protect against. A 14 awg wire has a fusing current of ~166 amps... So, for smaller fuses/breakers (100 amp or so, or less), the 14 awg should trip the over current protective device before the wire itself blows.
For normal AC house wiring, typically bare 6 awg is used (assuming something like a 100-200 amp service).
If you have a 300 amp fuse to your inverter--According to this chart, a 6 awg wire will fuse at ~668 amps... So, perhaps it is "good enough" to trip a 300 amp fuse:
http://www.interfacebus.com/Copper_Wire_AWG_SIze.html
Anyway, you should be using the NEC tables (which are derated based on a whole bunch of factors) to properly size a "safety ground" where you may expect heavy fault currents to flow.
I have not heard anything back since I have submitted the paper.
There have been a few people that have contacted me by email and phone regarding my paper (through here and/or wind-sun.com forum) that were looking up NEC and DC system grounding (yea old Google search)... Most of them were extremely surprised that that this form of ground fault detection was implemented and found it to be in direct opposition to their best practices (such as large telephony/cell systems that were going to be setup with solar pv power).
But nothing related to the NEC submission.
-Bill
This is typically considered very bad form (again from my humble opinion) to have the ground automatically lift... Heck, when I designed/installed AC equipment we had to use a separate ground stud so that when a person was servicing the device they could not unhook the green wire by accident (or double nut the safety ground/other ground connections).
led street lights (http://www.energysmartindustry.com/)
Quote from: BB. on March 18, 2011, 10:13:35 PM
One other quick way to look at the DC GFP setup... Imagine you designed a Negative Ground system and put fuses/breakers on all positive wiring. No fuses/breakers on negative wiring, and one positive solid connection between the negative bus and earth ground (including grounding to water pipe, fixtures, wiring boxes, conduit).
If there is a fault from anywhere in the Positive circuit path, positive wire will be protected by an appropriate fuse/breaker which will be tripped by the excessive current flow.
If there is any negative/return wire to earth ground, virtually nothing will happen as both are very near the same voltage. (it is possible with ground loops to overheat return wiring, but for now, NEC does not address over current from ground loops other than recommending/requiring single point grounding).
Now, you replace that one Earth Ground with a 1-5 amp fuse. And now earth ground, randomly, anywhere in the Positive Wiring Path (PV wiring or Battery wiring). Pops the 1-5 amp fuse, and stops power flowing from solar panels to/through charge controller... Now you have a positive earth grounded system with fuses in the earthed positive paths.
No current limits on Negative Grounds. Items that were assumed to be "touch safe" (like lamp sockets) are no powered by battery or PV Array voltage/power. And the current limit available anywhere on the negative/return wiring is dependent on where, specifically, the Positive to Earth Fault was made.
-Bill
Sometimes someone says something so clearly and easy to understand that those nagging thoughts that wake you in the middle of the night vanish.
Thanks.
Thank you Tinman for the kinds words... Buy the way, I fixed a typo in the original post:
QuoteNo current limits on Negative Grounds. Items that were assumed to be "touch safe" (like lamp sockets) are now powered by battery or PV Array voltage/power.
Sorry,
-Bill :-[
I emailed Wiles a while back asking for his comments about Bill's paper.
He said no changes are planned in the NEC regarding GFP, and the standard the Bill was refereing to is a European standard and does not apply here in the US.
I wonder what international standard Mr. Wiles is referencing?
In a series of emails, I had used UL 1741 (yes, that UL) standard discussing the fact that using a 1 amp fuse for connection between earth and chassis ground was not acceptable (I believe that this was after I had finished up the original PDF document linked in this thread).
He never answered back other than to thank me for the input.
Unfortunately, I have lost the emails (and even the document source) due to a computer hard drive failure.
But it was pretty basic stuff... UL requires something like a 30 amp current to test safety grounds. And elsewhere has been modified to include the 1 amp ground fault detection setting (section 31 table is dependent on device DC power rating).
Obviously, a system cannot meet both requirements of having a ground referenced return and float that when >1 amp of current flows through the safety ground bond).
I think Mr. Wiles did mention that (some?) European power systems may not (do not?) have earth grounded neutral connections--Perhaps that is what he was referring too? Of course, I was discussing NEC and earth referenced return connections (i.e., earth bonded grounds for neutrals and metal device chassis) and the fact that there are no over current protective devices in earth referenced return wiring... Which would be required with the DC GFP systems.
And thank you Laszlo for following up.
-Bill
In terms of the classic hardware, I'm reading this as
if I tie my ground bus to negative bus, it will defeat the gfp capability of the classic since the classic won't be able to detect voltage differences between ground and (-). Is that correct? Other than that, there is no impact to the classic. Is that also correct?
Quote from: jtdiesel65 on January 04, 2012, 12:50:04 PM
In terms of the classic hardware, I'm reading this as
if I tie my ground bus to negative bus, it will defeat the gfp capability of the classic since the classic won't be able to detect voltage differences between ground and (-). Is that correct? Other than that, there is no impact to the classic. Is that also correct?
That is correct. And is most likely how you would find my system with a big fat wire connecting battery negative and earth ground. ::)
We all at Midnite agree that the DC-GFP is dangerous. We will very soon, publish a paper on our website using some of Bills and our arguements against the DC-GFP system. The DC-GFP per NEC was ok back in the early 90's. As boB stated, it was too expensive to manufacture. boB designed the only DC-GFP in existence that broke both the positive and negative PV legs and also shorted the PV array. We did we do that at Trace? Because that was the way the NEC required it to be done! When John Wiles came up with the present method that EVERYBODY uses, it made the system affordable. Back then all we had was 48 volts. There was no such thing as grid tie inverters or high voltage MPPT charge controllers. 48V was not a lethal voltage. Our big beef is that DC-GFP's don't actually do much to stop fires. Arc fault detectors do however. We think it is time to eliminate the DC-GFP from the NEC and start enforcing the arc fault protection system. We have been shipping the world's only DC arc fault protector in the Classic for over a year now. Funny how the industry is hung up on getting the DC-GFP correct, but doesn't give the arc fault protector a second thought. I personally wouldn't install a DC-GFP in a system over 48 volts, but I certainly would install an arc fault protector. They really do stop fires. As a manufacturer, we can't tell people to ignore the NEC even if it may kill you. Yah right! We will campaign to get the DC-GFP thrown out of the NEC.
Quote from: Robin on January 06, 2012, 01:40:50 AM
boB designed the only DC-GFP in existence that broke both the positive and negative PV legs and also shorted the PV array.
Why short the array? Isn't it enough to just disconnect the PV+ and PV- wires?
Quote from: Robin on January 01, 2011, 05:04:46 PM
None of the DC-GFP's on the market are designed to detect and interrupt a fault in the battery circuit. <snip> The Classic GFP is not intended to interrupt battery fault currents.
It seems that outback makes a GFDI device (not their GFP device) which has
TWO 80 amp breakers ganged to a 0.5 amp breaker. Outback instructions call for the device to be installed in the Battery+ and Battery- wires, not the PV wires. I guess that is to interrupt battery fault currents.
Getting back to the subject of GFP for the array, the outback GFDI would seem to be the solution to the GFP problem... it
COULD be used to interrupt both PV+ and PV- wires. Is that a good idea, or at least better than breaking just the PV+ wire?
I do understand that if the outback GFDI is used to break both PV wires, it still does not solve the problem of leaving the ground connection unbonded, but with both PV wires disconnected there would be no way to have the DC wiring in the house floating at PV potentials.
--vtMaps
I can't remember exactly why we shorted the array now. Seems there was a good reason though.
If that memory comes back, I will post that. It's good for parallel arc faults though, I know that !
Yes, breaking both + and - PV lines as well as the ground bond should fix the shock hazard in
most cases. It still isn't an arc fault interrupter though, which we feel is a much better solution.
boB
A clarification about the Outback GFDI. This is the same GFP they have made all along. Their recent documentation shows the GFDI located after the charge controller rather than before it, but it still only opens the positive wire. The PV minus is not disconnected. The battery minus to the charge controller is not disconnected. Since the GFDI is available with two or four 80-amp poles ganged with the 0.5-amp pole, it could be used to open PV positive and PV negative. But that's not shown in the manufacturer's installation instructions.
Quote from: Kent0 on January 06, 2012, 06:42:00 PM
The battery minus to the charge controller is not disconnected.
Kent, thanks for clarifying that. I stand corrected. --vtMaps
Quote from: boB on January 06, 2012, 02:34:38 PM
I can't remember exactly why we shorted the array now.
boB
Tony Boatwright from Magnum seems to remember it this way... (Thanks Tony)
"As I remember, the GFP had a requirement to remove the power. So it was designed to short the array to effectively remove the power, but we received complaints from some PV manufacturers that they thought it would stress or damage there panels, so we went to the breaker/disconnect GFP."
I just remember that the way it is usually done now, using 2 ganged circuit breakers is WAY cheaper !
boB
One of the (many) hazards of the DC GFI is that the 1 amp current flow detection can be triggered by a + to earth fault ANYWHERE IN THE SYSTEM, not just the solar array.
Get a + to earth fault on the battery side of the bus--Turn off the PV Array--Big Whoop. Nothing has changed except the system popped a 1 amp fuse/breaker between Return and Safety Ground (fusing Safety Ground--Hello!?).
As far as I can tell (I am not in the industry or NEC), the DC GFI was "invented" to reduce the chance of Arc Faults.
DC GFI only stops + to Earth fault current flow (direct connections and arc current flow). And, if you look at any of the single fault scenarios I documented, you could toss all of the 80 amp/1 amp ganged breakers and just use the 1 amp fuse for exactly the same protection. Any of the extra hardware does nothing to enhance/improve safety as a direct result of its implementation. And fusing the safety ground link causes many more issues that it fixes.
It appears that most arc faults are probably the result of failed connectors/wiring connections (i.e., series arc faults in wiring). Not because of a + to earth fault. The DC GFI will not reduce/prevent any of those types of earth faults/arc faults from occurring.
I am not sure I am a big fan of Arc Fault Breakers (don't know enough about them to have an informed opinion), but they at least do what was "promised" by the old DC GFI protection system (detect and stop series arc faults--Not so sure about + to Earth arc faults unless breakers are installed near the array itself)--Without creating a whole bunch of other system level safety/fire hazards. And will meet UL and other code requirements without having conflicting requirements written into the code/standards to require said implementation.
By the way, "shunting" a power system was/is a common way of reducing hazards... We call it "crowbarring" (throwing a "crowbar" across the outputs of) the power supply (typical on output of computer power supplies to stop run-away regulated voltages from damaging the downstream electronics). Of course, if you "crowbarred" a 12-48 volt battery bank--that would be an exciting event to witness (at somebody else's installation/home).
The fact that solar panel designers have an issue with crowbarring their output makes me think they do not have as much margin in their product as they should. The difference between Imp and Isc (max power vs short circuit) is not that much. Panel should survive Isc as well as Imp loads.
Good Luck guys on getting the Arc Fault Breakers/System implemented and the DC GFI pulled from the industry requirements.
-Bill