I am about to pull the switch on a new installation. I have two PV arrays each using 4 panels X 250Watts ea (~ 1000 watts/array OR 2000 Watts for the pair). Each array wires directly to a Midnite MNPV6 divided in half for two circuits of 8 amps each that each utilize a separate MNEPV15-300V breaker. Voc of each array is about 155 V room temp; nominal current is about 8 amps - full sun. From my Array Combiner mounted on one PV Pole, I run 2 pair of #4 Aluminum URD cables (one pair for each array @ 8 amps) about 200 feet to my MIDNITE "pre-assembled/pre-wired E-Panel. I change from Al to Cu at the Barn wall using bi-metal barrels and the appropriate "putty". Inside the barn wall, going into my E-Panel, I am entering with two short pair of #8 Copper wires (5' max). Everything inside the E-Panel is Copper.
Notably, there is 1 ea. MNEPV30A 300V breaker in the "Pre-wired" E-Panel to disconnect the PV array from the Charge Controller and I have another (perhaps redundant?) pair of 300V breakers at the PV array site 200 feet distant from the Barn wall where each each PV cable from the solar panels connects to the combiner. So there are 300 V MIdnite breakers at each end of the 200 ft long PV cable pairs... The two breakers at the array site each carry 8 amps and the single breaker at the E-Panel carries 16 amps... I restate they are all 300 V breakers.
I read, "NEVER OPEN A PV CIRCUIT UNDER LOAD". QUESTION: IS THIS TRUE FOR THIS TYPE OF BREAKER AT THE PV PANEL SITE, OR - the E-Panel site or BOTH? I think the self inductance of the lead cable running 200 feet to the E-Panel location would create very high voltages under any 'open circuit" event if it occurred under load. I am sure the arc created if opened under load would exceed 300V. Is there a capacitor inside the breakers sold to clip the HV transient? What about installing a 1000V .1 ufd across each breaker? What about arc risk to opening either of the two Breakers located very close to the PV arrays? There is only about 10 feet of wire length coming to the combiner from the solar panels; not much self inductance...
Can anyone say they OCCASIONALLY open MIDNITE HV PV breakers under load with no failure consequence?
Is the best practice to open the circuit if necessary only at night? What about putting a piece of cardboard over one panel to block light and force zero PV volts prior to disconnecting any series breaker if needed to be done during daylight hours? What is BEST PRACTICE in this case?
I also read extreme cautions for using "TOUCH SAFE FUSE HOLDERs"... is a MNEPV15-300V style breaker a far more robust type of disconnect than a "touch safe fuse holder" whatever that happens to be????? If a Touch Safe Fuse holder blows why not a 300V MNEPV breaker? How are they different with respect to arc faulting?
Thanks for any guidance offered...
clivusmul - KB7GX
Well I can say all of our breakers are rated as switches as well and are designed to open under full load Voltage and Amperage. I have opened them repeatedly myself with no issues. I do not really think 200ft of wire will cause a spike in voltage unless it was open, closed, opened fast?
I do want to point out do not open fuse holders under load though.
Ryan
Thank you Ryan. I really appreciate your always prompt replies.
What allows the Midnite PV breaker to survive when the "Touch-Safe" fuse holder always fails? Can you say how they are different in this regards? They both are exposed to the same arc-transient event...
I was thinking of using a "pull-out" disconnect, at the PV panel termination site, like the ones used behind an A/C compressor - essentially a copper bus bar pull out shunt. They come in weather proof boxes and would serve the purpose. However your Midnite PV combiner comes with 300 V PV Breakers...
I was more concerned about the PV breaker at the E-Panel due to the long cable run and that specific breaker handling 16 amperes. Anytime a fast open occurs with any self or mutual inductance (to some adjacent magnetic field) in a conductor carrying current, a fast high voltage event will occur. The energy discharged during the "open" event is associated with the arc event time, the driving inductance and current being interrupted. Remember, V = L x dI/dt... ANY inductance present will force a voltage transient that could be high for a brief moment; why you see an arc when you open a knife switch under these conditions. If the energy is small, there is usually no damage... My guess is the Midnite PV breakers are designed to absorb this type of "normal" arc-event energy at breaker open..
The Midnite PV Breaker must be designed with contact materials and/or gap spacings that accommodate some level of contact arcing at a "load open" event. I do have two SPARES in the event one fails. I just want to avoid a fire in my Midnite E-Panel. It is a beautiful piece of equipment... by far the best one available at any price!
Hi clivusmul,
The Classic CC has a Mode function in the main menu. Simply set the Mode to Off when you wish to open any of the DC breakers. With the Mode Off, the CC is resting, and essentially, no current is flowing through the Breaker(s).
There must be some incductive kick on opening the DC breaker at the CC with current flowing, but IMHO, this is small compared to the effects caused by the DC current that may be flowing. As Ryan noted these MN breakers are rated for switching DC currrents up to the maximum rating rating of the breaker. I like to avoid, if possible, switching breakers if current is flowing. Vic
The difference between a breaker and fuse holder is huge. The fuse holder is just that and there is no attempt to have contacts, arc shutes or magnets to extinguish arcs. The breaker on the other hand is designed specifically to extinguish the arc under worst case scenario. I can not comment on inductance in the line causing voltage rise but the manufacturer of the breaker has not put a wire distance on there breaker and I have several running on 400ft of pv wire at 30 amps 200 volts and have opened the 150 volt breaker several times. Even held it in the arcing position during Arc fault testing.
Ryan
Great thanks to both Vic and Halfcrazy... I appreciate your thoughtful responses... I will report next week on how the 'beast' fired up! :)
bj-clivusmul
At work for resetting a regular 20 amp 120 v AC breakers they now require us to wear long sleeve shirts, leather gloves, and safety glasses. I could see that for some really old crappy breaker that has seen much better days , but for newer breakers that seems like overkill safety precautions. I guess if one breaker anywhere in the world ever arc flashed then everyone everywhere is subject to these new rules. I know arcflash is serious stuff - like when you have the panel cover off and are poking around inside a panel - shut the power off and be careful !
But breakers should be designed like Ryan says to not blow up from the simple act of breaking their intended load.
We have a document somewhere on our website that talks about the ETL testing done years ago on our din rail breakers. It was an eye opener. Basically we took a 63 amp. 125VDC CBI breaker and repeatedly broke 104 amps at 165 volts. 50 times I think. Then we had to watch the breaker trip when shorting out the 13 group 27 batteries in series. We had a few thousand amps flowing for those repeated tests also. These breakers are tough. There isn't anything you can possibly do to hurt them with only 4 solar panels!
Quote from: Robin on November 20, 2013, 02:41:09 AM
It was an eye opener. Basically we took a 63 amp. 125VDC CBI breaker and repeatedly broke 104 amps at 165 volts. 50 times I think.
These breakers are tough. There isn't anything you can possibly do to hurt them with only 4 solar panels!
Robin, is this a good thing when we are using wire physically
smaller than 4 ga wire inside the E-panel to the CC? Where we IMHO really DO want the breaker to break at ~ 60 amps?
Quote from: Westbranch on November 20, 2013, 03:22:12 PM
is this a good thing when we are using wire physically smaller than 4 ga wire inside the E-panel to the CC? Where we IMHO really DO want the breaker to break at ~ 60 amps?
This was about testing breakers. The breaker will still trip at 63 amps.
Breakers trip based on time AND current. As a testing procedure you will find that if you apply 104 amps to a 63 amp breaker, it will trip sooner than if you apply 63 amps to it.
--vtMaps
Thanks VT, I think I have it, so in this case:
we took a 63 amp. 125VDC CBI breaker and repeatedly broke 104 amps at 165 volts
Robin is talking about an instantaneous tripping of the CB...?
I still have trouble understanding how they can get to that high a current AND Voltage before tripping occurs.
Quote from: Westbranch on November 21, 2013, 11:55:38 AM
Thanks VT, I think I have it, so in this case:
we took a 63 amp. 125VDC CBI breaker and repeatedly broke 104 amps at 165 volts
Robin is talking about an instantaneous tripping of the CB...?
I still have trouble understanding how they can get to that high a current AND Voltage before tripping occurs.
The 63 amp breaker is "supposed" to not trip AT 63 amps but should trip at currents above that.
The higher the current, the faster it should trip. The amount of time it takes to trip is somewhat
variable though.
If you have 104 (or so) amps running through the 63 amp breaker, it should trip and open
up fairly soon (seconds) and then as it is tripping/opening, there is a a DC arc across its
arc-schute internally which is voltage across it. This takes (hopefully) milliseconds.
Then, when it is completely open, that maximum voltage, in this case,165 volts DC is across it
but no current passing through it. It is that combined current AND rising voltage that
is powering that DC arc (power = voltage X current) that has to be withstood until
it completely opens. It has to do this at least 50 times during that test.
So, the 165 volts is open circuit voltage (Voc) and not at 104 amps. That would
be 165V X 104 = about 16,500 watts ! It can't handle that sustained but maybe
for milliseconds.
boB