MNEPV**-300 Breakders have two connections?

Started by keithwhare, December 02, 2010, 11:17:25 PM

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boB

#15
May I suggest that when one wants to disconnect the CC from the battery using the battery side breaker, that they first disable the Classic in the MODE menu by turning the MODE, OFF.  Then there will be very little current through that breaker.   This also goes for turning off the input breaker if possible just to make it easier on everything.  The current that normally trips the battery breaker when a CC breaks is FROM the battery into the charge controller.

It's typically that high, unlimited battery current that breaks things the worst.

I find though that when the usual currents go through these DC breakers, even in the reverse direction, they work OK.   It's the unusually high currents that seem to make them work harder trying to turn off the current. When I say unusually high, I mean like, several times their normal current carrying amount but less than their AIC rating.

Just  don't use them as a disconnect very often in their reverse mode and they should last a long time.  Circuit breaker returns are VERY rare BTW and AFAIK are even rarer for reasons of reverse polarity.

boB
K7IQ 🌛  He/She/Me

RegGuheert

Quote from: boB on March 02, 2011, 01:18:35 AMMay I suggest that when one wants to disconnect the CC from the battery using the battery side breaker, that they first disable the Classic in the MODE menu by turning the MODE, OFF.  Then there will be very little current through that breaker.   This also goes for turning off the input breaker if possible just to make it easier on everything.
That sounds like very good advice!  I will try to do this as a matter of practice and I may even put a placard by the breakers to let the homeowners know, too.
Quote from: boB on March 02, 2011, 01:18:35 AMThe current that normally trips the battery breaker when a CC breaks is FROM the battery into the charge controller.
Understood.
Quote from: boB on March 02, 2011, 01:18:35 AMIt's typically that high, unlimited battery current that breaks things the worst.

I find though that when the usual currents go through these DC breakers, even in the reverse direction, they work OK.   It's the unusually high currents that seem to make them work harder trying to turn off the current. When I say unusually high, I mean like, several times their normal current carrying amount but less than their AIC rating.
That's what I have been getting at:  due to the fact that the the PV panels limit their own current the fault conditions you face in the PV Combiner tend to be more forgiving than those associated with disconnecting from the batteries.
Quote from: boB on March 02, 2011, 01:18:35 AMJust  don't use them as a disconnect very often in their reverse mode and they should last a long time.  Circuit breaker returns are VERY rare BTW and AFAIK are even rarer for reasons of reverse polarity.

boB
Thanks, boB!  I think we are on the same page now.

In the end, I guess selecting DC circuit breaker polarity comes down to trying to choose the lesser of two evils:

- For the DC breakers connected to the battery you have no choice on polarity since any faults will be fed by very high currents from the battery:  You MUST connect the positive terminal to the battery side or it very likely will not be able to clear a fault to ground.  You then are left to live with arcs in the opposite direction during daily use as a disconnect switch.  Extinguishing those arcs is much easier due to the lower current levels, so the breaker may be able to handle quite a few such disconnects before it fails.  On the other hand, these arcs are much more common than arcs from faults.  Best practice, therefore, is to reduce the current through the breaker as much as possible before throwing it using the paddle.

- For the DC breakers used in the PV combiner, the fault current for a shorted string is limited to Isc for a single string times N-1, where N is the number of parallel strings.  The breaker is capable of clearing a fault of this magnitude in the direction opposite of the design direction, as has been certified by UL and NEC testers.  So judgment here goes to ensuring that repeated disconnects of Isc or less by using the paddle do not incrementally damage the breaker to the point where it can no longer clear the fault.

Am I on the right track to understanding how you have arrived at this arrangement?

boB

Quote from: RegGuheert on March 02, 2011, 06:30:01 AM
Quote from: boB on March 02, 2011, 01:18:35 AMMay I suggest that when one wants to disconnect the CC from the battery using the battery side breaker, that they first disable the Classic in the MODE menu by turning the MODE, OFF.  Then there will be very little current through that breaker.   This also goes for turning off the input breaker if possible just to make it easier on everything.
That sounds like very good advice!  I will try to do this as a matter of practice and I may even put a placard by the breakers to let the homeowners know, too.
Quote from: boB on March 02, 2011, 01:18:35 AMThe current that normally trips the battery breaker when a CC breaks is FROM the battery into the charge controller.
Understood.
Quote from: boB on March 02, 2011, 01:18:35 AMIt's typically that high, unlimited battery current that breaks things the worst.

I find though that when the usual currents go through these DC breakers, even in the reverse direction, they work OK.   It's the unusually high currents that seem to make them work harder trying to turn off the current. When I say unusually high, I mean like, several times their normal current carrying amount but less than their AIC rating.
That's what I have been getting at:  due to the fact that the the PV panels limit their own current the fault conditions you face in the PV Combiner tend to be more forgiving than those associated with disconnecting from the batteries.
Quote from: boB on March 02, 2011, 01:18:35 AMJust  don't use them as a disconnect very often in their reverse mode and they should last a long time.  Circuit breaker returns are VERY rare BTW and AFAIK are even rarer for reasons of reverse polarity.

boB
Thanks, boB!  I think we are on the same page now.

In the end, I guess selecting DC circuit breaker polarity comes down to trying to choose the lesser of two evils:

- For the DC breakers connected to the battery you have no choice on polarity since any faults will be fed by very high currents from the battery:  You MUST connect the positive terminal to the battery side or it very likely will not be able to clear a fault to ground.  You then are left to live with arcs in the opposite direction during daily use as a disconnect switch.  Extinguishing those arcs is much easier due to the lower current levels, so the breaker may be able to handle quite a few such disconnects before it fails.  On the other hand, these arcs are much more common than arcs from faults.  Best practice, therefore, is to reduce the current through the breaker as much as possible before throwing it using the paddle.



Quote from: RegGuheert on March 02, 2011, 06:30:01 AM

- For the DC breakers used in the PV combiner, the fault current for a shorted string is limited to Isc for a single string times N-1, where N is the number of parallel strings.  The breaker is capable of clearing a fault of this magnitude in the direction opposite of the design direction, as has been certified by UL and NEC testers.  So judgment here goes to ensuring that repeated disconnects of Isc or less by using the paddle do not incrementally damage the breaker to the point where it can no longer clear the fault.

Am I on the right track to understanding how you have arrived at this arrangement?

I think you're pretty close  there...   One nit-pick...  If the whole array (minus one string) is ganging up on one string with one shorted module, then the current going into that one faulty string will not be at 0 Volts, but just above the Voc of the remaining string of series modules of that faulty string.  Depending on how large a portion of the entire string voltage that shorted module was, the current at that voltage might be close to Isc because Isc isn't that much higher than Impp of the remaining non-faulty strings.

This scenario has just never been a problem that I know of and shorted modules are just extremely rare.

You could always use fuses, but you can't open those under load.

I KNEW there was a good reason for an OFF mode !  I don't think that any other controller has an off mode ??

boB

K7IQ 🌛  He/She/Me

RegGuheert

Quote from: Robin on March 01, 2011, 11:47:38 AMIf there was such a thing as a non polarity DC breaker, we would be using them, but they do not exist.
There IS such a thing.  If I am not mistaken, YOU sell some of these breakers.  I went to check the polarity of the 100A DC breaker that I installed between the Classic 200 and the battery and I found NO polarity markings on it.  Vendor describes it as "MidNite Solar EDC-100-150vdc Panel Mount Breaker".  If I missed the polarity marking on this, please let me know, as I want to make sure this one is correct, since it might one day be called upon to break a very large current!

I have also found some PV combiner DC breakers for overseas applications which are non-polarized.  However, they were for high-voltage application and they seem to be designed to switch both the positive and negative legs of the circuit.  Perhaps that is code overseas?

All that said, I do not see any non-polarized breakers for the PV combiner application at hand.

Thoughts?

Reg

ericwahl

Quote from: Robin on March 01, 2011, 11:47:38 AM
If there was such a thing as a non polarity DC breaker, we would be using them, but they do not exist.

Well they do appear to exist, based on a quick google search,

http://www.dksh.com.au/htm/620/en_AU/800V-DC-circuit-breakers.htm?Id=109790&BUId=24905


boB

#20
Quote from: ericwahl on April 25, 2011, 11:57:04 AM
Quote from: Robin on March 01, 2011, 11:47:38 AM
If there was such a thing as a non polarity DC breaker, we would be using them, but they do not exist.

Well they do appear to exist, based on a quick google search,

http://www.dksh.com.au/htm/620/en_AU/800V-DC-circuit-breakers.htm?Id=109790&BUId=24905


Thanks for finding that !   Those are Alan Bradley breakers.

Unfortunately, I don't think those are usable in our systems due to price and size problems.

boB
K7IQ 🌛  He/She/Me