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AC circuit breakers on DC strings of PV


PaulinNorthcliff

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Hi all,

Years ago when building a solar system I balked at paying for DC circuit breakers to switch out strings of panels. I installed AC circuit breakers just before the DC fuses and after the surge arrestors (so a pretty well-controlled system in general). At the time DC circuit breakers for 400VDC (double DIN units) were about R500 each. I reckoned that even though DC arcing would ruin the switches with multiple on-off repetitions, these would suffice in a situation when I would very infrequently need to switch a string on or off.

In the last week I had a string play up so tried to 'hard reboot' several times. The smoke escaped... and left the string offline.

I again went looking (kak timing) for DC CBs only to find they are still exorbitantly expensive. I have dropped in a cheap R75.00 AC CB as I contemplate the cost/benefit of DC CBs.

All the literature rages against using AC where DC switches should go, but if the smoke escaping after 3 years is the worst that can happen, where's the harm?

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18 minutes ago, PaulinNorthcliff said:

Oh, and DC breakers are unobtanium at this time of the year.

I agree with the AC breakers, ideally you switch them off before power is being produced and there's no harm, if you need to switch with power being produced, shade one or two of the panels in the series string you're about to switch off and again, no major power should flow and no major smoke should be produced... but don't talk too loud, a few others on here will surely jump down your and my throat for suggesting such sacrilege...

 

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11 minutes ago, Kalahari Meerkat said:

I agree with the AC breakers, ideally you switch them off before power is being produced and there's no harm, if you need to switch with power being produced, shade one or two of the panels in the series string you're about to switch off and again, no major power should flow and no major smoke should be produced... but don't talk too loud, a few others on here will surely jump down your and my throat for suggesting such sacrilege...

 

Indeed.

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1 hour ago, PaulinNorthcliff said:

In the last week I had a string play up so tried to 'hard reboot' several times. The smoke escaped... and left the string offline.

You will make fire every time you open the AC breaker under load, not just after 3 years. I think all the other times you were just lucky and the string was not under load.

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24 minutes ago, P1000 said:

You will make fire every time you open the AC breaker under load, not just after 3 years. I think all the other times you were just lucky and the string was not under load.

Probably true. I understand the concept and the DC vs AC spark. Frankly, at the price differential, it still pays.

Or, am I wrong?

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21 minutes ago, PaulinNorthcliff said:

it still pays.

Or, am I wrong?

Well, if you have a fire at your home, and the insurance investigators figure it out, that DC breaker will look like a real bargain!

On inverters with a DC breaker (like the popular SunSynk/Deye/GoodWe models), you only need to fuse the panels, the breaker on the inverter is sufficient if mounted at the correct height.

Edited by P1000
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7 minutes ago, Kalahari Meerkat said:

explain this one to me, please, panels are rated for XAoc, let's say 10A, so you put in a 10A fuse, when will this fuse ever blow?

The fuse is to protect the wires from making flames. In some inverters there are fault conditions that can connect your battery directly to the PV in. You also want a second way to make sure the panels are disconnected and not loaded when doing maintenance.

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2 minutes ago, P1000 said:

In some inverters there are fault conditions that can connect your battery directly to the PV in.

OK, if that's the case, then that would really only apply to the low DC Voltage PV in inverters like the 145V max PV in Axperts, but not really to any of the high DC Voltage 450V max PV in, inverters, since the battery is far removed Voltage and circuit wise from those.

As for

5 minutes ago, P1000 said:

The fuse is to protect the wires from making flames.

it would only protect the wires to the solar panels and only again, applicable to the low DC Voltage PV in inverters, the wire between the fuse and the inverter could still conceivably go up smoke before the fuses blow and since the low(er) Voltage PV side has much higher current requirements, I can think that it likely would still let the wires go up in smoke, even towards the solar panels, if the manure were to strike the fan and the battery end was connected through to the panels...

Thanks, I have no fuses in line here and have had 2 occasions where I had to do work and disconnect the solar panels, both times was done after sunset with nary a crackle pop or snap on the switching...

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5 minutes ago, Kalahari Meerkat said:

Thanks, I have no fuses in line here and have had 2 occasions where I had to do work and disconnect the solar panels, both times was done after sunset with nary a crackle pop or snap on the switching...

Yeah, the fuses are also a legal requirement for battery inverters. If you don't have a battery, you can do without fuses, as long as the wires are oversized to greater than 1.56x Isc. Not installing fuses with a battery inverter should mean no CoC. But even if you have a CoC, in the event of a fire, the insurance might not pay out.

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17 hours ago, P1000 said:

Well, if you have a fire at your home, and the insurance investigators figure it out, that DC breaker will look like a real bargain!

On inverters with a DC breaker (like the popular SunSynk/Deye/GoodWe models), you only need to fuse the panels, the breaker on the inverter is sufficient if mounted at the correct height.

Ed Zachery. I have the Deye 8kw. The big DC rotary switch disconnects ALL of the strings. I have 4 strings of panels... each has separate surge protection on positive and on negative, then each has a breaker, then each has a DC fuse, before the inverter and the big rotary switch.

This way I can isolate each string.

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23 minutes ago, PaulinNorthcliff said:

This way I can make plasma flames.

I fixed it for you.

Remove the AC breakers to make it compliant.

To disconnect, make sure the inverter is not generating power, use the DC disconnect on the inverter to disconnect all the PV, open the fuse of the string you want to work on, and then you can work on that string.

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