I was chatting to a mate who does installations and he was saying that he is planning on replacing his DC fuses between his panels and inverter with DC breakers instead.

He had heard that fuses were a fire risk and breakers are safer. That is the exact opposite of what I have heard and there are numerous reports from the Aussies about breakers/isolaters catching fire.

If I look at commercially available combiner boxes they normally just have breakers.

I watched a video where an installer was explaining that breakers cant always handle the load when they get turned off while the sun is shining and the panels are producing. The sparky in the vid explained that the contacts in a DC breaker are too small to stop that power safely. Just like he said, when he turned the breaker off under power it melted down and caught fire.

I have fuses rated according to my panel manufacturer and no breakers - the Sunsynk has a contactor for disconnecting PV. 

So to all of the clever people who have experience in these matters, which is the correct one to use? should it be fuses into breakers?

On 2023/04/11 at 10:02 AM, Superfly said:

Can you post the link to that? DC breakers are safe if sized correctly, same for fuses - never heard of them catching fire other that due to arc-ing which happens when an AC breaker is used on DC.

There are quite a lot of recorded cases in Australia, here is one vid I watched on the topic:

I agree that there shouldnt be a reason if there is a arc diffuser like in DC breakers, but the argument that the contacts being disconnected under load can fuse and melt down is a little scary.

Is the solution to have DC breakers *and* fuses? seems a bit overkill to me.

23 minutes ago, RocketBoy said:

There are quite a lot of recorded cases in Australia, here is one vid I watched on the topic:

I would have loved to see a multi-meter indicating correct polarity before he did that test. If I recall, there were no instances of fires attributed to breakers wired with the correct polarity - only when they were installed backwards.

Be careful with certain fuses. This is what happened to a friend less than 24hrs after installation and at about 16h30 far from peak power. Fuse holder melted that one could not even get it to open around the pivot point. Now this is a 1000V DC fuse but what about the holder? 

6 minutes ago, Superfly said:

Those din isolators are useless. I'm pretty sure a fuse would have blown with that short circuit current anyway always use a DC breaker with Short-Circuit and Overload protection.

Without seeing the fuse myself the rating was higher than the panel Isc. Further if blown it would have been during peak full sunshine day not 16h30. My take is just a hot connection in the fuse holder. 

On 2023/04/11 at 9:18 AM, RocketBoy said:

I was chatting to a mate who does installations and he was saying that he is planning on replacing his DC fuses between his panels and inverter with DC breakers instead.

He had heard that fuses were a fire risk and breakers are safer. That is the exact opposite of what I have heard and there are numerous reports from the Aussies about breakers/isolaters catching fire.

If I look at commercially available combiner boxes they normally just have breakers.

I watched a video where an installer was explaining that breakers cant always handle the load when they get turned off while the sun is shining and the panels are producing. The sparky in the vid explained that the contacts in a DC breaker are too small to stop that power safely. Just like he said, when he turned the breaker off under power it melted down and caught fire.

I have fuses rated according to my panel manufacturer and no breakers - the Sunsynk has a contactor for disconnecting PV. 

So to all of the clever people who have experience in these matters, which is the correct one to use? should it be fuses into breakers?

Sorry but, why will any one break the circuit under load? shouldn't you first switch off all the loads before isolating the panels? In MHO, it doesn't matter whether you use fuses or isolator or both. I prefer using both 

1 hour ago, hoohloc said:

Sorry but, why will any one break the circuit under load? shouldn't you first switch off all the loads before isolating the panels? In MHO, it doesn't matter whether you use fuses or isolator or both. I prefer using both 

All your loads could be off. Also the inverter can be off as well but charging will still take place based on the PV input and charge setting thus under load. 

11 hours ago, Scorp007 said:

All your loads could be off. Also the inverter can be off as well but charging will still take place based on the PV input and charge setting thus under load. 

how will charging take place when the batteries are off and isolated? Again, it is always best and safe to remove all the load before switching off. The load includes batteries as well. Think safety first and you will not go wrong 

It is a legal requirement that there be a 'switch-disconnector' on each supply into a device.

So it is required that at least one of the disconnection devices (be it a circuit breaker or fuse) be rated to disconnect under load.

So this symbol (line with circle - either on its own, or in combination with others) should appear on one of the devices:

These disconnectors serve 2 purposes:

1) safely isolate all power sources when working on a device, and

2) emergency disconnect of power to a device when something goes wrong.

The second case requires something that can switch under load.

5 hours ago, hoohloc said:

how will charging take place when the batteries are off and isolated? Again, it is always best and safe to remove all the load before switching off. The load includes batteries as well. Think safety first and you will not go wrong 

The advice was to switch off loads but not battery. In such a case the battery is still connected and the panels should then not be isolated under load while charging the battery. 

3 minutes ago, Scorp007 said:

The advice was to switch off loads but not battery. In such a case the battery is still connected and the panels should then not be isolated under load while charging the battery. 

The advice is to switch off all the loads. When all loads are off, that include batteries, you will not have current flowing from the PV panels and it will be safe to switch them off. Always work safe 😉

On 2023/04/13 at 7:19 AM, Scorp007 said:

All your loads could be off. Also the inverter can be off as well but charging will still take place based on the PV input and charge setting thus under load. 

^This.

DC doesnt play games like AC does in terms of waveform so there are no dips which makes it difficult to break. In theory you should never turn any of that DC off under load but what happens if there is a fault and you have no choice?

The inverter will limit the use of solar if there is nothing to do with it, but the full DC current is still reaching right through to the MPPT controller.

18 minutes ago, RocketBoy said:

^This. In theory you should never turn any of that DC off under load but what happens if there is a fault and you have no choice. 

It is an abnormal condition so no time to think I should not switch off. Use any means to switch off-even if it means cutting a wire/cable and risk another flame. 

This is what I will do. 

You cannot use a breaker as a switch on pv dc due to arcing. You use a fuse, with a dc breaker and surge protection device tgat needs to be earthed for lighting protection 

If there is a fault, the fuse will blow to protect the breaker. 

 

 

 

You cannot use a breaker as a switch on pv dc due to arcing. You use a fuse, with a dc breaker and surge protection device tgat needs to be earthed for lighting protection 

If there is a fault, the fuse will blow to protect the breaker. 

 

 

Interesting reason for the fuse. 

Just a question. If a panel with a Isc current 11.68A as in 1 has a 20A fuse as in 2 per the manufacturer how will the panel be able to blow the 20A fuse? 

 

 

Edited September 19, 20232 yr by Scorp007

 

If a panel with a Isc current 11.68A as in 1 has a 20A fuse as in 2 per the manufacturer how will the panel be able to blow the 20A fuse?

The fuse is primarily to protect a single string when multiple strings are in parallel (i.e. in a combiner box), where the sum of the currents from other strings can exceed the current limits of the bypass diodes in a bad string.

This is also why fuses are recommended, as the fault current could be a reverse current. So you would need non-polarised breakers in a combiner.

AFAIK only Australia has a regulation requiring fuses instead of breakers.  I can't see any local regulation requiring a fuse instead of a breaker.

I hope someone can explain the following to me. We have a solar system, and the contractor installed 16A DC fuses on all panels strings positive and negative size. And then again at the combiner box both live and negatives cables had fuses in. He had a bad connections on MC4 connectors and eventually the join box started to burn in total there were 18 strings in this box. After we extinguished the fire none of the fuses at the combiner box and at the panels were blown.

Why?

On 2023/04/13 at 11:04 AM, hoohloc said:

how will charging take place when the batteries are off and isolated? Again, it is always best and safe to remove all the load before switching off. The load includes batteries as well. Think safety first and you will not go wrong 

To stop the load of the panels the batt must be switched off before you can switch the panels off. Now the batt which is charged by the panels must be switched off which is under load. So now the batt switch is going to arc? 

2 hours ago, Herbie. said:

To stop the load of the panels the batt must be switched off before you can switch the panels off. Now the batt which is charged by the panels must be switched off which is under load. So now the batt switch is going to arc? 

I assume the battery switch turns off the BMS which shuts down the battery as opposed to it being a switch that disconnects the current by introducing a break in the cable. If the switch did the latter the battery would remain on as it can power itself. 

My batteries do not have on/off switches, so not to sure

Edited December 7, 20241 yr by I84RiS

Sorry what I mean is the switch that cuts the battery cable to the inverter. This switch is under load because the PV is connected.

Also what happens if you switch the batt off with its own built in breaker with PV connected. Remember it says to disconnect batt first before PV. 

3 hours ago, Herbie. said:

So now the batt switch is going to arc? 

Since the Voltage on the battery side is likely to in the 50V range, unless you're talking about a HV battery setup, the arc would be a lot less than the potentially 150+V on the solar end, personally I have never needed to disconnect the solar panels yet, although, with lightning at times, it might not be a bad idea, but then I would want to include latching relays on the panel side, to disconnect the panels from the cabling, making its way to the Inverter/MPPT and a contactor on the Inverter/MPPT side of the cable, no lightning induce energy should then be an issue...

I also thought about the voltage being to low for an arc.

So it would be safe to switch it off.

Thank you. 

On 2024/12/04 at 3:39 PM, Stiaan Botha said:

I hope someone can explain the following to me. We have a solar system, and the contractor installed 16A DC fuses on all panels strings positive and negative size. And then again at the combiner box both live and negatives cables had fuses in. He had a bad connections on MC4 connectors and eventually the join box started to burn in total there were 18 strings in this box. After we extinguished the fire none of the fuses at the combiner box and at the panels were blown.

Why?

This is the biggest hole in PV regs at the moment, worldwide.

It is not the current that is the problem, but a bad connection which causes an arc. An arc can cause tremendous heat and a fire without there being an overcurrent.

Many countries are now mandating that solar strings be protected by an Arc Fault Circuit Interrupter, which will detect arcs and break the circuit. Many modern MPPTs now include this as standard. But older installations will always be at risk.

For installations without AFCI, the only solution is regular inspections and maintenance of all DC junctions.

1 hour ago, Herbie. said:

I also thought about the voltage being to low for an arc.

So it would be safe to switch it off.

Thank you. 

Your typical arc welder works at around 50V and can pull a nice fat 3cm arc...

By law the batteries must have a disconnect device rated for switch-disconnect (isolate under load).

35 minutes ago, JustinSchoeman said:

Your typical arc welder works at around 50V and can pull a nice fat 3cm arc...

A DC rated switch should have some form of plasma extinguishing feature and since rule of thumb in my book spark gap of 1kV/mm should be a reasonable value to sustain a plasma arc, on a 100+A switch for the batteries, I'm sure the gap between the contacts are probably much more than a mm and even if it were to create a small arc at 50V, it would probably die an infant death and not do too much damage, if any at all.... now if you're running 300V panel string at 18A, well, that looks a whole lot more damaging, perhaps... after all, when you are disconnecting the batteries, are they even providing or taking any current at the time, if not, no spark/arc will occur....