I share this to raise the question because this is the 3rd or so time I have seen this used on a PV installation. These are 32A AC isolators found in a PV DC Combiner Box. 

The more concerning thing is the Array string in this Installation are all High Voltage, so they are all close to 400V DC. 

To me this is a hazard, especially if you disconnect these isolators under high load with the fuses still in tact. The arcing air gap distance for AC Vs DC is not the same. AC the air gap is spaced closer. The DC air gap requires more gap so meaning this can arc & can potentially cause a fire 🔥. 

Please, share your thoughts on the subject...

Just now, Steve87 said:

Please, share your thoughts on the subject...

since you can easily open the fuse, then open the isolator, there should be no issue, yes, sure its not exactly the way it is supposed to be, but once the fuse is open, there's no more current flowing and the AC isolator would not be problematic... just a switch under no load...

Big difference between DC en AC. After one opening under load you can chuck those AC Isolators.

Big fire hazard.

The rated voltage of these breakers are also to low for the circuit.

Waiting for fireworks.

Edited September 2, 20223 yr by Jacques Ester

I get your reasoning, but there is a reason there is DC rated specified fuses. I have since removed these because they are not the correct spec & are not required because the fuses serve as isolators & fuses in one. I removed them also because I installed DC SPDs & this was not previously installed.

 

This video sums it up quite nicely:

 

1 hour ago, Steve87 said:

I share this to raise the question because this is the 3rd or so time I have seen this used on a PV installation. These are 32A AC isolators found in a PV DC Combiner Box. 

The more concerning thing is the Array string in this Installation are all High Voltage, so they are all close to 400V DC. 

To me this is a hazard, especially if you disconnect these isolators under high load with the fuses still in tact. The arcing air gap distance for AC Vs DC is not the same. AC the air gap is spaced closer. The DC air gap requires more gap so meaning this can arc & can potentially cause a fire 🔥. 

Please, share your thoughts on the subject...

Hi Steve. That’s nothing. I have seen AC breakers used between batteries and inverters. I’m still not sure why one would use a breaker and a fuse as the way the fuse is designed is to act as a breaker as well. Obviously good practice dictates that there should never be current flowing when the fuses are opened or closed. 

23 hours ago, Kalahari Meerkat said:

since you can easily open the fuse, then open the isolator, there should be no issue,

Except that the fuse holder is not rated for disconnection under load, especially under DC load. Often they will be labelled as such. You could easily have an arc follow you from the conductors inside the fuse holder and kill you. 400 VDC arcs can continue to about 400 mm, once an arc is started. High voltage DC is very dangerous unless the equipment is designed to work with it.

52 minutes ago, Coulomb said:

Except that the fuse holder is not rated for disconnection under load, especially under DC load. Often they will be labelled as such. You could easily have an arc follow you from the conductors inside the fuse holder and kill you. 400 VDC arcs can continue to about 400 mm, once an arc is started. High voltage DC is very dangerous unless the equipment is designed to work with it.

Thanks pointing this out. We can get used to our high voltages at not too high current. Once we see the flames of 400V we will think twice at what equipment we use and the danger of DC flames.

The most important has always been a blow out magnetic field to break the DC flame during opening.

I once turned on a DC rated PV breaker while an Axpert inverter was under load and the inverter blew up with a code 9 fault. So whether it’s a DC breaker or a fuse, it’s best to disconnect when there isn’t any load. 
 

Similarly, nobody disconnects a battery fuse when the battery is supplying current to an inverter at whatever voltage. 

3 hours ago, Leshen said:

I once turned on a DC rated PV breaker while an Axpert inverter was under load and the inverter blew up with a code 9 fault. So whether it’s a DC breaker or a fuse, it’s best to disconnect when there isn’t any load. 
 

Similarly, nobody disconnects a battery fuse when the battery is supplying current to an inverter at whatever voltage. 

At times it's good to read the manual as the Axpert manual makes it clear to have the inverter switched off when one connects/disconnects AC, battery or PV panels. It does not take that long - LOL. Not the the circuit breaker causing this error.

Makes sense to apply it on other inverters as well?

Edited September 3, 20223 yr by Scorp007

2 hours ago, Scorp007 said:

At times it's good to read the manual as the Axpert manual makes it clear to have the inverter switched off when one connects/disconnects AC, battery or PV panels. It does not take that long - LOL. Not the the circuit breaker causing this error.

Makes sense to apply it on other inverters as well?

At times it’s also a good idea to have common sense. What you clearly don’t understand is that the only reason the inverter blew up is due to the load on the inverter. The inverter doesn’t need to be switched off as I have turned PV on and off while an Axpert is on without load and no damage was done. 

38 minutes ago, Leshen said:

At times it’s also a good idea to have common sense. What you clearly don’t understand is that the only reason the inverter blew up is due to the load on the inverter. The inverter doesn’t need to be switched off as I have turned PV on and off while an Axpert is on without load and no damage was done. 

I does not mean just because many times there is no damage it's a good thing. I would rather adhere to warnings provided. It's like driving without making an accident by intoxicated. But what one might not know is how many accidents by others one caused.

For me I will rather take precaution.

I grew up with my father checking if a battery has power by shorting out poles with thick wire. He never had a battery exploding. I for one would never even try it once.

In short I would say due to the high back EMF of DC circuits when interrupted this causes insulation of electronic devices to fail. For relays it's easy by just fitting a reverse biased diode to clamp the reversed voltage back EMF. Not sure if this is always fitted in all inverters.

It remains a matter of choice. 😄

AC breaker can be rated for DC as long as inline with the spec, Almost all hager breakers can be used at lower voltage and amps.

37 minutes ago, Dylan said:

AC breaker can be rated for DC as long as inline with the spec, Almost all hager breakers can be used at lower voltage and amps.

That's true , for the same the dc amp rating of the contact , we must de-rate the dc voltage  by an order of magnitude lower . Example for a given relay i have looked at the other day:

• AC contacts rating: 10 A / 240 VAC
• DC contacts rating: 10 A / 28 VDC

 

The dc voltage for the same current rating is 10% of the ac voltage .  Just to indicate perspective here.

Edited September 19, 20223 yr by BritishRacingGreen
Correction

8 hours ago, Dylan said:

Almost all hager breakers can be used at lower voltage and amps.

And they used to quote the DC ratings in the datasheets. I haven't looked lately, but they seem to have stopping doing that, sadly.

From memory, it was something like 60VDC per pole, so a two pole breaker could be used for 2S panels on ELV PV systems. But of course, if it's not in a datasheet, you can't use them. 

4 hours ago, Coulomb said:

And they used to quote the DC ratings in the datasheets. I haven't looked lately, but they seem to have stopping doing that, sadly.

From memory, it was something like 60VDC per pole, so a two pole breaker could be used for 2S panels on ELV PV systems. But of course, if it's not in a datasheet, you can't use them. 

The save way remains as per the markings on the face or side. If it does not show DC then the save way is DON'T USE IT.

Refer how this MCB has no DC rating although 415V AC. Refer to the minimum voltage.