240V ac MCBs on 12v DC?

[Big Steve]

As per the heading, will a 240volt MCB work at 12v DC. I can't see why it won't, it's tripping on over current, not volts and they're rated at 240 and 440v ac. I can see that the contacts won't last long switching DC, but then, it's not being used as a switch is it?

 

If it won't work, why not?

 

I've tried a 5A one and it did seem to work OK, was this just a fluke? Tripped at about 8A on a slowly increasing load.

 

Cheers

 

Steve

[Keeping Up]

There is a possibility that the contacts will strike an arc from the DC, and not break the circuit under fault conditions. Having said that, I believe the possibility to be small and I'm sure that any good quality MCB will be just fine (that's what I use)

[wonderdust]

As per the heading, will a 240volt MCB work at 12v DC. I can't see why it won't, it's tripping on over current, not volts and they're rated at 240 and 440v ac. I can see that the contacts won't last long switching DC, but then, it's not being used as a switch is it?

 

If it won't work, why not?

 

I've tried a 5A one and it did seem to work OK, was this just a fluke? Tripped at about 8A on a slowly increasing load.

 

Cheers

 

Steve

 

 

Try reading this http://www.canalworld.net/forums/index.php...e+ratings\

[DHutch]

We make use of a 240vac consumer unit for our lowvoltage (24vdc) ciructry and it appears to work fine. Although i can not suggest eather way what i would recomend you to do.

 

Photo in this post http://www.canalworld.net/forums/index.php...ost&p=43539

 

 

 

Daniel

[Gibbo]

OK..............

 

All other things being equal, AC breakers don't need such a large gap between the open contacts because as they open, the supply is alternating so the spark quenches itself quite quickly - less than 10mS absolute maximum at 50Hz.

 

This doesn't happen with DC because it isn't alternating. So DC breakers open to a much wider gap and some of them also use magnetic quenching where a magnetic field is used (generated from the fault current) to force the spark into a longer arc thus quenching it.

 

However we have to compare like with like. The above would be the case for an equivalent 230 volt AC breaker and a 230 volt DC breaker. The DC breaker would need wider contact spacing and probably also magnetic quenching.

 

But we're taking here about using a 230 volt AC breaker for 12 or 24 volts DC not 230 volts DC

 

The chances of a 12 or 24 volt DC system causing sufficient arcing to cause a 230 volt AC breaker a problem are slim to say the least. In fact I'd be close to saying impossible.

 

I would have no problem whatsoever using them. They are cheaper and easily obtainable. Perhaps the only issue would be that they are not as physically tough but they survive quite happily in boats on AC systems.

 

The breaking current, the way they operate etc are completely compatible with both systems.

 

Gibbo

[frangar]

The only problem I would see would be a possible confusion between 240V ac and 12/24V dc if they are fitted in similar consumer units unless clearly marked.

 

Cheers

 

Gareth

[Guest]

Some MCB's are guaranteed to operate at 12V DC some aren't. I suspect this means that some haven't been tested at 12V DC. I would expect them all to operate under these conditions. I use 240V MCB's for the 12V DC on my boat.

[Big Steve]

Thanks for the feedback chaps. I did suspect they'd be OK, but have no experience of this application, nice to see some of you have.

 

[Mac Man]

Thanks for the feedback chaps. I did suspect they'd be OK, but have no experience of this application, nice to see some of you have.

 

 

 

Hi,

 

There was a thread back in Jan 2008 where this was covered in depth. I printed it out as I was in the market for MCB for my boat Holdfast. MEM type B, Square D C60 range and Hager 10 amp MTN110 all stated they were suitable for 12 volts DC. Crabtree and MK stated theirs were not suitable for 12 volts DC.

Hope this helps.

 

Mac Man