240VAC MCB's for protecting 12VDC circuits?

[kayDee]

Hi all...

Can standard 240VAC MCB's be used to protect 12VDC circuits? This sounds a bit odd to me!

Any help appreciated!

Kay

x

[Theo]

Hi all...

Can standard 240VAC MCB's be used to protect 12VDC circuits? This sounds a bit odd to me!

Any help appreciated!

Kay

x

 

My guess is that they would work fine. They are current sensors. Unless they need to AC for some magnetic reason.

 

I am sure that someone will pop up and give you the definitive answer soon!

 

Nick

[Guest]

Some are OK and tested at 12V DC, and some are not Kay. I suggest you find the particular make and email the technical people. They are mostly very helpful. I have used 240V MCB's on my 12V (can't remember make, but I checked them out first.)

[Keeping Up]

Almost all MCBs are fine at 12v dc, but only a few manufacturers will guarantee them for this type of usage (for several reasons).

 

Personally I don't know of any that are definitely unsuitable - unless anyone else here knows differently I'd say use a reputable make and just go ahead.

[paulcatchpole]

use a reputable make and just go ahead.

 

That's what I've done, although I'm not saying I'm right!

 

I've a mix of different current ratings in an eight-way board, all for 12v.

 

PC

[wonderdust]

Hi all...

Can standard 240VAC MCB's be used to protect 12VDC circuits? This sounds a bit odd to me!

Any help appreciated!

Kay

x

 

Here is a list from a member who emailed some companies on whether their mcb's would be ok with 12v.

 

Link & Link to specific posts.

Edited November 15, 2008 by wonderdust

[kayDee]

Thanks for your help people. I just thought the trip mechanism inside it may have needed AC to allow it to operate or something.

I can't remember who manufactured the MCB's we've got but I will check that list that someone kindly gave me a link for and see!

Thanks again, I feel a bit better now...

Kay

x

[Guest]

Thanks for your help people. I just thought the trip mechanism inside it may have needed AC to allow it to operate or something.

I can't remember who manufactured the MCB's we've got but I will check that list that someone kindly gave me a link for and see!

Thanks again, I feel a bit better now...

Kay

x

Chances are they will all work OK as someone else stated, it is just that some haven't been tested at 12V DC. Personally I would use the ones that have been tested. Pretty sure mine are "Square D."

[down the north]

Kay,

try www.rapidonline.com , panel mounting circuit breakers,

pt nos, 26-7734, 5 amp

26-7742 10 amp

26-7744 15 amp

Its a mail order only but we deal with them all the time and theyre quick, these are only £1.55 +vat each ( schhhhh)

( in my catalogue)

and I think theres a min order amount of £30 exc vat for free delivery, otherwise £4.95 + vat.

 

I'm planning to use these, they are thermally operated only, and years ago I could only find Hagar MCB's that would trip on 12 volts. The above need a minute or so to cool down before a reset.

I think most 230V trips use both thermal magnetic to activate the trip, and with 12 v, there just aint enough magnetic bit to cause it to work ( some varieties)

Most mcb's have an adjust screw hidden somewhere where the sensitivity can be 'tweaked'.

I havent tried internet rapidonline.com yet but it might be worth a go.

martin

[Keeping Up]

I think most 230V trips use both thermal magnetic to activate the trip, and with 12 v, there just aint enough magnetic bit to cause it to work ( some varieties)

I can't see how that could possibly be correct. The voltage drop across an MCB, just below its tripping current, will invariably be insignificant compared with 12 volts (they'd get awfully hot otherwise). All they care about is the amount of current, they can't tell whether they're on a 12v or a 240v circuit.

 

Allan

[down the north]

I can't see how that could possibly be correct. The voltage drop across an MCB, just below its tripping current, will invariably be insignificant compared with 12 volts (they'd get awfully hot otherwise). All they care about is the amount of current, they can't tell whether they're on a 12v or a 240v circuit.

 

Allan

 

have a look at,,,

Hhttp://www.newtechmcb.net/miniature-circuit-breakers.html

It both gives the thermal, ( bi metal ) and the induced ( AC ) trip option dependant on surge current capabilities ( c type) or standard non inductive domestic loading ( b ) type.

So youre only going to get an induced trip if its on 230V AC ( DC cant induce),

 

re the rapid ones, I havent tried them yet, but will this week and see what volt drop I get at 100% current, and hence lost power through it,

 

martin

[Keeping Up]

have a look at,,,

Hhttp://www.newtechmcb.net/miniature-circuit-breakers.html

It both gives the thermal, ( bi metal ) and the induced ( AC ) trip option dependant on surge current capabilities ( c type) or standard non inductive domestic loading ( b ) type.

So youre only going to get an induced trip if its on 230V AC ( DC cant induce),

 

re the rapid ones, I havent tried them yet, but will this week and see what volt drop I get at 100% current, and hence lost power through it,

 

martin

Why not? Direct Currents induce a magnetic field just as well as Alternating Currents

[MoominPapa]

I can't see how that could possibly be correct. The voltage drop across an MCB, just below its tripping current, will invariably be insignificant compared with 12 volts (they'd get awfully hot otherwise). All they care about is the amount of current, they can't tell whether they're on a 12v or a 240v circuit.

 

Allan

I think that the magnetic section of 240v circuit breakers is there so that the breaker disconnects FAST with very large fault currents, to keep the total fault energy small enough to avoid melted cables. The thermal section is there for smaller but more prolonged overloads. When used on 12v it's less likely that even a short circuit will produce a high enough current to trip the magnetic section (the short would have to have 1/20th the resistance needed at 240v.) but by the same token the need to break the circuit really fast is less.

 

So, in essence, the magnetic section becomes redundant when used on 12v. that's why purpose-designed 12v breakers don't have it, but the thermal section of a 240v breaker provides the same protection as a dedicated 12v breaker.

 

As an aside, I'm sure I've read that standard mains breakers used in the US don't have the magnetic section: yet another reason why their houses are always burning down.......

 

MP.

[paulcatchpole]

Why not? Direct Currents induce a magnetic field just as well as Alternating Currents

 

Are you sure? There'll surely only be a field during a period when the DC current is changing rapidly, and 'cutting the flux' as it were? Induced field strength being proportional to the first differential of that?

 

It's not like you get far driving a transformer with DC, apart from spike when you initially apply DC - like a car ignition system?

 

Of course, I may be remembering my physics lessons with much rose-tinting.

 

PC

[MoominPapa]

Are you sure? There'll surely only be a field during a period when the DC current is changing rapidly, and 'cutting the flux' as it were? Induced field strength being proportional to the first differential of that?

 

It's not like you get far driving a transformer with DC, apart from spike when you initially apply DC - like a car ignition system?

 

Of course, I may be remembering my physics lessons with much rose-tinting.

 

PC

Not quite right: the field is proportional to the current, but the induced voltage in the secondary of the transformer depends on the first differential of the field. That's why transformers are AC only.

 

A magnetic circuit breaker is only a specialised form of relay, and they work fine on DC.

 

MP.

[paulcatchpole]

Not quite right: the field is proportional to the current, but the induced voltage in the secondary of the transformer depends on the first differential of the field. That's why transformers are AC only.

 

A magnetic circuit breaker is only a specialised form of relay, and they work fine on DC.

 

MP.

 

That's right, it comes flooding back - rate-of-change-of-cutting-the-flux, so second differential of current?

 

Ahha, I see what you mean about DC applied to a relay - sorry!

 

C- must try harder, for me, I think.

 

PC

[Gibbo]

I pondered this one for a while before posting.

 

If the circuit breakers are purely thermal breakers and rated for AC then using them on DC could cause problems.....

 

Like any switch or relay the breaking capacity relies on quenching the arc at the time of opening. If the supply is AC then the arc gets quenched anyway as the supply changes polarity. Thus the longest possible time the arc can contune for is just under 1/2 a cycle. This allows much smaller contacts to be used than an equivalent DC breaker. So unless the breaker is actually rated for DC it could be a problem.

 

However, as someone pointed out, most breakers use a combination of thermal and magnetic tripping. In this type, the magnetic field not only opens the switch but is also used to quench the arc (it's part of the design). The magnetic field blows the arc to the side thus forcing it into a longer path which kills it. The same technique is used in most high voltage breakers on the national grid pylons.

 

Now I accept that this sounds like complete theoretical bo**ox but it's actually a very old, well tried and tested, technique that is used in just about every type of magnetically tripped circuit breaker. See http://en.wikipedia.org/wiki/Circuit_breaker

 

So I'm pretty certain that magnetic types will be fine on DC even if they are not rated for it.

 

Gibbo

[Mitch - Soma]

I just bought any old off-the-shelf 240v jobs, tested them on a bench PSU at 12v and they worked fine. Pointed out to the BSC inspector as to what I had done, he didn't bat an eye, said it was perfectly alright and passed the boat.

[ditchcrawler]

I just bought any old off-the-shelf 240v jobs, tested them on a bench PSU at 12v and they worked fine. Pointed out to the BSC inspector as to what I had done, he didn't bat an eye, said it was perfectly alright and passed the boat.

How did you test them?

[Arnot]

Hi all...

Can standard 240VAC MCB's be used to protect 12VDC circuits? This sounds a bit odd to me!

Any help appreciated!

Kay

x

My take is that if they are already installed and seem to work leave them be. As others have posted, most (if not all) will work fine on 12VDC even if they are not rated for it.

 

My concern is that they are not intended to be reliable when subjected to the levels of vibration, change of temperature and change humidity that electrical equipment on boats usually needs to contend with. My suspicion is that they would work fine when new but could not be relied on in the longer term.

 

Of course this is why most vehicles tend to use old fashioned fuses... They tend to be reliable longer and if they do fail, they fail safe rather than jamming.

 

When an electrical sytem is tested, the MCB's are almost never tested except in installations where their operation is absolutley critical. The reason for this is that they are very difficult to test effectively and any testing routine would also strain other parts of the installation thus compromising future reliability.

 

In most cases on boats, the amount of power available from a generator, inverter or average shore line would not be able to cause overheating of the wiring by over current given it has been wired with the appropriate cable and it is the RCD that provides the majority of the protection for mains systems. Also it is unusual to find loads that draw higher continuous currents such as heaters, immersion heaters and electric cooking devices.

 

On the 12VDC systems this is not the case, there are a lot of devices that draw continuous higher currents (not least of which are inverters and power supplies for laptops and monitors) and the potential short circuit current of a battery is probably higher than anything that could be delivered by the mains. For these reasons as well, I would propose that fuses were a better option...

 

Regards

 

Arnot

[Guest]

My take is that if they are already installed and seem to work leave them be. As others have posted, most (if not all) will work fine on 12VDC even if they are not rated for it.

 

My concern is that they are not intended to be reliable when subjected to the levels of vibration, change of temperature and change humidity that electrical equipment on boats usually needs to contend with. My suspicion is that they would work fine when new but could not be relied on in the longer term.

 

Of course this is why most vehicles tend to use old fashioned fuses... They tend to be reliable longer and if they do fail, they fail safe rather than jamming.

 

When an electrical sytem is tested, the MCB's are almost never tested except in installations where their operation is absolutley critical. The reason for this is that they are very difficult to test effectively and any testing routine would also strain other parts of the installation thus compromising future reliability.

 

In most cases on boats, the amount of power available from a generator, inverter or average shore line would not be able to cause overheating of the wiring by over current given it has been wired with the appropriate cable and it is the RCD that provides the majority of the protection for mains systems. Also it is unusual to find loads that draw higher continuous currents such as heaters, immersion heaters and electric cooking devices.

 

On the 12VDC systems this is not the case, there are a lot of devices that draw continuous higher currents (not least of which are inverters and power supplies for laptops and monitors) and the potential short circuit current of a battery is probably higher than anything that could be delivered by the mains. For these reasons as well, I would propose that fuses were a better option...

 

Regards

 

Arnot

I see your point, and understand your concerns, but believe me when I say that in industry I have seen MCB's working in far worse conditions than any boat would offer! I have seen mcb's go faulty in these conditions, but generally they will stan up to lots of abuse. Much of my work was in an iron foundry, so you can imagine the conditions!

[Mitch - Soma]

How did you test them?

 

0-30 volt and 0-30 amp bench power supply. Set it to 12v, then put a breaker across it and slowly turned up the current till it tripped. 10 amp tripped at 11 amps, 16 amp tripped at 15 amps and 20 amp tripped at 22 amps - all well close enough for me.

[Safeworks]

I have used domestic mcb's on boat installations. You need to check them to ensure they trip on short circuit. CONTACTUM breakers do work. But I am sure other may also work. When I asked MK to comment about them, technical support said I have to tell you they are not designed for 12 volt DC. But I suggest you test them out on the bench. Which we did. Short circuit and excess load. I can't remember why we did not use Mk. But the Contactum mcb's worked. I think I liked the look of the enclosure, Metal clad with a nice lid.

From an installation point of view the Metal clad enclosure with a 100 amp isolator is wonderful. Unusual to see on a boat but it makes a very tidy installation. Also no adverse comments from boat safety.

[nicknorman]

I have used domestic mcb's on boat installations. You need to check them to ensure they trip on short circuit. CONTACTUM breakers do work. But I am sure other may also work. When I asked MK to comment about them, technical support said I have to tell you they are not designed for 12 volt DC. But I suggest you test them out on the bench. Which we did. Short circuit and excess load. I can't remember why we did not use Mk. But the Contactum mcb's worked. I think I liked the look of the enclosure, Metal clad with a nice lid.

From an installation point of view the Metal clad enclosure with a 100 amp isolator is wonderful. Unusual to see on a boat but it makes a very tidy installation. Also no adverse comments from boat safety.

 

There is no doubt that they will trip, the question is will the contacts actually open and/or continue to function afterwards. With DC, and depending on the source, load, and system inductance, it is possible for a very large current to build up very rapidly (within the activation time of the electromechanical bits) and thus the contact has to be able to open in spite of an arc developing. With AC of course, the current falls to zero 100 times a second so AC rated breakers need less robust contacts than those designed for DC.

 

So if you gently wind the current up to beyond the breaker's rating, sure it will trip satisfactorily. But if you put it across 440AH of batteries, will the outcome be the same? There has to be some uncertainty at least.

[Chewbacka]

There is no doubt that they will trip, the question is will the contacts actually open and/or continue to function afterwards. With DC, and depending on the source, load, and system inductance, it is possible for a very large current to build up very rapidly (within the activation time of the electromechanical bits) and thus the contact has to be able to open in spite of an arc developing. With AC of course, the current falls to zero 100 times a second so AC rated breakers need less robust contacts than those designed for DC.

 

So if you gently wind the current up to beyond the breaker's rating, sure it will trip satisfactorily. But if you put it across 440AH of batteries, will the outcome be the same? There has to be some uncertainty at least.

Some Hager MCBs are DC rated and have a break capacity of 6kA which should be enough for you battery bank.

http://www.hager.co.uk/downloads-services/technical-services/mcb-s-dc-rated/56346.htm

 

That said, DC fuses rarely blow, so I am quite happy with car blade fuses. Another advantage of blade fuses is you can pull the fuse and plug a current meter in if you ever want to know the current draw.