MCBs on low voltage circuits. - UK Vintage Radio Repair and Restoration Discussion Forum

Tractorfan · 26th Aug 2015, 9:23 am

Hi,
Just a quick question.
Will a standard Miniature Circuit Breaker still trip at its stated current when used in a 12 volt circuit? I need to replace two 30 amp car-type burnt out fuse holders on an underwater lighting scheme which has two 300 watt 12 volt lamps on separate circuits. They pull about 24 amps a piece and I thought that, maybe, 32 amp MCBs would do. Or would cartridge fuses be better?
Thanks.
Cheers, Pete.

cmjones01 · 26th Aug 2015, 9:29 am

Yes, the circuit breaker will still work, but take care: is the low voltage AC or DC? If it's DC, a standard 'mains' MCB is almost certainly not handle the arc that results when the breaker opens. There is likely to be a bang and a flash and certain amount of molten copper, which you don't want. If the 12 volt supply is AC, you'll be OK.

Another thing to watch is that the MCB may have a higher voltage drop across it than you'd like when 24A is flowing, which wouldn't bother a mains circuit at 230V but could make a significant difference to the performance of 12V lamps. Watch out also for the inrush current when switching the lamps on: they will draw much more than 24A for a moment as the filaments heat up, and you may need a slower MCB (type C or type D) to handle the surge.

Chris

broadgage · 26th Aug 2015, 10:34 am

In practice it will be fine.
DC is indeed far more destructive than AC due to arcing at contacts.
A "rule of thumb" is that switches, fuses, contactors, relays and so on if designed for AC are fine on DC provided that the voltage is limited to about 10% of the AC rating.

A 240 volt AC light switch on 24 volts DC ? certainly ! on 32 volts DC ? maybe if not fully loaded, on 110 volts DC, no way.

In this case, the supply is PROBABLY AC. The load is said to be a 12 volt 300 watt lamp, which strongly suggests a filament lamp. That in turn suggests AC, why use a relatively costly DC power supply when a simple transformer would do the job at lower initial cost.

I very doubt that the inrush current will trip a standard mains type 32 amp MCB. Remember that the impedance of the transformer will somewhat limit the inrush current.

I also doubt that voltage drop within the MCB will be problem, the voltage drop has to be limited to avoid excessive temperature rise. The limit is often about 2 or 3 watts at full load.
So at full load the voltage drop would be less than 0.1volt, and less still at 25 amps through a 32 amp MCB.

Leon Crampin · 26th Aug 2015, 2:56 pm

Beware that the "rule of thumb" above applies only to resistive loads.

Using an MCB rated for AC 240V on a 12/24V DC circuit containing, for example, a motor or an unsnubbed (nice word...) relay coil will result in a continuous and destructive arc.

Amongst other factors, the rate of separation of the contacts is too low to suppress a DC arc, which, in a low frequency AC circuit would self-extinguish near the zero crossings. Go carefully, low impedance circuits can pass large currents through an arc.

Leon.

Boater Sam · 26th Aug 2015, 3:31 pm

Maybe digressing slightly but in case the lamps discussed here are on DC it may be relevant.
Back when we used to install the power and backup supplies for Mercury exchanges, we always used standard 440/240v AC Merlin Gerin ( Now Schneider ) MCBs as they were the only make that would certify use on 48v DC.
Never had a problem with them and some were very high current.
I have used them extensively on boat wiring without failures too.

McMurdo · 26th Aug 2015, 8:26 pm

why not just use something like this?

http://uk.farnell.com/te-connectivit...30a/dp/9659161

Tractorfan · 26th Aug 2015, 11:04 pm

Hi,
Many thanks, as ever, your your input.
I forgot to mention that the circuit was 50Hz AC fed from a toroidal transformer. Apologies to all.
I more or less answered my own question this afternoon, as I tried what I thought was a 32A MCB, but was in fact a 15A one which tripped straight away. I've since found a couple of C32A ones in my junk box which will do nicely.
Cheers, Pete.

richard.cs · 4th Oct 2015, 10:17 pm

Probably a bit late for the O.P. but in case it's useful to anyone else some (maybe all, I don't remember) Hagar MCBs are rated to break d.c. up to 60 V, but it took a lot of digging around in the datasheets to find it.

The thermal trip will work at the same current but the magnetic trip will need more current than expected. e.g. a B type trips with a.c. current 3-5 times its rating, but responds to the peak of the waveform and therefore at d.c. won't trip until you get sqrt(2) times higher. This means it'll trip on magnetic at 4 to 7 times its rated current and is closer to a C type in behavior.

happytiger · 5th Oct 2015, 4:18 am

Quote:

Originally Posted by cmjones01

Yes, the circuit breaker will still work, but take care: is the low voltage AC or DC? If it's DC, a standard 'mains' MCB is almost certainly not handle the arc that results when the breaker opens. There is likely to be a bang and a flash and certain amount of molten copper, which you don't want. If the 12 volt supply is AC, you'll be OK.

Another thing to watch is that the MCB may have a higher voltage drop across it than you'd like when 24A is flowing, which wouldn't bother a mains circuit at 230V but could make a significant difference to the performance of 12V lamps. Watch out also for the inrush current when switching the lamps on: they will draw much more than 24A for a moment as the filaments heat up, and you may need a slower MCB (type C or type D) to handle the surge.

you wont get arcing on 12v.DC for arcing you want at least 24V.I have found the standard " mains" mcb works fine on DC, these may have "arc blow outs".
The MCB work fine on DC, I use then on 120v DC mains. they trip and clear instantly,without arcing.
The trouble with 12v is that is that you may not have enough fault current to trip quick enough,since these MCB are thermal/magnetic, you should get the specs of the mcb,which will give trip times with overload. Of course small MCBs up to 6a should be ok,even 10a if the wiring and supply is up to it.
On DC Arcing will occur from about 60w upwards, AC type switches will not open the circuit unless the current is very small, an arc will quickly destroy the switch,and could start a fire. A proper switch will have an arc shute,quick make and break contacts and made of incombustale material (IEE regs) "AC switches" often dont comply with these regs.

Kala_12 · 10th Oct 2015, 9:20 pm

Quote:

Originally Posted by Tractorfan

Hi,
... the circuit was 50Hz AC fed from a toroidal transformer.

Am I missing something? Can the primary not be fused or have an MCB on it?
Stuart

matthewhouse · 11th Oct 2015, 8:56 pm

I looked at a fault a few years ago on a pool cover motor, it was tripping the MCB. The motor was 12V DC fed from a C 50Amp Merlin MCB on the secondry of the transformer. The primary was protected by the MCB at the distribution board. (Which wasn't tripping) The setup seemed to work fine.

26th Aug 2015, 9:23 am	#1
Tractorfan Dekatron Join Date: Mar 2008 Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.) Posts: 3,184	MCBs on low voltage circuits. Hi, Just a quick question. Will a standard Miniature Circuit Breaker still trip at its stated current when used in a 12 volt circuit? I need to replace two 30 amp car-type burnt out fuse holders on an underwater lighting scheme which has two 300 watt 12 volt lamps on separate circuits. They pull about 24 amps a piece and I thought that, maybe, 32 amp MCBs would do. Or would cartridge fuses be better? Thanks. Cheers, Pete. __________________ "Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..."

26th Aug 2015, 9:29 am	#2
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Re: MCBs on low voltage circuits. Yes, the circuit breaker will still work, but take care: is the low voltage AC or DC? If it's DC, a standard 'mains' MCB is almost certainly not handle the arc that results when the breaker opens. There is likely to be a bang and a flash and certain amount of molten copper, which you don't want. If the 12 volt supply is AC, you'll be OK. Another thing to watch is that the MCB may have a higher voltage drop across it than you'd like when 24A is flowing, which wouldn't bother a mains circuit at 230V but could make a significant difference to the performance of 12V lamps. Watch out also for the inrush current when switching the lamps on: they will draw much more than 24A for a moment as the filaments heat up, and you may need a slower MCB (type C or type D) to handle the surge. Chris __________________ What's going on in the workshop? http://martin-jones.com/

26th Aug 2015, 10:34 am	#3
broadgage Nonode Join Date: Jan 2010 Location: North Somerset, UK. Posts: 2,130	Re: MCBs on low voltage circuits. In practice it will be fine. DC is indeed far more destructive than AC due to arcing at contacts. A "rule of thumb" is that switches, fuses, contactors, relays and so on if designed for AC are fine on DC provided that the voltage is limited to about 10% of the AC rating. A 240 volt AC light switch on 24 volts DC ? certainly ! on 32 volts DC ? maybe if not fully loaded, on 110 volts DC, no way. In this case, the supply is PROBABLY AC. The load is said to be a 12 volt 300 watt lamp, which strongly suggests a filament lamp. That in turn suggests AC, why use a relatively costly DC power supply when a simple transformer would do the job at lower initial cost. I very doubt that the inrush current will trip a standard mains type 32 amp MCB. Remember that the impedance of the transformer will somewhat limit the inrush current. I also doubt that voltage drop within the MCB will be problem, the voltage drop has to be limited to avoid excessive temperature rise. The limit is often about 2 or 3 watts at full load. So at full load the voltage drop would be less than 0.1volt, and less still at 25 amps through a 32 amp MCB.

26th Aug 2015, 2:56 pm	#4
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: MCBs on low voltage circuits. Beware that the "rule of thumb" above applies only to resistive loads. Using an MCB rated for AC 240V on a 12/24V DC circuit containing, for example, a motor or an unsnubbed (nice word...) relay coil will result in a continuous and destructive arc. Amongst other factors, the rate of separation of the contacts is too low to suppress a DC arc, which, in a low frequency AC circuit would self-extinguish near the zero crossings. Go carefully, low impedance circuits can pass large currents through an arc. Leon.

26th Aug 2015, 3:31 pm	#5
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: MCBs on low voltage circuits. Maybe digressing slightly but in case the lamps discussed here are on DC it may be relevant. Back when we used to install the power and backup supplies for Mercury exchanges, we always used standard 440/240v AC Merlin Gerin ( Now Schneider ) MCBs as they were the only make that would certify use on 48v DC. Never had a problem with them and some were very high current. I have used them extensively on boat wiring without failures too.

26th Aug 2015, 8:26 pm	#6
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: MCBs on low voltage circuits. why not just use something like this? http://uk.farnell.com/te-connectivit...30a/dp/9659161 __________________ Kevin

26th Aug 2015, 11:04 pm	#7
Tractorfan Dekatron Join Date: Mar 2008 Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.) Posts: 3,184	Re: MCBs on low voltage circuits. Hi, Many thanks, as ever, your your input. I forgot to mention that the circuit was 50Hz AC fed from a toroidal transformer. Apologies to all. I more or less answered my own question this afternoon, as I tried what I thought was a 32A MCB, but was in fact a 15A one which tripped straight away. I've since found a couple of C32A ones in my junk box which will do nicely. Cheers, Pete. __________________ "Hello?, Yes, I'm on the train, I might lose the signal soon as we're just going into a tunn..."

4th Oct 2015, 10:17 pm	#8
richard.cs Triode Join Date: Feb 2011 Location: Southampton, Hampshire, UK. Posts: 46	Re: MCBs on low voltage circuits. Probably a bit late for the O.P. but in case it's useful to anyone else some (maybe all, I don't remember) Hagar MCBs are rated to break d.c. up to 60 V, but it took a lot of digging around in the datasheets to find it. The thermal trip will work at the same current but the magnetic trip will need more current than expected. e.g. a B type trips with a.c. current 3-5 times its rating, but responds to the peak of the waveform and therefore at d.c. won't trip until you get sqrt(2) times higher. This means it'll trip on magnetic at 4 to 7 times its rated current and is closer to a C type in behavior.

11th Oct 2015, 8:56 pm	#11
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: MCBs on low voltage circuits. I looked at a fault a few years ago on a pool cover motor, it was tripping the MCB. The motor was 12V DC fed from a C 50Amp Merlin MCB on the secondry of the transformer. The primary was protected by the MCB at the distribution board. (Which wasn't tripping) The setup seemed to work fine.