240v mcb's as 12v circuit breakers

lumphammer

lumphammer

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I see in a recent PBO about electrical systems in boats that the writer appears to have used standard mains type mcb's to protect against current overload. I've always thought that these weren't designed for dc use because of problems with arc suppression when operating under overload conditions.
As one of my winter jobs is replacing the automotive fuses currently fitted the idea of using the mains mcb's is quite appealing.

Are these mcb's something that others would use?
 
I have used MCB (merlin gerin) on my previous boat over 12 years in use and my current boat with not a single problem. I have found that MCD do have a DC rating but not easy to find in the spec and when you consider that most MCD in the range we use on boats are up to 63A rating and trip with in half the AC cycle the arcing happens on both AC and DC with a break rating in the order 5000A, I don't think much problems will happen.

I also use my MCB to control the on/off of a number of circuits so have an increase useage over just being switch when an overload current flows.

I would not use a MCB on my bow thruster with a max rating of 600A but they are not designed for that load anyway.

Old boat and new boat MCB switch panels
 
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For the past 6 months in every issue on PBO, there have been electrical installations and methods that I would personally not do on any customer's boat. I am finding that their new "electrical expert" is advocating bad practice, e.g. drilling out a ring crimp terminal to make it fit a bigger post, and this month's reader idea of using XLR connections for sockets... mmm.

Any thermal circuit breaker should have a DC rating for up to 50volts DC, 250V AC but the size, weight and design of domestic breakers means they are not best suited for marine use.

Mark
www.boatdoctorni.com
 
Rogershaw said:
I have used MCB (merlin gerin) on my previous boat over 12 years in use and my current boat with not a single problem. I have found that MCD do have a DC rating but not easy to find in the spec and when you consider that most MCD in the range we use on boats are up to 63A rating and trip with in half the AC cycle the arcing happens on both AC and DC with a break rating in the order 5000A, I don't think much problems will happen.

I also use my MCB to control the on/off of a number of circuits so have an increase useage over just being switch when an overload current flows.

I would not use a MCB on my bow thruster with a max rating of 600A but they are not designed for that load anyway.

Old boat and new boat MCB switch panels
Click to expand...

Roger these panels have a spookily familiar look and feel to them, Merlin Gerin MCBs, red LEDs and white printing on black perspex (mine also says "by FonZ" on the bottom of the panel). You didn't do a similar job on a SA built Sadler 26 back in the early 90's by any chance did you?


Not the best photo I'm afraid but I'd be fascinated to know if you did.
 
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There are better things now than xlr's, big range of multipole neutrik connectors, some with some meaty current ratings

Best panel I have seen :-
 
Spelling was never my good point and having trained as a mechanical engineer I have an excuse.

Yes the one on my old boat was fitted by fonz before I purchased her and I followed the same style. In fact most boats built in Durban were wired up by fonz up until the downturn in boat building in Durban.
 
Rogershaw said:
Spelling was never my good point and having trained as a mechanical engineer I have an excuse.

Yes the one on my old boat was fitted by fonz before I purchased her and I followed the same style. In fact most boats built in Durban were wired up by fonz up until the downturn in boat building in Durban.
Click to expand...

Thanks for responding Roger. I'm slowly filling the gaps in our boat's history. I have to say the switch panel is first class and works extremely well.
 
It must be 16 years or more since I wired a mates net S&S using 240 V a/c circuit breakers, though I seem to remember back then we had a little getting the ones with a wider gap, theory at the time was that this would reduce arching.

She still has the same box today, all circuits individually earthed and all tinned wire, most globes were soldered in as well.

How things have changed.
.
 
markdj said:
For the past 6 months in every issue on PBO, there have been electrical installations and methods that I would personally not do on any customer's boat. I am finding that their new "electrical expert" is advocating bad practice, e.g. drilling out a ring crimp terminal to make it fit a bigger post, and this month's reader idea of using XLR connections for sockets... mmm.

Any thermal circuit breaker should have a DC rating for up to 50volts DC, 250V AC but the size, weight and design of domestic breakers means they are not best suited for marine use.

Mark
www.boatdoctorni.com
Click to expand...

Power-XLRs surely? Got a load of kit at work that has power-XLR connectors and XLR audio. You learn to not plug it up the wrong way.
 
markdj said:
For the past 6 months in every issue on PBO, there have been electrical installations and methods that I would personally not do on any customer's boat. I am finding that their new "electrical expert" is advocating bad practice, e.g. drilling out a ring crimp terminal to make it fit a bigger post, and this month's reader idea of using XLR connections for sockets... mmm.

Any thermal circuit breaker should have a DC rating for up to 50volts DC, 250V AC but the size, weight and design of domestic breakers means they are not best suited for marine use.

Mark
www.boatdoctorni.com
Click to expand...

Quite right too. As you appear to be a profesional applying recognised electrical practices for customers this is of corse correct. However, the individual who usually has a good understanding of what vairious standard domestic equipment is capable of is a different matter. Here he will know for example that a thermal MCB designed for AC will operate acceptably on DC and also have understanding arcing problems in DC switch gear. He will also know price differences. Let people decide what to do on their own property. If they get it wrong, so-what. They will have to try again and will perhaps learn a lot more and have more fun in the process.
 
Do not use AC designed products for DC applications they work in a slightly different way....like for instance as you say "arcing"
And there is at least 2 types of circuit breaker.
Earth Leakage and RCCB (or RCD).
and these work on very different princples.
The safety ground on a boat is not the same system as in a house.
Please do not do this not even for yourself it is not correct.
Stay safe with electricity.
 
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