Jump to content

240VAC MCB's for protecting 12VDC circuits?

kayDee

By kayDee
in Living Afloat

 Share

Featured Posts

Safeworks Gongoozler

Safeworks

Posted
Posted

I do understand domestic mcb's are not designed for marine use. However they are often fitted to generators for marine installation, so I assume the environmental conditions have been taken into consideration, in the design of the breaker. Regarding the current carrying capacity between AC and DC. I have found that relay manufacturers reduce the voltage handling capacity by 50% to compensate for DC switching. Using their calculation a 240v AC mcb should be well capable of switching a 12 volt DC load.

Given some of the poor quality breaker and fuse boards I have pulled out of pleasure craft over the last 40 years. Sold at inflated prices in the name of marine equipment. I would go for the domestic breaker any day. Looking at some of the narrow boats at the Crick boat show so do some of the boat builders.

Link to comment
Share on other sites
WotEver Veteran II

WotEverDonate to Canal World

Posted
Posted

Derating an AC switch by 50% for DC use really doesn't work. Take a look at the RS range of toggle switches as an example, many AC switches don't have a DC rating but for those that do it's common to see 16A AC switches derated to 2A for DC.

 

Personally, if a breaker didn't have a specifically noted DC rating then I wouldn't use it. Nothing to do with a marine environment and everything to do with the possibility of the contacts welding shut.

Link to comment
Share on other sites
Chewbacka Veteran

Chewbacka

Posted
Posted

My understanding is that DC switches must have more contact separation than a pure AC switch otherwise an arc can be struck and sustained for long enough to cause contact burning and pitting. This is turn leads to a much reduced contact area which will have a higher resistance the next time the switch is closed which will either burn or indeed weld it shut. So simply derating an AC switch and assuming it will be ok for DC use is a rash assumption.

Link to comment
Share on other sites
Safeworks Gongoozler

Safeworks

Posted
Posted

Derating an AC switch by 50% for DC use really doesn't work. Take a look at the RS range of toggle switches as an example, many AC switches don't have a DC rating but for those that do it's common to see 16A AC switches derated to 2A for DC.

 

Personally, if a breaker didn't have a specifically noted DC rating then I wouldn't use it. Nothing to do with a marine environment and everything to do with the possibility of the contacts welding shut.

 

Derating an AC switch by 50% for DC use really doesn't work. Take a look at the RS range of toggle switches as an example, many AC switches don't have a DC rating but for those that do it's common to see 16A AC switches derated to 2A for DC.

 

Personally, if a breaker didn't have a specifically noted DC rating then I wouldn't use it. Nothing to do with a marine environment and everything to do with the possibility of the contacts welding shut.

Yes you are right I got it wrong.

Link to comment
Share on other sites
Guest

Guest

Posted
Posted (edited)

Somewhere on the boat I have a "marine switch" sold by a well known chandler for use on boat DC lighting. When I inspected it closely, it is an "ac only" switch inside. I did post a photo of it some time in the summer of this year. I also have light fittings from the same supplier with soldered connections and ac switches within.

My previous boat had a purpose made panel with screw in fuses and switches, all labelled, very neat etc. Same with that, all ac switches when I looked inside (supplier unknown.)

I used all dp 20amp domestic switches on my current boat, 6A breakers behind them. 14 years and good. Also mains style MCB's on my boat for 12V circuits, but I did check with the manufacturer first. Again 14 years and good.

Edited by Guest
Link to comment
Share on other sites
WotEver Veteran II

WotEverDonate to Canal World

Posted
Posted

Somewhere on the boat I have a "marine switch" sold by a well known chandler for use on boat DC lighting. When I inspected it closely, it is an "ac only" switch inside. I did post a photo of it some time in the summer of this year. I also have light fittings from the same supplier with soldered connections and ac switches within.

My previous boat had a purpose made panel with screw in fuses and switches, all labelled, very neat etc. Same with that, all ac switches when I looked inside (supplier unknown.)

I used all dp 20amp domestic switches on my current boat, 6A breakers behind them. 14 years and good. Also mains style MCB's on my boat for 12V circuits, but I did check with the manufacturer first. Again 14 years and good.

I've put the best part of your post in bold :)

Link to comment
Share on other sites
  • 4 weeks later...
adrianh Community Regular

adrianh

Posted
Posted

If you do a google search for DC Din Rail Circuit Breakers, you will get a listing of the correct ones.

Quite a few manufactures make MCBs of this type for industrial/solar use, but they tend to me more expensive.

Using AC rated components for any safety type electrical component on DC is not a very good idea, they may work but the contact faces are probably the wrong type of alloy and will burn/pit should they trip or create to high a resistance for low voltage circuits. This then means that you then have poor contact at low voltage/heavy current, resulting in overheating and burn out of the contact. This can be quite dramatic under load for a DC current, fortunately equipment of this type is made from fire retardant plastic, but it will still smoulder!

The latest electrical regs now require that metal enclosures are used instead of plastic, I would advise this for low voltage installations from a fire point of view.

This is why switches always show a much lower current rating for DC, particularly at very low voltages.

Link to comment
Share on other sites
mross Influencer

mross

Posted
Posted

Bimble are selling DC circuit breakers from 20A to 300A

 

http://www.bimblesolar.com/index.php?route=product/search&search=dc%20circuit%20breaker


Earlier a post suggested some MCBs rated at 6kA. but with a large battery bank this might not be enough. "If being used on a battery bank, check the total short-circuit potential of your battery bank is under 6kA - this is generally battery banks of under approx 600Ah." is a recommendation from Solar and Wind Store.

Link to comment
Share on other sites
Guest

Guest

Posted
Posted

Just for information if anyone is thinking of using AC breakers on there DC system check out the HAGER site www.eurolecenergy.com/pdfs/hager-mcb.pdf The pdf file give you all you need to know.

Useful link thanks for posting.

 

Pretty much what they told me a few years ago: "For use with DC: Because of their quick make and break design and excellent arc quenching capabilities Hager circuit breakers are suitable for DC applications. The following parameters must be considered. 1. system voltage: Determined by the number of poles connected in series 2. short-circuit current: 3. tripping characteristics: - the thermal trip remains unchanged - the magnetic trip will become less sensitive requiring derating by Ö2 the ac value."

Link to comment
Share on other sites
  • 2 months later...
Safeworks Gongoozler

Safeworks

Posted
Posted

I have now found a manufacturer of miniature circuit breaker designed for Dc use up to 70 volts. With a range starting at 1 amp. CHINT CIRCUIT BREAKERS they can be found on line. I found a wholesaler EXPERT ELECRTICAL on line or call on 01706860011 They sell the range of MCB's and the enclosures

  • Greenie 1
Link to comment
Share on other sites
Giant Proficient

Giant

Posted
Posted
14 hours ago, Safeworks said:

I have now found a manufacturer of miniature circuit breaker designed for Dc use up to 70 volts. With a range starting at 1 amp. CHINT CIRCUIT BREAKERS they can be found on line. I found a wholesaler EXPERT ELECRTICAL on line or call on 01706860011 They sell the range of MCB's and the enclosures

Thanks for this Safeworks, I'd been on the lookout for a good source of DC rated DIN rail breakers.

Link for anyone else looking: https://www.expertelectrical.co.uk/circuit-protection/chint-miniature-circuit-breakers

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.