Circuit breaker sizing for DC loads

[jonesy29847]

How do you properly size a circuit breaker to operate at 24VDC? We are now using 24VDC control circuit voltages and I want to be sure circuit protection is correct.

Thanks

 

[jOmega]

Suggest you use a search engine, such a

http://www.google.com

and search for DC Circuit Breakers.

You'll find more than enough information and vendors of such equipment in the search results.

 

[jonesy29847]

Thanks for your fast reply.

Sorry I should have mentioned this is an Industrial Application. My concern goes more to properly rating an AC breaker for DC use, assuming my supplier does not make an industrial DC breaker. As an example the system I am currently following up is using standard industrial DIN rail mounted breakers rated at 277V 6A, which if we don't think about AC for a moment, is about 1662 watts. The relay contacts that are being protected by the CB are rated at 24VDC @ 6A or 144 watts. The question I have posed to the vendor is do we need to derate the CB, my gut says yes, thus I would need about a .5A 277 volt relay. I need some other opinions to get a warm fuzzy feeling.

 

[TWW]

Squared sells DIN mounted QOB circuit breakers that are dc rated.

 

[PUMPDESIGNER]

jonesy29847
Not expert here and always wanting to learn.

Why you are considering the circuit breaker as protection for the relay? I have always considered the circuit breaker as protection against overheating due to overcurrent and have always considered this to involve mainly the wire. Circuit breakers can do this job very well but I have always thought that relay protection is not predictable, which is why UL requires actual testing for group use listing (breakers and contactors).

ant to consider breakers designed for the aerospace industry. Many are DC rated.


PUMPDESIGNER

 

[rbulsara]

There is no formula to rate AC breakers for DC rating if that is what you are looking for. Only way to know is they have to be listed (marked on the unit) for use on DC.

Many AC breakers are listed for DC use (of course different current /voltage than AC). As TWW indicated search and ask Square D, GE, Cutler Hammer etc.

 

[jbartos]

Suggestion: Some circuit breakers listed in manufacturers catalogs have DC voltage aligned with their current rating for AC voltage. This means that the circuit breaker is good for and application at the rated current shown on the circuit breaker and for the voltage indicated in the catalog, e.g. 250VDC.
The manufactures provide more information. There are four pole circuit breakers that have recommended two and two contact connections in series to be used for DC applications.

 

[jonesy29847]

Thank you all for your help, it turns out the breakers used are indeed rated DC, the DC rating was not visible from the installed position.

PUMPDESIGNER

I am protecting relay contacts rated at 6A, so the relay contacts end up being the 'weak link' in terms of current capacity. In the past when using a typical control relay the rating was 10 A. We are now using Safety Relays which have an 8A fast blow rating and a 6A slow blow rating and we are now using 24VDC for our control voltage.

 

[jbartos]

Suggestion: Visit

http://www.ferret.com.au/articles/2c/0c019b2c.asp

for DC Circuit Breakers, 3 and 6ADC

http://www.ferret.com.au/articles/3b/0c00de3b.asp

for DIN - T series miniature Circuit Breakers