What's new
What's new

OT - Use DC Circuit Breaker for AC?

Jeff_M_PA

Cast Iron
Joined
Oct 31, 2008
Location
San Diego, CA
In a moment of stupidity, I bought on eBay what I thought was a used 500A molded case circuit breaker for 480v 3-phase service (should have known it was too good of a deal). When it arrived today, my maintenance guy said, "hey dummy, why'd you buy a DC breaker?"

It's a (really nice) Square D Cat #MHL3650032DC Series 2. So, I did a little Googling, and I found out, in caveman terms, that DC breakers have to be much more "King Kong" to break x amount of DC current than an equivalent AC breaker. Lots of references online say "no, you can't use that AC breaker on your DC system", but no references I found address the opposite situation of using a DC breaker on AC.

Yes, I know the right answer (given that this is obviously a safety issue) is probably to buy the correct breaker, and just re-sell this one, and that's probably what we'll do, because I would imagine no inspector is going to pass it anyway. But, I'm curious; it looks like it should work.

If it matters, this will be a 400A 480V 3-ph main service, and was intended to replace our last remaining fused disconnect. We have two other 400A services and one 800A now.

Jeff
 

davycrocket

Hot Rolled
Joined
Jul 10, 2005
Location
UK
Seems a strange question.

Surely a DC breaker will have either ONE pole or Ideally TWO poles .
The contacts can certainly handle AC as well as, if not better than DC, but for 3 Phase you need a THREE pole switch. Or did you manage to buy a THREE pole DC breaker ?

Davycrocket
 

Forrest Addy

Diamond
Joined
Dec 20, 2000
Location
Bremerton WA USA
Yup, gotta be three pole like Crokett says but here's the clincher: If you are working in a commercial setting you have to use NEC approved circuit protection devices rated for your electrical service. The DC breaker you bought may be superior in every way to an AC breaker but it's not AC rated unless it says so right on it. If you install it and the OSHA people catch it, they will gig you. Your electrican has a perfect right - actually a legal obligation - to refuse to install it in an AC distribution system.

Same deal in a residence but there's less chance of getting caught.

Put it on the "Aw, $hit!" shelf, shrug, and get a breaker rated foir AC.
 

Jeff_M_PA

Cast Iron
Joined
Oct 31, 2008
Location
San Diego, CA
Davy,

I'm dumb, but that THAT dumb. It has three poles (see photo). I actually thought, for some reason, that they had used an AC breaker for DC by putting that gigantic jumper on there. Don't ask me where i got that idea. But it turns out that the yellow sticker on the left side of the breaker has a diagram that shows two ways to configure the jumper to make the unit either 300A or 500A. Go figure. I know someone on here understands this and can enlighten us all on the situation.

Forrest - of course I know you're right. At this point, it's probably strictly of academic interest.
 

Attachments

  • DC Breaker.JPG
    DC Breaker.JPG
    54.7 KB · Views: 1,247

macona

Diamond
Joined
Jun 20, 2006
Location
Beaverton, OR
DC breakers usually have magnetic arc quenching to extinguish the arc that occurs with DC. In an AC breaker the arc will extinguish itself in the dead time. DC wont, magnets near the contact blow the arc out. Thats why large DC breakers also have polarity noted on them.
 

Jeff_M_PA

Cast Iron
Joined
Oct 31, 2008
Location
San Diego, CA
Technical Support

I thought WHHJR's suggestion was good, and on the Schneider Electric website I found a Support inquiry system. They responded within a few hours, but the news was not very enlightening:

Thank you for contacting Schneider Electric !

This is a dc only thermal/magnetic breaker. It is not approved for ac operation.

Sorry.

Regards
Jeff Dixon
Square D/ Schneider Electric
Cedar Rapids PSG Sr. Product Support Specialist

Heck, Forrest told me that already! Anybody need a nice DC breaker? $200 it's yours.
 








 
Top