DC vs AC circuit breaker

As a young electricians apprentice, I was working on a Southern California Ed_son steam turbine generator overhaul, and noticed a remarkable size difference in the AC & DC breakers.

I was told that there are significant changes in the breakers because of the way that the current (of AC & DC circuits) will engage and dis-engage (when switched).

On the larger (2000 amp) DC breakers, the contacts were solid (yes solid) Silver. I was told that it was because of the heat would deteriorate copper or copper-clad in no time what so ever.

If you are insistent on trying to use the breaker then I would go to this link: http://www.ul.com/regulators/index_tech.cfm, and see if the topic has already been covered.

If it isn't posted in a spot on the UL website that is easy to locate, you could email (and) ask them what they think.

Hope that this helps.

It would pay to check with the circuit breaker manufacturer's data, but in general no, you cannot replace a DC circuit breaker with an AC type of the same voltage rating. There will usually be a de-rating that must be applied because DC tends to arc when the circuit is broken, much more so that the same AC voltage.

What is often done is to use a 2-pole AC breaker with both poles wired in series.

(Assuming you're not talking about the coil ) I think it's generally ok to use a DC rated breaker for AC - as has been pointed out, AC type contacts are de-rated for DC, so going the other way should be alright.

Don't hold me to this though. In fact, don't hold anyone on CR4 to their word on things like this - always check back to source (manufacturer's data etc.). I may be a raving loony!

Hello chinjim

The answer to your question is Yes, it can be replaced as you say.

But it will not be safe, nor will it work properly.

Do the job properly, get a DC Circuit Breaker.

The DC Circuit Breaker is more costly, but it will do the proper designed job for your DC power.

An AC circuit Breaker is quite likely to have severe internal arcing, and maybe explode or catch fire, not a thing you need, is it?

Kind Regards....

You cannot use AC breaker in a DC circuit.

The problem is the electric arc. When you turn the breaker off, in an AC circuit the arc disappears at the moment of zero-current but in a DC circuit the arc remains burning and you have to extinguish the arc with an additional magnetic field which continuously lengthen the burning arc till it "dies"

I believe that there are respondent tangents that are not addressing chinjim and his question:

"Dear all, Can a DC circuit breaker of 30 amps, 240V be replaced with an AC circuit breaker of 30 amps, 240V for a main drive?."

Our inquirer yet needs to provide the mfr and P/N of the breaker he has, and just what is the "main drive" (inductive?). Then we can better proceed.

I fail to understand why resondents move to reference 2000 Ampere breakers and whatever when it may be out of context. It just may be a whole different arena and subject area. Too, I fail to understand just what another comment was about a "coil", not knowing just what chinjim is making reference to.

....but the answer, in spite of your post, is still the same. That is a resounding:- "NO"!!

Its far too dangerous as many have said due to the lack of proper magnetic "Arc Snuffing" that DC requires......

I agree with Bill ML you need to check the specific ratings of the breaker you are using. Depending on the Mfg, you can probably find this info on the internet.

In general most 600 Volt AC rated molded case circuit breakers have a 250 Volt DC rating of 10 kAIC. Some 240 volt AC rated breakers have a DC rating but it is usually 5 kAIC.

It is a UL requirement the ratings be on the label. If you are outside North America, or it is a very old breaker it may not be there.

Good Luck

The answer is NO.

Dc circuit breakers differ from AC CB's by the distance of the contact opening, once activated.

DC voltage has a higher tendency to arc, especially when contacts are being opened slowly, or under inductive loads. Therefore, you must have a larger contact distance for the same voltage. A rule of thumb is, de rate the AC by 35%. to get the DC. (Or DCV+50%). In your case than you will need a 380VAC circuit breaker if you want to use it in a 240DCV line.

But as one of our poster has wisely told you, Better use the proper fuse.

Wangito

Dear seeker,

there is a reason that there is a difference between a DC circuit breaker and an AC circuit breaker. If it was only an issue of expense, the cheapest (least expensive one would win out.) However, experience has caused a design difference. I would err on the side of caution, until I could verify it. Especially if the main drive is an expensive component. What is the main drive for?

"there is safety in a multitude of counselors" (not my quote originally) good luck!!!

Depends on the breaker manufacturer, I sugggest you check with Square D as I know that they do offer a 30A AC/DC dissconnect for photoelectric installations.

The big concern is DC is capapble of jumping a much larger gap than AC. Lack of impedence, I think, been awhile since reading. DC breakers are listed as such, and usually the AC rating is also given.

I advise, check is various manufacturers or I you want email me and we can discuss the details, and I hope I can be of assistance.

My knowledge comes from photovoltaics as an instructor and the us of AC/DC breakers up to 600V AC/600DC

Good luck and be safe