Is it safe to use an AC circuit breaker in a DC circuit?

Thread Starter

Rahulk70

Joined Dec 16, 2016
511
Hi,

Let's say I have a 20Amp GE AC circuit breaker. So, let's say after testing it in a DC circuit with some loads and ammeter + clamp DC amp meter I find the current at which the breaker trips. Let's say this is 45Amps. So, is it possible to use the AC circuit breaker in the DC circuit on a permanent basis? I've seen on a youtube clip where a person is using an AC 6Amp 230V breaker with a DC circuit and it tripped at 13Amps when loaded.

Are there any significant safety hazards with this technique? I mean I was thinking since it's the current that matters maybe these breakers can be used with 6V/12V/24V circuits where the currents are generally going to be drawn only under 20Amps in the above mentioned case.
 

MisterBill2

Joined Jan 23, 2018
7,235
Hi,

Let's say I have a 20Amp GE AC circuit breaker. So, let's say after testing it in a DC circuit with some loads and ammeter + clamp DC amp meter I find the current at which the breaker trips. Let's say this is 45Amps. So, is it possible to use the AC circuit breaker in the DC circuit on a permanent basis? I've seen on a youtube clip where a person is using an AC 6Amp 230V breaker with a DC circuit and it tripped at 13Amps when loaded.

Are there any significant safety hazards with this technique? I mean I was thinking since it's the current that matters maybe these breakers can be used with 6V/12V/24V circuits where the currents are generally going to be drawn only under 20Amps in the above mentioned case.
Circuit breakers have a lot of different parameters aside from rated current. First comes the voltage, which relates to how far and how fast the contacts open. Then there is current versus time to trip, That covers a wide range of performance and applications. Then, also, there are at least three fundamentally diffeent tripping mechanisms, Thermal, Magnetic, and Electronic. Also combinations of them. There is often a dual specification as well, the "Must hold" current and the "Must Trip" current.
An AC breaker may function in a DC circuit but the performance may not be adequate.
So really it is not the best choice to use a breaker that is not designed for the application.
 

SamR

Joined Mar 19, 2019
2,770
You might also note that there are 6 classes of DC circuit breakers with very different trip specifications. Class A, B, C, D, K, Z. Somewhat similar to the different types of fuses, Fast Blow, Standard, Slow, etc. There are significant differences in arc suppression and extinguishing between AC and DC breakers due to the difference in the nature of the inputs between a flat and sinusoidal voltage. Do NOT use AC breakers for DC service as their arc suppression is inadequate.
 
Last edited:

boostbuck

Joined Oct 5, 2017
15
I made use of a range of 240V AC breakers in a low voltage DC operation. The manufacturer's data sheet for those breakers stated that they were rated to 48V DC, and provided the derating information for the current trip point when in DC applications. With this information I could ensure that they were used safely within the manufacturer's specification. Even so, it would still have been better to use breakers designed specifically for DC.
 

Lectraplayer

Joined Jan 2, 2015
116
Although not a breaker, I did find out one time exactly what happens when you switch 120V DC (in an MRI installation) with an AC switch. We tried because the DC rated switch was much more expensive and none of us knew what the difference was. After installing a common heavy duty wall switch (120VAC, 20 amps) and trying to turn it off, the lights flickered noticeably and then the switch did nothing. The lights remained on. Obviously the contacts had welded together. Something you DEFINITELY don't want a breaker to do.
 

SamR

Joined Mar 19, 2019
2,770
Some manufacturers do make dual rated breakers to handle both AC & DC. Don't think I ever saw one though and it could be buried in the PDF, so read it.

Edit: At least in the US, I would think it would clearly be marked as such on the label.
 
Last edited:

Thread Starter

Rahulk70

Joined Dec 16, 2016
511
Thank you very much for posting that video clip. That was very informative. I too tried an experiment today. I had two power supplies and the current outputs they can handle is quite high. In the end, I decided to add a higher current rectifier like a 350 to 400A rectifier to protect the DC as those ones can handle short circuit currents exceeding 1000Amps. I'll add the breaker to the transformer low voltage-high current AC side. I did test it with an old GE 20A breaker in the storeroom. I performed a dead short from the transformers of the two supplies and the breaker tripped. The first supply transformer is very large and can output above 75 to 80 amps easily in a dead short. That caused the transformer to trip in 39-49 ms and the second charger in a similar dead short but it can output a max of only 50 amps in dead short tripped it in like 1.17 seconds.

I tried the dead short technique again with the smaller charger on the DC side after the rectifier and even after 4 seconds the breaker was intact and I could feel both the transformer wires & rectifier getting warm. The breaker did also get hot. I immediately shut the test and decided to abandon this idea.

One question though, do you think arching like in the above video but to a smaller extend will occur from 7.1 VAC and 15.8VAC will occur during interruption?
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
511
Some manufacturers do make dual rated breakers to handle both AC & DC. Don't think I ever saw one though and it could be buried in the PDF, so read it.

Edit: At least in the US, I would think it would clearly be marked as such on the label.
Most likely those would be expensive as they usually make it the home circuit breakers with a single purpose in mind right. I mean it's always 120/240VAC. Only if there is a solar panel the need for DC breakers arises. Well, I haven't come across one yet, but as you say they may exist.
 

MaxHeadRoom

Joined Jul 18, 2013
21,601
The switches and relays I use on power DC circuits have arc quench blow-out magnets fitted, otherwise a plasma arc ensues!
As seen in the video.
Max.
 

nsaspook

Joined Aug 27, 2009
7,723
One question though, do you think arching like in the above video but to a smaller extend will occur from 7.1 VAC and 15.8VAC will occur during interruption?
As the contacts open the electrical resistance at the remaining point of contact increases with the resulting I/R loss in the circuit at that point causing heating at a smaller and smaller cross section of conductor until it generates a tiny molten hot spot that continues the current flow and heating as a plasma (ion and electron flow) even as the contacts physically separate due to a breakdown voltage across the arc. The source voltage and current to sustain a continuous arc must exceed this breakdown voltage or the arc will quelch quickly. At those voltage you will usually only have a small pit burn on the contact instead of a continuous arc.
https://en.wikipedia.org/wiki/Electrical_breakdown#Persistent_arcs
 
Top