AC vs DC circuit breakers?

[oak_box]

Professor Google proclaims that AC and DC circuit breakers are NOT the same.

There are some less expensive models, though the Blue Seas 7210 15A breakers claim to be AC / DC.

Are not all breakers AC/DC? Are the Blue Seas 7210 breakers likely to match what I have in an '83 Ocean Alexander 40?

 

[Comodave]

They will work for either. I have used them for both.

 

[BruceK]

There is only one true "AC/DC".

 

[av8r]

I can’t confirm the differences but I’ve used a couple of 7228 breakers as drop-in replacements in my ‘83 OA.

 

[DDW]

Many are rated for both. But usually, the ratings are different. Not the current specification for trip point, but the voltage and current interrupt ratings. AC is much easier to switch off, after all you have a zero crossing in the current and voltage waveforms 120 times a second. That makes them arc much less when you switch them or (or they trip). On a DC circuit, it will arc and try to weld itself when you trip it. Usually, and AC/DC circuit breaker will have a much lower DC voltage rating than AC.

 

[STB]

DDW has this exactly correct. Mechanical switches of any kind, including circuit breakers, need to be rated for DC to be used with DC and should not be used in excess of their DC rating.

The concern is that, as the switch breaks under load, the contacts of the switch form electrodes. As they are drawn apart the current is concentrated and the arc grows. And, just like the arc from a DC welders torch formed the same way, the current concentration forms a plasma and very localized very high heat, which can weld metals together. This includes welding together the metals of the switch contacts acting as electrodes. Once that happens, the switch can't switch off. Bye, bye, circuit protection. Hello energized failed and potentially dangerous circuit. Hello critical safety device turned fire hazard.

With AC the voltage's sine wave, it goes to 0v twice each cycle. And, 0 volts can't form or support an arc at any distance. So, as long as the contacts aren't welded in 0.083s, the arc is broken before they welding happens.

Also, although we most often talk about AC voltage as it's DC equivalent RMS value, it spends a lot of time rising and falling when it is below the RMS voltage and can support less of an arc (and some above, when it can support more).

The big thing is that AC voltage allows the arc to get broken before it gets hit enough to be damaged or weld in situations where the same DC voltage, even comparing RMS, would support a continuous or more prolonged arc.

 

[sunchaser]

STB
You and DDW raise valid points when using an AC switch for DC. Which though raises the question, can an DC breaker be used for AC?

 

[psneeld]

From Blue Seas web site..... either AC or DC

Not sure about this but some are certified to be used as switching devices too.

 

[STB]

STB
You and DDW raise valid points when using an AC switch for DC. Which though raises the question, can an DC breaker be used for AC?

Not necessarily.

Many modern breakers have advanced circuitry in them for things like load-side arc fault or ground fault protection. Devices rated only for DC may not include an AC rating because these aspects of these devices may not be compatible with AC. I wouldn't use an DC-only rated GFCI intended for a solar system for AC voltage, for example.

The same is also often true for relays and other devices with advanced features for integration, etc. A lot of things that we may think of as "just switches" may actually be small computers with switches!

Some devices can also contain noise filtering components or surge suppression components that will behave differently under AC vs DC. They could also have simple things like LED indicators with DC drivers.

So, it is really best to use the device as rated. If it isn't rated for AC, there is likely a reason. The same is true if it isn't erated for DC.

Also note that what we call 120v AC is actually 120v RMS. In other words, it is 170v peak. We talk about the RMS voltage vs peak voltage, because it is the average voltage over time, so it gives a better idea of how much work can be done over time with a constant resistive loads. Arc hazard distance is proportional to voltage

It is possible that a high AC voltage could present an arc hazard in ways other than to the contacts that would make it unsafe in an installation or for the type of PPE where it would be fine if it were a DC load at the RMS equivalent DC voltage. This probably wouldn't come.up with these voltages, though.

I'd stick with using devices as rated. Always.

 

[Turtle Blues]

To answer your last question, does it look like this?
Photo of typical marine AC/DC breaker repackaged by many resellers.

 

[DDW]

From Blue Seas web site..... either AC or DC

Not sure about this but some are certified to be used as switching devices too.

Most (but not all) of the Blue Seas breakers (which are actually made by others such as Carling) are rated for both AC and DC. If you click into the specs though, you will see that the AC and CD ratings are different.

I agree with STB, no reason not to use a breaker rated for what you are switching.

 

[Capt Ray]

All I know is I'd rather be shocked by 120 VAC compared to 120 VDC.

60 Htz means that the AC current returns to zero 120 time each second!
DC... just keeps on burning ... full bore.

Good topic!

 

[STB]

All I know is I'd rather be shocked by 120 VAC compared to 120 VDC.

60 Htz means that the AC current returns to zero 120 time each second!
DC... just keeps on burning ... full bore.

Good topic!

I don't think so. Not at all.

1st, if that is 120vac, it is probably 120v RMS AC, which means that the area under the current curve over time is the same as for 120v DC.

2nd, 120v AC is probably 120v RMS AC, which means that it is 170v AC peak-to-peak, so, the current spikes will be higher, even if the average is the same.

3rd, the physiological effects of the alternating current are such that it it is more likely to introduce muscle spasms, sweating, and arrhythmia. The sweating could make the shock worse. The muscle spasms could expose other hazards. And the arrhythmia, well, that's how one dies.

4th, although there isn't a huge difference between 120vdc and the 170v peak of 120v RMS, there is a difference, so the AC can arc across a greater air gap to get to you.

5th, although it doesn't matter much for 60hz and 120v, the body actually has capacitance, so AC can shock you even when you aren't grounded. DC can never do that. For the curious, a simple model is considered to be 100pF capacitor in series with a 1.50kΩ resistor, which is used to make touch sensors work.

I'd be careful when playing with either sparky or arc!


Just my 2 cents.

-Greg

 

[Capt Ray]

Thanks Greg. Thrilled to stand corrected. Your RMS measurement of AC makes perfect sense.

 

[DDW]

DC is generally considered to be more dangerous because it freezes the muscles. But difference is probably small. Above about 24V, extra care is always a good idea.....

 

[STB]

DC is generally considered to be more dangerous because it freezes the muscles. But difference is probably small. Above about 24V, extra care is always a good idea.....

I agree that DC is more likely, proximal to the current path, to cause muscle tetanus than muscle spasm.

Whether that spasm shakes a hazard free or exposes someone to a more severe or compounding hazard depends upon environment, situation, and length and location of current path. The same is true w.r.t. whether or not it extends the duration of the shock. An arrhythmia from AC can also cause immediate disability, even if not fatal.

So, I'm going to respectfully disagree. All circumstances being equal and unknowable, I'll take 120v DC over 120v ac rms for the very, very significantly lower risk for debilitating or fatal arrhythmia, by itself.

Again. Be careful around either sparky or arc! There is no such thing as a good shock!

 

[psneeld]

Most (but not all) of the Blue Seas breakers (which are actually made by others such as Carling) are rated for both AC and DC. If you click into the specs though, you will see that the AC and CD ratings are different.

I agree with STB, no reason not to use a breaker rated for what you are switching.

Also says right on the breaker..... they are rated for both types of current.

 

[twistedtree]

Many are rated for both. But usually, the ratings are different. Not the current specification for trip point, but the voltage and current interrupt ratings. AC is much easier to switch off, after all you have a zero crossing in the current and voltage waveforms 120 times a second. That makes them arc much less when you switch them or (or they trip). On a DC circuit, it will arc and try to weld itself when you trip it. Usually, and AC/DC circuit breaker will have a much lower DC voltage rating than AC.

What he said

 

[twistedtree]

Also says right on the breaker..... they are rated for both types of current.

That's the way to do it. Just use parts rated for the voltage and current you need.


It won't matter in most cases, but in the Blue Sea info page you posted you can see that the DC voltage rating is only 60VDC compared to the AC rating. When that starts to matter is if you are installing solar panels where it's pretty easy for the DC voltage to go above 60V. Then it starts to get harder and harder to find breakers, and they start to get really expensive.

 

[psneeld]

A lot of tech talk since post #1...but as the OP asked...for most electrical systems on small vessels AC/DC systems........

 

[STB]

A lot of tech talk since post #1...but as the OP asked...for most electrical systems on small vessels AC/DC systems........

I think the OP's original post was answered very early on. This thread is well into "bonus round", I think!