DC and AC circuit breaker equivalence

[jack11]

I believe the heating effects (the amount of heat delivered to resistive load - Joule's law) of DC and rms AC currents are equivalent.
So, assuming the DC and AC circuit breakers trip based on the heating effects (a heat-based switch inside the breaker), a fast-acting AC circuit breaker normally operated at 120 VAC rms and trip-rated at X AAC rms should trip if we use it in a DC circuit and run X ADC current through it at 120 VDC, right or am I missing something here?

What about an AC current rating adjustment factor if we operate this AC breaker at DC voltages different from 120 VDC (higher or lower)? In other words, if we operate this 120 VAC rms AC breaker at say 60 VDC in a DC circuit, then what rms AC current rating stamped on the breaker should it have to trip at the same X ADC?

Someone's asked me why they can't go to Home Depot and use their stock AC breakers in their solar system. But, let's not worry for now about the impact on the electric code, and focus on the electric/tripping properties of the DC and AC breakers.

[TomW]

There is more to it than heating to trip at X amps. DC will sustain an arc and can burn or weld contacts. AC basically switches off (zero crossing) 60 times a second so the arc does not last.

That is the first thing that will bite you using AC breakers / switches on DC power.

Others will probably jump in with better info.

Tom

[OperaHouse]

Amps are the same, volts are not.  When DC opens the contacts may sustain an arc.  Up to about 30V there isn't a problem.  Silver is super bad to use as a dc contact.  I imagine at 120V DC there would be some issues.

[Bruce S]

I worked with 110Vdc here not too long ago, it liked to jump to the meters probes. I had no problem with 120Vac
As others have said, it's the 0 crossing point for AC.

[birdhouse]

TOM W!!

good to see you posting! 

adam

[Flux]

Most breakers have an instantaneous overload trip and a lower level thermal trip. As others have said, this is not the issue although the instantaneous trip may be different on dc.

The issue is has been stated, dc above 30v forms an arc that is difficult to break as there is no zero volt crossing.

True dc circuit breakes are very expensive things with magnetic blowout coils and arc chutes to lengthen the arc path to get it to extinguish. Even at 48v I suspect an ac breaker would need serious de rating. At 120v or more an ac breaker will be lethal if used on dc and if there is any significant fault current it will literally explode and melt. Some cheap manufacturers try to use ac breakers with multiple contacts in series, this sometimes works for easy applications but if the thing is ever really asked to perform it will still end up as a pile of rubble.

Just don't risk it, if you have high voltage dc use the correct and very expensive dc breakers or stick to HRC fuses. Dc is not at all user friendly above 40v.

Flux

[Mary B]

ANL fuses are a DC rated fuse that is heavily used in the car stereo large amplifier business. Rated to 80 volts DC and current capacity of 35 to 750 amps. I use them on all cables coming off battery banks including between my parallel strings and on my inverters. Very easy to find on ebay at reasonable prices.

[97fishmt]

I had a 600 volt 250 amp double pole breaker used just for a disconnect fail on one side operating at 30 volts.
I'm glad I caught it when I did.  It was on plywood and when I found the problem and took it out of the system
the breaker was to hot to touch and the plywood black behind it. 

I was playing with different transformers and letting the turbine run unloaded and then connecting to see differences in prop speed and amps out on the previous visit to the cabin.  I was lucky.  I have been leaving it up and on now when I'm away.  I was over at my cabin last week and I couldn't believe it said 450 Amp-hours since I was there last.  Kicking but on solar this time of year.

Yea, cool to hear from TomW and Adam and Happy New Year to all.

Mike

[jack11]

Thank you all for this useful info.

We can't have the contacts weld together and then the breaker not open when it needs to. I do some welding and can confirm that the AC and DC arc characteristics are quite different.

That fellow that asked me about the Home Depot breakers ended up buying a 8-to-1 combiner box for his PV array, with 8 Midnite Solar 15ADC 150VDC din rail DC breakers. He paid $10.70 for each breaker - not all that much.
I wonder what kind of technology they use in these DC breakers to overcome the DC arc problems, surely cannot be as expensive as some of the methods described above. Maybe thicker contacts, different contacts shape, contacts plating, some simple arc suppression techniques, ..., does anyone know, or ever disected a DC breaker?

[Flux]

I don't know about small breakers for 120v dc, I  know that some manufacturers try to get away with several contacts in series to increase the arc length as it opens.

I do know about real high power dc breakers for 500V. There are several approaches, the simplest used copper contacts as the main current carrying element but in addition there are spring loaded carbon contacts. The contacts are mounted to pivot in such a way that when open the copper contacts are separated by a couple of inches but the carbon tips open much wider. As it opens the copper contacts separate but leave the carbon ones carrying the current, the final break takes place on the very fast moving carbon tips and they separate far enough to extinguish the arc which can be a foot long at break.

More modern and compact breakers used magnetic blowout coils with arc chutes containing several baffles. The magnetic field forced the arc to follow the chute and the baffles lengthened tha arc path to make it go out.

It may be that modern dc breakers could have permanent magnet blow out, permanent magnets have come a long way in the last few years.

If you can keep the fault current low fairly simple breakers will suffice so the demand for small power systems may not be too serious. PV is a current limited source and will not be as challenging as clearing a fault on a half megawat dc supply.

Flux

[phil b]

Square D makes a QO breaker that's rated for AC and DC. The DC rating is about half the AC rating. It uses a thermal magnetic tripping mechanism.  They are available at several chain stores like Lowes and Home Depot. 

Pricey as compared to Midnite Solar and Outback's DC breakers. You get 100% amp carrying capacity with these.

[cardamon]

Square D makes a QO breaker that's rated for AC and DC. The DC rating is about half the AC rating. It uses a thermal magnetic tripping mechanism.  They are available at several chain stores like Lowes and Home Depot. 

Pricey as compared to Midnite Solar and Outback's DC breakers. You get 100% amp carrying capacity with these.

Are you thinking of the QOU breakers? Actually the regular QO breakers are rated for DC also, up to 48V.  I dont think those QOU breakers are any different, they are just expensive because they are unit mounts, less used lower production items.  A QO breaker used on a DC system will have the same thermal (overload) trip time as shown on the standard trip curve.  The instantaneous time will be a little longer.  This bulletin from square D gives the correction factors.

http://static.schneider-electric.us/docs/Circuit%20Protection/DC%20Rated%20Circuit%20Breakers/0601DB0401.pdf

[Xan]

I have recently been messing around with a relay diverting excess power from the solar panels from one 24 volt system I have to a second system. The first relay didn't go so well as at one point it was switching in & out too much (before I settled the circuit down) and the contacts became a bit burned. To the point where they would sometimes stick.

I replaced this relay with an identical unit, but this time connected a 6 uF florescent light capacitor across the contacts. Probably overkill, but I wanted to be sure it would work. And it certainly seems to have done the trick nicely.

So it got me thinking: Would connecting a capacitor such as this across an AC circuit breaker make it more effective for handling DC switching/breaking...? It seems to me that a cap in this role would divert the current long enough for the contacts to open without arcing providing it was large enough. Anyone have any thoughts on this matter?

[joestue]

the problem with a capacitor is the discharge when you turn the circuit breaker on.

if however you put a diode in series with the cap, and a resistor across the cap to discharge it, then it solves that problem.

another option is to put a a TVS diode (rated for not many more volts than your dc system) across the contacts and see if that works.

When you open a dc circuit, the current continues to flow for as long as it must to dump all the energy out of the inductance of the system, and the voltage will rise to a theoretically infinite value if it has to. there is usually nothing but the parasitic capacitance of the wires that limits this voltage, so i suspect the biggest issue with dc breakers is the leakage inductance of the system.

[OperaHouse]

I have grave doubts that a TVS would have any effect in cases where you are you are essentially switching one DC source to another.  There just isn't that much inductance and what there is can't store that much energy. Relay material is a big contributor, silver is very bad for this.  Once the arc is initiated it needs distance to extinguish it.  A capacitor is a good idea as it will allow continued current flow as the contact distance increases. In effect reducing the voltage across the contacts for a while.  The book GASEOUS CONDUCTORS by James Dillon Cobine is a good read, still in print after about 50 years.  I happen to have a couple books from his personal library.  That will get you cup of coffee with another $4. I would do the parallel diode resistor with capacitance values that high or at least add a small  value resistor.

[Johann]

You may want to watch this youtube,https://www.youtube.com/watch?v=HLX9cdB4TFQ

Notice, that high voltage DC was used and that the fire did not start until the breaker was turned off that created the arc inside the breaker that started and feed that breaker fire.

For safety use a DC rated breaker.