Just what is the difference between a circuit breaker rated for DC and one rated for AC?

I had always thought that DC had a greater ability to weld the contacts together or pit them, but I never had confirmation on that.

Is the DC resistance of the thermal element different? Low voltage systems (which are usually the DC ones, but that's hardly a rule) are affected more seriously than higher voltage systems (such as 125VAC). Any truth to that theory?

 

When a pair of contacts open, a spark occurs across the gap, the higher the current the bigger the spark, as the current flow tries to continue.

With DC the contacts have to open wide enough for this sparking to stop, but with AC the voltage falls to zero every half cycle, so as soon as the voltage falls to zero (or below the critical voltage) the sparking stops.

This allows contacts to have a far higher current rating for AC than DC, as sparking will be far less.

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You sure? I'm used to the idea that AC has a greater capability to ionize air due to the dV/dT characteristic, thus can create longer arcs for a given voltage and sustain them for longer times. (Can't do a good Jacob's Ladder with DC).

 

Nigel is right...

 

I agree that DC breakers need beefier contacts and larger air gaps for the same current and voltage.

Just for fun though look at this link -all AC breakers (I'm a bit skeptical about some of the comments though :!: )

http://205.243.100.155/frames/longarc.htm#Disconnect

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DC is a constant, AC 60Hz 16.67mS per cycle or every 8.335mS is a zero crossing, 50Hz 20mS per cycle or every 10mS is a zero crossing.

Current has a profound affect on gap spark, as this is used in proportion to voltage applied and relays, circuit breakers are made with specific ratings. If the current is minimal and voltage large the gap would still need to be larger for DC, same if the current is over voltage. Compared to a AC relay of about the same rating the gap would be somewhat smaller. Although I have relays that can use both AC and DC on the switching side usually the DC rating is alot smaller than the AC equivalent, this does not apply to Solid state relays as the must be made for AC or DC.

Jacob's Ladder, we are talking high voltages here like in the kilo volt range 2kVac. If you applied 2kV DC on a dead short across the Jacob's Ladder using a piece of wire (most likely the wire would vaporize to break short) to the gap required for AC ionization the DC spark would jump at the narrowest part of the ladder and continue to do so untill the gap is broken or to wide to allow DC to pass.

Alot of things change and the physics of electricity bend as we get into extremely high voltages.

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