Two Poles in Series for DC

[SG-1]

I am looking at a Cutler-Hammer GHC breaker that states " Use two poles in series for DC."
"125/250 VDC"

125 VDC circuit.

Am I correct thinking that the 2 pole breaker should be wired thus: The positive DC comes in the top left hand pole. The bottom of the left hand pole is connected to the top or bottom of the right hand pole and then out of the other side of the right hand pole on to the circuit it is protecting ??

 

[LarryFine]

Yep. Placing two contact sets in series effectively opens the gap twice as far, twice as fast, and reduces contact damage from arcing.

Less pitting means greater contact life. DC is harder on contacts than AC, because the AC passes through 0 volts twice per cycle, extinguishing the arc faster.

 

[SG-1]

Is this correct ?

If one were insistant on using this breaker and wanted to break the positive & the negative together, they would need this breaker in a 4P configuration.

The system is ungrounded & as stated before 125 VDC nominal.

 

[LarryFine]

That's not necessary. Remember, in a series circuit, the relative location of each component doesn't matter.

You can break one line with two poles, and the other with one. In effect, you'll have three contacts in series.

Added: The above presumes we're talking about a 3-pole breaker.

 

[dicklaxt]

I have always heard that too but never totally understood how all that worked and why............somebody needs to draw a picture for this dummy.How about it Larry,,,,,,,,,

dick

 

[LarryFine]

Not a pic, but think about it: if each contact opens 1/4" in 1/100 of a second, three in series open 3/4" in the same amount of time. That's effectively three times the speed.

 

[dicklaxt]

Well I'm thickheaded and it takes a while and this must be one of those times,,,,,,,,,,,,,1/4 inch vesus 3/4 inch either way the circuit is broken so I've got to go do some reading I guess:grin:

dick

 

[jim dungar]

It has to do with 'lengthening' the arc as you break the DC circuit.
In AC the zero crossing of the waveform stops the arc, and as long as the contacts have moved for enough apart there is no restrike. In DC the arc has to be stretched until it self extinguishes. There are many different techniques used to break DC current (i.e. putting 2 poles in series), but in the end they almost all come down to distance.


And yes, placing a pole in both the negative and positive legs, counts as having 2 poles in series.

 

[dicklaxt]

Well I finally found an explanation of why and how,I wish I would have ear marked it but I didn't.It basically explained it this way.

An AC circuit breaker is designed to open as the voltage passes thru zero thus it follows current also passes thru zero and there will be a minimum arc across the contacts allowing it to clear the fault and not damaging the contacts except at least minimally.

Now DC doesn't have this alternating of voltage and current as we all know so that a DC breaker must operate faster,,,,,,so to minimize the size and duration of an arc,,,,,,,,,,,,it is accomplished by putting two or more contracts in series, it divides the arc across these contacts,,,,,so less arc size allows faster extinguishing of the arc itself thus faster clearing,less contact damage etc.

I'm satisfied,thanks for this post it was educational chasing down an answer that I understood.

dick

 

[SG-1]

Jim says I just need one contact in the positive leg & one contact in the negative leg & I am covered.

Larry says I need two contacts in one leg & one contact in the other leg & I am covered.

There is an inadvertent ground at the battery rack, on the negative leg, before ( ahead of ) the 30A breaker in the control circuit.

The control circuit develops a ground on the positive leg after the 30A breaker in the control circuit.

My 2 pole breaker has a contact in both the positive & negative leg, but only one pole is involved in the fault.

My 3 pole breaker was wired such that the double contact is in the negative leg. Only the contact in the positive leg is involved in the fault.

 

[SG-1]

I have been digging into the specs on this breaker.

The single pole has a maximum rating of 125VDC.

The double pole has a maximum rating of 125/250VDC

I have the double pole.

The Use two poles in series for DC is in reference to the 250VDC rating. The data sheet ( slightly too large to upload ) says to use the 2 outside poles for 250 VDC on the 3-P breaker.

I am concluding that I am covered on a 125VDC circuit with the 2-pole breaker connected with positive on the first pole & negative on the second pole.

What say ye ?

 

[LarryFine]

Jim says I just need one contact in the positive leg & one contact in the negative leg & I am covered.

Larry says I need two contacts in one leg & one contact in the other leg & I am covered.

Either will work. Using the third pole merely extends contact life. How important that is depends on how often the contacts must interrupt how much current.

There is an inadvertent ground at the battery rack, on the negative leg, before ( ahead of ) the 30A breaker in the control circuit.

The control circuit develops a ground on the positive leg after the 30A breaker in the control circuit.

Obviously, one of these groundings must be prevented.

My 2 pole breaker has a contact in both the positive & negative leg, but only one pole is involved in the fault.

My 3 pole breaker was wired such that the double contact is in the negative leg. Only the contact in the positive leg is involved in the fault.

It looks like you need to permanently lift the inadvertent ground.

 

[LarryFine]

I am concluding that I am covered on a 125VDC circuit with the 2-pole breaker connected with positive on the first pole & negative on the second pole.

What say ye ?

Works for me, once you correct the above-referenced inadvertent ground.

 

[SG-1]

Works for me, once you correct the above-referenced inadvertent ground.

Thanks Guys as always !! &
You will never see that unadvertent ground again !

 

[adelle]

That's not necessary. Remember, in a series circuit, the relative location of each component doesn't matter.

You can break one line with two poles, and the other with one. In effect, you'll have three contacts in series.

Added: The above presumes we're talking about a 3-pole breaker.

+1
I just put in two large UPS systems and the battery breakers are three pole and wired exactly as you say.

 

[LarryFine]

I just put in two large UPS systems and the battery breakers are three pole and wired exactly as you say.

I see the same thing in multi-supply fork-lift-type battery chargers. When changing for different supply voltages and/or phases, you need to change the input breaker wiring, the power-transformer primary connections, and the control-circuit transformer primary tap.