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breaking an arc

breaking an arc

breaking an arc

(OP)
I have a contactor circuit (basic start stop etc.).  An interposing relay is used between the operator controls and the contactor.  

My question is when do I need to place 2 contacts of the interposing relay in series when they are being used to trip a conactor?  I realize the point is two have each contact open some of the arc... but in this case the contactor only draws a couple amps (of course there is also an inrush).... and the relay contacts are rated for 20A at rated voltage?

Is there a rule of thumb?

Regards,
TULUM

RE: breaking an arc

No. Hardly at the levels you are describing. I assume that you have AC since you mention inrush. If you have DC, it is quite another game.

Gunnar Englund
www.gke.org

RE: breaking an arc

(OP)
to explain further..

The rule in the office is 125vdc and up use 2 contacts...

My thoughts are if the contacts are rated for the full voltage and full current... why?

Regards,
TULUM

RE: breaking an arc

So you have DC! Then you have to watch out.

We have talked this over and over again before. Do a search on kick-back, arcing, snubbers, free-wheeing diode etcetera.

In short: DC never goes to zero, so there is no natural arc extinction. Also, an inductive load (like a coil) has a time constant. Trying to reduce current to zero in shorter time induces a kick-back voltage that tries to keep current up (Lenz's Law about action and reaction). Once an arc has established itself it can last forever. It is a question of energy and driving voltage.

As I said, search the fora or google. Or - why not? - have a look in a text-book on electric circuits.

Gunnar Englund
www.gke.org

RE: breaking an arc

(OP)
Thanks gunnar...

I will hit the books a bit later... just was looking for some practical insight...

Although the rule stands here... I have another colleage that works for a DC locomotive company that says if the voltage and current ratings are adequate they do not double up the contacts... especially if the current is 10x the rated current...

Regards,
TULUM

RE: breaking an arc

I guess that that is because one rating is AC and the other DC.

Gunnar Englund
www.gke.org

RE: breaking an arc

(OP)
Huh?  both are DC??

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