breaking an arc

[tulum]

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

 

[Skogsgurra]

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

http://www.gke.org

 

[tulum]

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

 

[Skogsgurra]

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

http://www.gke.org

 

[tulum]

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

 

[Skogsgurra]

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



Gunnar Englund

http://www.gke.org

 

[tulum]

Huh? both are DC??

 

[Skogsgurra]

DC AND resistive?

Gunnar Englund

http://www.gke.org