seriesed 125VDC contacts to switch 250 VDC ??

[leitmotif]

I thought I had posted this question here previously and tried looking it up - to no avail. If someone else can find it well thank you.

BACKGROUND belong to this electrical vehicle website and got into argument regarding using two swithces to switch 250 VDC.
Some people who I believe (generally) know what they are talking about say it can be done AND make the implication it can be done safely.

ASSUME a contactor two pole single throw rated at 125 VDC.
Both contacts are NO and rated for 20 or 25 amp. Not a fancy unit say cost about $25 so no blowout coils arc chutes or stuff you would find on larger units.
ADDED AFTER Chistoff 84 responded cause I forgot -- and assume contactor made by reputable outfit ie P & B Square D Allen Bradley etc - sorry Chris

QUESTION Can you safely use two contacts on a double pole relay that are each individually rated at 125 VDC
AND connect them in series to switch 250 VDC?

I have never seen this done (just dont recall it?) but that does not necessarily mean it can or cannot be done.

Seems to me that the logic I was told here before went
"both contacts will NOT open simulaneously therefore on is going to open the full voltage ie 250 which is double the rating of the contact"

I know that a lot of motor starter relays use a bar with contacts at each end which in theory supports the argument BUT these are designed from ground up to operate as designed.

Seems to me the answer is simple --
buy a contactor whose contacts are designed to switch the designed voltage and current.

Just checking to see if there is something I forgot or is there an exception to the rule??
Dan Bentler

[Christoff84]

As far as I can see, this would be like buying a garden hose rated for 300 PSI and trying to use it as a high pressure line on a hydraulic cylinder running at 5000 PSI. It might work for a short time but in the end you're going to have excess arcing due to the insulation not being good enough. The problem is, say this is a cheap unit built with a safety factor of 2....at 250V you have no safety factor left. In the end, not the best idea to try. Even more so, it would violate the electrical code to use a device not rated for the application.

[mellis]

I've seen this done before. Wiring more than one contact in series for DC. I think I understand the benefit, but I don't believe the equipment supplier is going to allow you to double the voltage rating. If you don't care about the warranty, test it and see what happens.

If you have a 2 or 3 pole contactor and you only need to break a single circuit, wiring the contacts in series should make the contacts last longer. As long as the other contacts open before the arcing stops on the first contact, they should help. Since they are mechanically linked, it should be pretty fast. (i.e. we don't worry about single phasing a 3-phase motor when we pull in and drop out its contactor, right?)

While it may be tempting to say that the total air gap is additive and that determines the max voltage, once one contact welds, it ain't gonna open.

[Mickey]

I thought I had posted this question here previously and tried looking it up - to no avail. If someone else can find it well thank you.

This one?

http://www.plctalk.net/qanda/showthr...trical+vehicle

[leitmotif]

Originally Posted by Mickey

This one?

http://www.plctalk.net/qanda/showthr...trical+vehicle

Mickey

Well I did post that one. It is irrelevant to todays question. HOWEVER it was worth while reviewing and jogging my memory. Good try however.
Thanks
Dan