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Say I have a DPDT relay, like T92S7D12-24. The contacts of this relay are rated for 30A, but there are two sets of contacts. Can I parallel the contacts to get an effective 60A relay? Further, could I parallel two (or more) relays and get even more current capacity?

I see two possible problems.

  1. Current may not be shared equally between sets of contacts and between relays. One set of contacts could take more than their share of the current and overheat.
  2. Switching times may vary between contacts and between relays. When breaking with current through the contacts, the last set of contacts to open may be carrying far more than their recommended current at time of break. This could cause damage.

Are these problems? Are there other problems? If so, can they be quantified and worked around? Or is paralleling contacts and relays always bad design practice?

In my specific application, I'm using these relays as part of precharge for a capacitor bank. They are not expected to switch current. They make once the caps are charged, then hold. They should never open under current flow. Under these specific circumstances, should I still expect problems?

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3 Answers 3

up vote 6 down vote accepted

No you should not do this. Sometimes it is explicitly allowed on the data sheet (but not that I can see on this data sheet), and when it is, IME you never get as much as double the capacity.

Paralleling physically separate relays is worse again because they're not physically moving together- expect welded contacts etc. if you tried that.

If you can split the load (for example, instead of a 40A heater use two 20A heaters) then you can get an equivalent functionality.

You could think about ballasting the loads (wasting power to roughly equalize the currents) and fusing each contact separately, but I don't think that's a good idea at all.

Note that using the relay at the maximum rated current will lead to a pretty short life (only 100,000 operations for a resistive load), which might be only weeks or months if it's switching continuously. At 3HP (motor load), the life is only 1,000 operations, so at once per minute it won't last a single day.

Edit: With the added information that you're using the relay to switch effectively at a relatively low DC voltage and you're mostly concerned about carrying current.. I can't say categorically this is really a horrible idea with a single relay, but I think I'd get on the horn to the manufacturer and see if it's possible to get any buy-in. It comes down to variability in contact resistance vs. the resistance of the connections (plus whatever, hopefully balanced, resistance you add externally). When one of the contacts inevitably fails first, I think I would prefer the relay to not emit excessive amounts of smoke or flames). I think you're okay at 40A (with AgCdO contacts) given the UL508 rating, but beyond that is in question.

If you really need such a high carrying current, the Omron G7Z appears to explicitly allow paralleling the 40A contacts without derating, for 160A total capacity, but perhaps not with the blessing of safety agencies.

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I suppose under some conditions, the relay's ability to break the current (and not just continue arcing) could be a problem, too. ie, two parallel relays gets you half the resistance and twice the thermal mass, but it doesn't get you any more contact separation. –  Phil Frost Apr 3 at 1:45
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Paralleling contacts to pass a higher rating, than the individual contact rating, is a bad practice.

Sometimes paralleling of contacts is done for redundancy. Should one set of contacts degrade current can flow through the other set of contacts. When this is done the normal current load does not exceed the rating of the individual contacts. I've seen this practice recommended by NFPA in Electrical trade magazines where the system is critical and a failure of the relay can lead to another hazard.

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Relay current-handling ability is limited by two factors: the ability of the contacts to pass current when continuously "on", and the ability of the contacts to handle the stresses associated with switching. In general, if two relays are connected in parallel, the continuous current-handling ability of the combination will be almost equal to the sum of the individual relays' abilities, but the switching ability of the combination may be that of the worse relay, and in many cases won't be much better than that of the better one.

If one's application never entails opening or closing relays under loaded conditions, it may be reasonable to use paralleled relays to boost steady-state current-handling ability. In general, however, one should only use parallel contacts to boost "live" current-switching abilities if a relay manufacturer specifically allows it.

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