relay contacts in series for DC current break capacity

danw · Jun 16, 2015

A question on another forum brought up something I'd never heard before (no replies on that forum) so I thought I'd throw it out here:

1) How does putting two relay contacts in series increase the DC current break capacity ?

2) Is the implication increased contact life?

Samples of claims from spec sheets:

Tom Jenkins · Jun 16, 2015

Seems bogus to me. Maybe "parallel" was meant?

jraef · Jun 16, 2015

Quenching the DC arc of a contact opening is a function of the length of the arc and the applied voltage across the gap. By putting the contacts in series, you are in effect increasing the arc length. This allows the use of contacts in series to increase the use of the device on voltages that exceed the rating of one set of contacts alone. It does not really extend the life of the contacts if the voltage exceeds the rated votlage of the device, it just allows them to be used at all. It does however extend the life of contacts that ARE already rated for the applied voltage, but also increases the resistance in the circuit which may be an undesired side effect.

Lancie1 · Jun 16, 2015

If you consider current breaking capacity (ability to open a circuit with x amount of current), then it is different that the ability to conduct or carry x amount of current.

Each time a contact opens under current, some of the contact material is burned away. If you use 2 contacts in series, then first one is burned more than the other, then the next time the other one has less resistance so it is burned more the next opening, and so on alternating back and forth, achieving about 2 times more opens than for one contact. But most likely the same thing would happen with parallel contacts, and the advantage with parallel contacts is that they will also carry 2 times more current when closing or closed.

JesperMP · Jun 16, 2015

This is a common technique, not just for DC but also for AC.
And not just for contactors, but also for MCCBs. Maybe more so for MCCBs, because you can never have too much short circuit breaking capacity.
The method may have been mentioned in connection with DC, since DC is harder to interrupt than AC.

The reason why two contacts in series is better than one (of the same size) is that the air-gap that the arc has to pass is doubled.

James Mcquade · Jun 16, 2015

What the chart does NOT say.

1. type of relays used.
2. relay contact rating for each contact.
3. relay contact rating for 2,3,4 contacts wired in series and parallel wiring. I have used relays in the past that give the ratings for series and parallel wiring.
4. actual Load / voltage conditions for the test.
I had an application in the past where I wired contacts in series and that did not solve the problem because both contacts had the same rating and were operating at the same time. we eventually used a heavy duty relay with a much larger contact make / break rating.

regards,
james

Tom Jenkins · Jun 16, 2015

OK, Lancie, your rationale makes sense. However, doesn't that assume the contacts both open at exactly the same time? Is that a reasonable assumption?

Lancie1 · Jun 16, 2015

I think my parallel scenario assumes that the contacts open at approximately the same time, give or take a few milliseconds. The one that opens first is going to be burned off more than the later one, evening up the resistance through the circuit and again causing alternation of the current flow.

Saying that 2 contacts in series lengthens the air gap is hard to visualize. The arcs I have seen jumped from contact point to nearest contact point. Why would the arc suddenly jump from contact point 1 over 2 contacts to get to a (longer higher-resistance path) 4th contact? It sounds good in theory but breaks down upon closer inspection. It could be that there is another physical characteristic in play that does increase the series current-break capacity, but I doubt it is because of lengthening the air gap.

JesperMP · Jun 16, 2015

I on my part cannot understand the part of the "arc jumping" (?).
edit: You are referring to when you heve several contacts in parallel, maybe even many contacts ?

The arc that builds when opening a contact is made by the current that flowed when the contact was closed.
If you have 2 contacts in series, it is the same current and also the same arc that builds simultanously over the 2 contacts.

PapaSmiff · Jun 16, 2015

Perhaps it would have something to do with the air gap sufficient to stop the arc would form twice as fast with two contacts in series.

Lancie1 · Jun 16, 2015

If you have 2 contacts in series, it is the same current and also the same arc that builds simultanously over the 2 contacts.

If it was the same arc, wouldn't it be the same length? If it is twice as long, would not it have different characteristics?

Perhaps it would have something to do with the air gap sufficient to stop the arc would form twice as fast with two contacts in series.

Maybe the theory is that because two contacts in series produces a gap twice as wide, so the arc would have to be 2 times longer to jump the gap, thus causing a higher current-break capacity. But for most relays, the reality of a series connection for 2 contacts will be 2 gaps of the same length separated by some other conductors.

JesperMP · Jun 17, 2015

One air-gap will require a certain voltage to bridge.. 2 air-gaps will require double the voltage. When the voltage required is higher than the generator voltage then the current will begin to decline and the arc(s) will eventually be extinguished.

Liam Moran · Jun 17, 2015

I have done this many times in the past with relays switching Hydraulic Solenoid valves, on a lifting application in automated Palletisers. The control voltage was 24VDC and the switching current was 2 Amps. Normal 8 pin relays were supplied as original fitment, and I found these needed replacement every 4-6 months. I modified the wiring of the relays to have 2 contacts in series and they then lasted several years.

dmargineau · Jun 17, 2015

A bit off the OT:

Another option of switching high DC loads are the Solid State Relays.

http://ab.rockwellautomation.com/Relays-and-Timers/Solid-State-Relays
have done a much better job than the main stream electro-mechanical ones.

Since there isn't any air gap the "wear" is kept to a minimum.

dogleg43 · Jun 17, 2015

I ran into this same issue about 12 years ago while working with an AB/Rockwell Drives startup guy.

If I remember correctly, there was a real small DC motor we were starting and stopping with a 700N relay. It burned up the contacts right away. The drives guy said to wire two contacts in series and that solved the problem.

He pointed to an AB tech note on this subject. I should have kept it.

relay contacts in series for DC current break capacity

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