Hey guys,

I was talking with someone at work today about amp rating of relay contacts. In particular, relays with multiple contacts. What this person did was run a load off of two parallel relay contacts because the load was too much for one contact. His thinking is he could run twice the current rating between the two e.g. 12A across two 6A contacts (hypothetical). I told him I didn't think that would be right because of switching time between contacts. What other reasons would this not be a good practice?

Thanks

 

The contacts will not open or close at the same time. Normally the time difference between opening and closing will be small, so there is little problem under quiescent loads. The spark generated by the interrupted current from first opening contact will be much greater than that for rated load, thus the arcing will shorten the life of that contact. Eventually the first opening contact fails and you are left with a 2X load on the remaining contact. The closing current surge causes similar damage. Inductive loads will severely impact contact life.

 

Trying to parallel contacts for high current is equal to setting up a reliability problem. It will last a little bit longer than only one inappropriate relay, but not nearly as long as a properly sized relay.

There is a use for parallel contacts, but it is not for higher current, it's for very low current and causes better reliability. When the voltage is not high enough to puncture an oxide layer and the current is not enough to melt the surface a bit, you use special contacts. Some people call them, "pilot duty". I needed to continuously carry milliamps at 24 VAC. You don't use a 30 amp contactor for that! I had a 4PDT relay with gold plated contacts. I wired them all in parallel and it's been running without error for 13 years.

A bad use of contacts is demonstrated in a Kenmore refrigerator. When the Mfg switched to a microprocessor, the old 10 amp temperature sensor was not replaced with pilot duty contacts. The microprocessor can not supply enough power to keep the old contacts working, so the refrigerator fails to defrost.

 

There is a use for parallel contacts, but it is not for higher current, it's for very low current and causes better reliability. When the voltage is not high enough to puncture an oxide layer and the current is not enough to melt the surface a bit, you use special contacts. Some people call them, "pilot duty". I needed to continuously carry milliamps at 24 VAC. You don't use a 30 amp contactor for that! I had a 4PDT relay with gold plated contacts. I wired them all in parallel and it's been running without error for 13 years

This "pilot duty" thing that you're talking about... I had to learn it the hard way while working with TTL signals... through countless headaches and endless trials and errors until I reached the same conclusion that you mentioned. An extremely thin layer of oxide was being formed on the contacts and was wrecking havoc with the signal. I later discovered that there are microswitches with magnetic-actuated reed switches in them. That solved my problem, since reed switches are normally encapsulated in glass and are therefore completely isolated from outside humidity.

 

How come it's the contact that opens first? Isn't it short circuited by the parallel contact that hasn't yet opened? How does an arc form?
I'd vote for the contact that opens last as the vulnerable one in the parallel set.

 

This "pilot duty" thing that you're talking about... I had to learn it the hard way while working with TTL signals... through countless headaches and endless trials and errors until I reached the same conclusion that you mentioned. An extremely thin layer of oxide was being formed on the contacts and was wrecking havoc with the signal. I later discovered that there are microswitches with magnetic-actuated reed switches in them. That solved my problem, since reed switches are normally encapsulated in glass and are therefore completely isolated from outside humidity.

I call to mind the mercury wetted relay and the related concept of intentional contact wetting by means of a residual current.

 

This "pilot duty" thing that you're talking about... I had to learn it the hard way An extremely thin layer of oxide was being formed.

That's why I babble on about this stuff. Noobs are watching.

 

In a worst case, if one relay fails to close then there will be double the current running in the other relay. Now instead of an open circuit due to a fail to close, you have a fire hazard due to over current in the one that did close. Not safe. Fuses would help with this problem, but you would still have the problems described above. Two relays will never reliably open or close at exactly the same time.

 

Note: All relays should be energized prior to connecting loads. No hot switching.

 

Note: All relays should be energized prior to connecting loads. No hot switching.

AKA Dry Contact.
Max.

 

Note: All relays should be energized prior to connecting loads. No hot switching.

I'm not sure how that can always be achieved without exception.
Relays & contactors often switch full load currents. The direct-on-line (DOL) contactor (essentially a relay) for induction motors is an extreme case in point, where several times the rated load current is switched through the contacts with the motor at standstill.

 

Note: All relays should be energized prior to connecting loads. No hot switching.

Completely disagree.
If I turn off the 240 VAC to the relay that feeds power to the 10KW electric furnace, what do I use to turn the power back on?
Walk to the breaker box and flip the breaker every time the thermostat starts or stops the furnace?

 

Usually the high duty contactors that repeatedly switch high currents such as Plasma cutters, HVAC, large motors for e.g. use what is known as Special Purpose Contactors, they have some extra features for high duty use.
Max.

 

Usually the high duty contactors that repeatedly switch high currents such as Plasma cutters, HVAC, large motors for e.g. use what is known as Special Purpose Contactors, they have some extra features for high duty use.
Max.

Very true - but I doubt the exceptions mentioned here prove the 'rule' postulated in post #9.

 

Thanks guys, great answers. Although I am a bit confused on the term "pilot duty". Anyways, I wasn't planning on doing this but the thought did occur and it seems my coworker had already thought of it and is practicing it and I just wanted to get some clarification. I am planning on controlling some heating elements so I'll just find a higher rated relay or run the elements separately on each contact

 

What is the power rating of your heating elements?

Have you considered using solid state relays?

 

I am planning on controlling some heating elements so I'll just find a higher rated relay or run the elements separately on each contact

Generally once you get into the power switching and out of the 'Control' realm, they are known as contactors, which is just a heavier version, you may want to look at the special purpose rated type as I mentioned.
The SSR just mentioned is a good idea also.
Max.

 

Completely disagree.
If I turn off the 240 VAC to the relay that feeds power to the 10KW electric furnace, what do I use to turn the power back on?
Walk to the breaker box and flip the breaker every time the thermostat starts or stops the furnace?

One switch or one breaker is fine if it is rated for the overall current. When using several mechanical switches in parallel they inherently won't be open or closed at the same time. Instantaneous current will be greater in some switches. I ran a cross this in a test stand a fellow test engineer was testing high current aircraft power supplies he turned on the power supply then with software flipped 5 switches in the test stand which connected the supply to the loads. The test stand was burning out switches . I had had him connect the load then turn on the supply. Never burned out a switch since.

 

Note: All relays should be energized prior to connecting loads. No hot switching.

If you are talking about insufficiently rated switches or relays in parallel, say so instead of making a blanket statement that can't possibly be implemented in the real world.

 

Hey guys,

I was talking with someone at work today about amp rating of relay contacts. In particular, relays with multiple contacts. What this person did was run a load off of two parallel relay contacts because the load was too much for one contact. His thinking is he could run twice the current rating between the two e.g. 12A across two 6A contacts (hypothetical). I told him I didn't think that would be right because of switching time between contacts. What other reasons would this not be a good practice?

Thanks

Question, what are you doing still working with electromechanical relays? Shouldn't it be far better if you were to substitute them for SSRs ??? I'm just sayin'