Relay Failure Possibilities

[Lhottle]

TL;DR Summary

Relay failure (thermal event on common terminal) within specified manufacturer current on NO/NC contacts.

Trying to troubleshoot a field failure on a purely resistive load on a 30A rated relay utilizing only a 24A load (cycled off/on for 45 min, 1x/day). Our wiring/load has been verified but this failure has been occurring sporadically but impact has been large. My question is, could a tin-plated brass female connector be a potential failure cause on a nickel-plated relay contact tab (humidity and corrosion I do not believe to be an issue)? I know a snug fit is essential but I'm curious if the opposing metal interaction could be a potential cause as well...any thoughts on this matter would be greatly appreciated. Thank you in advance!

 

[hutchphd]

The failure is in the relay contact or an external terminal contact (pardon my lack of knowledge here)?. And the relay is NO and closed for 45 minutes once a day? What is the resistive load (inrush currents on light bulbs are nontrivial)...?

 

[Lhottle]

There is a 16A load on the NO contact and 8A on the NC contact (opposing operation so not 24A passing at the same time on a 30A rated relay). It was originally built with an "automotive" female terminal at the connection point (I should mention it's an open tab relay utilizing crimped connectors). Just curious about the conflicting metals (nickel-brass vs tinned-brass) and if that could've been a factor as well.

 

[Averagesupernova]

If the conflicting metals are the issue is it possible to set up a test of some sort without cycling the relay? This would eliminate inrush. Some of these types of relays even have multiple terminals on the same contact so you could eliminate current through the contact if your relay is built this way.

 

[Tom.G]

Any chance you can post a photo of the connections, both good and failed?
Also, what is the environment, your living room, a chemical plant, oil refinery, marine, ?

So far it sounds like either a poor mechanical contact or a chemical reaction from the environment.

Bad crimp connections are quite common, as are a mismatch in thickness of the male and female spade (slip-on) connectors. A thickness mismatch can occur when the mating terminals are from different manufacturers, one may be built to English units and the other to Metric standards. (That was so unexpected that it took several days to track that one down!)

Note that Tin plating does not do well under vibration. Being a soft metal, it will wear away to the base metal which can then oxidize/corrode. Nickel plating is better suited for high vibration and/or high temperature operation.

Google found over 4 000 000 hits with:
https://www.google.com/search?&q=electrical+reliability+of+tin+and+nickel

Much information in this one, from a connector manufacturer:
https://www.ramoem.com/uploads/4/4/0/7/44075859/tin_commandments.pdf

Cheers,
Tom

 

[DaveE]

Summary:: Relay failure (thermal event on common terminal) within specified manufacturer current on NO/NC contacts.

My question is, could a tin-plated brass female connector be a potential failure cause on a nickel-plated relay contact tab

Very unlikely. This external connection can be inspected in failure analysis. So don't ask us, look at it.

1) Relays wear out. You didn't tell us how old it was. OTOH, at one cycle per day, it should last a long time.
2) DC load or AC? Since you didn't say, we'll assume AC. There is a reason the DC ratings are much lower than AC.
3) You also didn't tell us if you have any transient protection at/near the relay. Things like RC snubbers or transient suppressor diodes.
4) Temperature is a big factor in lifetime. Those current ratings assume some set of environmental conditions, verify that you meet the whole specification, not just the number stenciled on the case. It is the relay temperature that matters, not the temperature of the room the equipment is in.
5) As others have said, surge currents and inductive loads during switching are very stressful. Look at the real current and voltage, especially when contacts are opening.
6) Get a detailed datasheet for it and read every word, including the footnotes.

 

[Frodo]

Please upload the relevant parts of the circuit diagram so sensible suggestions can be made.

If it is a DC circuit then you are almost certainly overloading the contacts. DC does not quickly (within 10ms for 50Hz) extinguish any arc by going to zero as AC does, so much more damage is done to the contact.

A given contact is rated much lower for DC than AC.

Are you using an arc suppressor?

 

[Henryk]

I had some relays failtures. It turned out that solder joints were cracked. Check all the joints, specially the high currrent ones unde a good magnifying glass.

 

[Averagesupernova]

Some of the posters here are assuming the failure is burning contacts but as I read it this is not the case. The failure is at the connection where the spade terminal on the relay connects to female spade connector on the wire.

 

[DaveE]

Some of the posters here are assuming the failure is burning contacts but as I read it this is not the case. The failure is at the connection where the spade terminal on the relay connects to female spade connector on the wire.

Yes, that sounds right. I missed it the first time. " thermal event on common terminal "

Summary:: Relay failure (thermal event on common terminal) within specified manufacturer current on NO/NC contacts.

My question is, could a tin-plated brass female connector be a potential failure cause on a nickel-plated relay contact tab (humidity and corrosion I do not believe to be an issue)? I know a snug fit is essential but I'm curious if the opposing metal interaction could be a potential cause as well...any thoughts on this matter would be greatly appreciated. Thank you in advance!

I still don't think those platings are the real problem. I think you just need to do some thermal testing. Of course any platings will suffer if they are overheated.

30A is a lot for spade terminals, but the big ones can do that if you've done everything right. The relay manufacturer wouldn't be able to keep those parts in production, or (probably) get safety agency approvals, if the terminals couldn't handle the rated current when properly installed.

Check your BOM/datasheets to see if they are truly designed to mate well. This is why good (i.e. big) equipment manufacturers have component engineers and qualified suppliers. If your manufacturing people are using whatever thing they can buy that seems to fit, that might be a problem.

As you said tight fit is essential as well as large contact area and lack of motion when installed. Ideally the contact points are tight and stable enough to keep oxygen out. You also should verify that they are crimped properly.

The terminals are cooled by heat conduction along the wires and into the relay. Verify that you wiring isn't too small and/or too hot; this can be an issue with bundled high current wires. The ampacity tables are pretty rough guides for equipment wiring since the applications are so variable. They were really made for electricians, that have very predictable installations. Even so a good ampacity table is pretty complex with lots of footnotes and different circumstances. Testing the actual worst case temperatures is the way to go. You need to test at worst case (everything: ambient temp, airflow, load, even altitude can be an issue), since heating is pretty non-linear in the real world.