Contact rating questions and verification methods

[Rusty Shackleford]

My company performs 3rd party testing to verify equipment performance. We typically do not know the end use of the equipment so we test all features. In the case of electrical contacts (I'm dealing specifically with a NEMA size 1 contactor auxiliary contact blocks), manufacturers publish the make, break and continuous ratings for the contacts. I have the following questions:

1) Wouldn't a system designer only need to look at the break rating when evaluating components since this is the lowest rating? What is the purpose of publishing the continuous rating? Are there scenarios where the contact would be subjected to a higher continuous current that the contact would not break?

2) To verify the 'current capacity' of the auxiliary contact, my company typically performs a temperature rise test at the continuous current rating. Going back to my first question, if the contact was selected such that it would never see continuous current higher than the break rating, is the temperature rise test really appropriate? It is my belief that break testing would be appropriate, performed per NEMA ICS 5.

3) I have no problem performing a temperature rise test as well, but for financial reasons would prefer not to if it is unnecessary.

Please keep answers simple as I'm a mechanical engineer...Thanks in advance!

 

[itsmoked]

1) Continuous is ohmic heating over time possibly overheating the structure or taking the spring temper out of the contact system.

2) Everything associated with motors tends to have large instantaneous currents during boundary conditions of closing or opening. A contact could easily be designed to take larger momentary currents than continuous ones. Almost everything associated with motors lives in this dynamic realm.

3) I'm pretty sure you need to keep with the temp rise test as it's all about temperature and how high it rises during an inrush or current-break event which can weld contacts and the continuous state as that is when I[sup]2[/sup]R heating occurs.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

All true, but the process of "rating" the contact design is somewhat simpler.

If a load is purely resistive, then the Make, Break and Continuous ratings are all identical and referred to ar the "Resistive" or "Thermal" rating. So for example a contact with a NEMA A600 rating (as most starter aux contacts will have) dictates that the 120VAC "Thermal" current is 10A, but if the load is inductive in any way, the Make rating is 60A and the Break Rating is 6A. In a control circuit, the action of the devices in the circuit might include solenoid coils, which will be inductive, ergo the "Break" rating comes into play and the maximum coil current rating it can handle becomes 6A. if you look at an AC coil, you will see an Inrush and Holding power rating, in VA, and they are vastly different. For example on a Size 4 starter coil, Inrush is 1490VA, sealed is 96VA. So the "Make" current for that coil is 12.42A, which exceeds the 10A thermal rating, but is of short duration. So the "Make" rating of 60A on the contacts more than covers that. The "Sealed" coil rating of 96VA however is only 0.8A, so 10A continuous is fine, as is the 6A "Break" rating.

If you are designing a circuit in a way that the starter Aux. contact is in there to ISOLATE, but will never actually SWITCH a coil circuit off, then you may not need to consider the Break rating limitation of 6A, you can use the continuous rating of 10A. As an example, one of the Aux contacts may be involved in the "Hold In" circuit for another coil, in which case it MIGHT be called upon to break the coil circuit, so it must be considered to be useful for only 6A max. But if there is ANOTHER device in the circuit, let's say a power relay, and this starter Aux Contact is NEVER going to be the contact that actually interrupts current to the coil because the power relay always does that first, then the Aux contact could be used at the full 10A.

Then if another Aux contact on that same starter may be driving 8 pilot lights in various panels and control stations, each one taking 1A. those are all resistive loads, so 10A is fine there too.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[davidbeach]

Protective relays typically have output contacts that can make the circuit breaker trip coil circuit, including its inrush, and can hold the "steady state" current of the trip coil (it will burn up in fairly short order at steady state) but are woefully inadequate in the break rating. Seems like a problem. But no, there's a really simple solution; every circuit breaker trip circuit includes an 'a' contact in the circuit. As soon as the breaker opens, the 'a' contact opens and then the relay output contact opens with zero current to interrupt. As long as it has a make of 30A, a hold of 6A, that contact can have a break of essentially 0. You certainly couldn't evaluate it simply on a break rating of nearly zero.