To break current, an arc is created - when the arc is long enough, the resistance of the arc is great enough to reduce the current, cool the arc and extinguish the arc. With two contacts in series, the arc is twice as long. If the concern is the "make" current, the contacts need to be in parallel.
I saw an expensive failure due to interrupting ratings. An engineer was queried as to his selection of a 250 HP rated contactor for a 400 HP motor. His reply was not polite. He worked his engineering doctorate into the harangue. He did mention that they had done extensive testing on the starter and found it able to carry the rated current of a 400 HP motor without overheating. What they failed to test was the ability of the contactor to interrupt the current of an overloaded 400 HP motor. When the contactor failed it "crow-barred" the motor which resulted in a bent and damaged motor shaft, a bent fan shaft and pieces of exploded coupling in every part of the fan room.
Your answer, the current rating is based Not on what the contacts can safely carry but on the current that the contacts can safely interrupt.
Bill
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"Why not the best?"
Jimmy Carter
Are those special relays? I wouldn't think you could rely, in general, on two contacts opening up at the same time and having the same "gap vs. time" relationship.
They're for interlocking a clutch control circuit (relay logic) of a mechanical power press to a robot. The contacts are switching solenoids and other control relays, not motor loads. I contacted the mfg of the relay and they said the same thing as dpc.
Please forgive me for I am new to the electrical industry...
The NEMA ratings for the relay contacts show "Thermal Continuous Test Current" and then there is make / break current. The break current is much lower than the thermal. Can someone explain the thermal?
The thermal current is the long term rating of the relay, the current it is able to carry indefinitely. It is determined by the temperature rise of the contacts and connections, and it is a function of ambient temperature, i.e. if you use the relay in a hotter environment than the one at which it was rated then it can carry less current.
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If we learn from our mistakes I'm getting a great education!
That's what I would assume too - but I keep getting surprised with standards and ratings from other parts of the world so I guess I'm getting cautious in my old age.
