Electrical – Some questions about a relay life


This electromagnetic relay will be used/wired in normally-open(NO) mode. It will be ON most of the time by a continuous DC control signal and will be open/OFF only when a fault happens(the control signal will go off and the relay will open and stop AC current).

In the data-sheet there is both mechanical and electrical life as below:

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What is the difference between mechanical and electrical life?
Most importantly in my application the relay will be in closed position i.e it will be passing a motor driver's current most of the time. Can this relay withstand continuous rated current for half a day for instance? Electrical life has anything to do with it?

Best Answer

Mechanical life is the longevity of the moving stuff, pivots, hinges, and contacts when there's no current through the contacts. This is simply mechanical friction and wear.

Electrical life is the lifetime of the contacts when opening carrying the full rated current. Every time the contacts open under load, there's a spark which can erode the contact surfaces. This is why the electrical life is so much shorter than the mechanical life.

The coil can be powered-on coil indefinitely. There's no identifiable mechanism like wear or arcing that will limit the lifetime simply due to the coil being powered. It does raise the temperature, which can accelerate other faults. However the temperature rise should be modest if using the relay with its rated drive.

The rated current can be passed through closed contacts indefinitely.

If you stick to the rated coil drive and the rated contact current, you should reasonably expect your relay to be able to open 10^5 times. It would of course be prudent to operate at less than the rated spec, there is then more likelyhood of exceeding the rated lifetime.

Make sure you are sticking to the rated contact conditions. A motor is inductive, and causes a larger and longer duration, that is more damaging, arc when it's opened. Some relays have different lifetime specifications for inductive and resistive loads. In your case, the rated single phase motor load is 550W, less than 2.5A.