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51 Device Coordination

51 Device Coordination

51 Device Coordination

(OP)
So as I understand, the recommended time deference in coordinating electronic relays in series is 0.3 Seconds. I am wondering at what point this time difference should be measure on the TCCs since the delta increases with time due to the log scale. Should it be measure at a particular time?

RE: 51 Device Coordination

When is the timing difference most critical?
Wouldn't it be when fault current is causing the relays to operate quickly?

If that is true, then find the highest short circuit current that is seen by both relays and adjust the settings to achieve the required timing margin at that current.

At all lower currents, the margin between the two trip curves should be greater.

RE: 51 Device Coordination

You need to consider the point where the two curves have the smallest time separation. Don't forget to consider the interval between a downstream instantaneous trip and upstream inverse time.

RE: 51 Device Coordination

...and make sure you consider the actual operating time of the device itself.

If you have a 3-cycle breaker or recloser, that's 50 milliseconds, AFTER the relay decides to trip, that you need to add in.

I have seen that overlooked on many occasions.

RE: 51 Device Coordination

In the era of numerical relays the discrimination time of 0.3s is little too much. I normally provide 0.15s to 0.2s only.

RE: 51 Device Coordination

DT or IDMT?
for DT is usually used from 0.18 up to 0.3sec.
depend on the terminal type and breaker.

For IDMT calculation is another.

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