What is the practice for the use of arc detection relay trip time in the arc incident energy calculations? NFPA-70E recognized arc detection relay but kept under "Other Methods" as opposed to "Incident Energy Reduction Method". Does this means the arc flash relay tripping time should not be used for the arc incident energy calculations?

Regards,

It really isn't all that different from other relays. Typically you would take the relay operate time and add in any other time such as breaker operate time, LOR time etc. Some people will also add in additional time for conservatism. This will all add up to the arcing time.

Any reason why NFPA kept arc flash relay in different category? The network can operate without arc flash relay (e.g. when one of the sensor out of service) but not with the standard protections. Is this the reason for keeping arc flash relay in different category?

Arc flash relays are only different in the sense that it uses a different measurement technique and trigger mechanism. A differential or a distance relay for instance is very different from a simple 50/51 relay. If you read the literature on them at first they appear to be some kind of totally new and unique device but in the end they work roughly the same way as a differential (87), ground fault, or instantaneous overcurrent relay in that the response time is not on a curve (inverse time)...if the input conditions are met, the relay triggers the output. The only intentional "delay" that you may encounter is that many of them look for both a light input and a current slope (di/dt) trigger and the second trigger is a bit slower to detect, just as instantaneous overcurrent detection requires a minimum of about 1/4 cycle and more often uses more than that for detection.

For these reasons SKM as an example simply allows you to assign a 1 cycle relay operation time for the purposes of performing arc flash calculations then adds on time for the circuit breaker. If you have a lockout relay or similar delays built into your relaying chain you'd have to add these as well.

This may sound trivial but in terms of the thought process when designing relaying, all of these types of "instantaneous" relays can be somewhat tricky to understand. The place that they end up on the TCC (time-current curve) is a horizontal line...there is a specific time that they trigger at. You can still achieve coordination but you do so by inserting intentional delays into the relay operation time. This is fundamentally simpler to understand than inverse time curves but because there is such a heavy emphasis on inverse time curves in relaying, it actually makes understanding a much simpler case harder.