Protection discrimination times (grading margin) for LV circuit breakers

[electrical429]

When doing protection grading studies for HV and EHV networks usual grading margins are 200-350ms depending on relay types (fast numerical vs slow electromechanical) and circuit breaker types (fast new vacuum vs slow old oil). When doing protection grading on LV networks I believe the rule of thumb is that grading margin can be 100ms as there are more devices to be graded and you don't need to take into account Current Transformer errors, relay overshoot, relays errors and circuit breaker operating times when grading just fuses. What about when you have MCCBs or ACBs on LV networks. Do they trip faster than HV breakers? Can you apply 100ms grading margin between LV ACB and MCCB, or do you need to consider ~300ms same as on high voltage network?

 

[dpc]

At least in North America - the published time-current curves for low-voltage devices include BOTH the trip unit and the breaker so should include all error bands, tolerances AND breaker opening/clearing times (unlike protective relays). So in general, devices should coordinate as long as the curves do not intersect or overlap. There should be no need for additional time to be added. We tell engineers that as long as they can "see daylight" between LV curves, they should coordinate. LV fuses are a little more complicated due to pre-loading and other factors - but generally not enough to worry about. That said, many people are more comfortable with some additional separation between curves, but certainly nothing like those intervals used for HV relays and breakers.

Cheers,

Dave