Which short-circuit current to use when checking coordination time intervals (CTI)
Which short-circuit current to use when checking coordination time intervals (CTI)
(OP)
When performing a coordination study on a MV system, we can use many types of short-circuit currents in the TCC curves.
If we use the ANSI C37 standard for calculation, we have many types of short-circuits to show in the TCC. Some of these are:
1. Momentary: Calculated using the first-cycle network
1.1 Asymmetrical (Total) RMS.
1.2 Asymmetrical (Total) Peak.
2. Interrupting: Calculated using the interrupting network
2.1 Symmetrical RMS 2 Cycle.
2.2 Symmetrical RMS 3 Cycle.
2.3 Symmetrical RMS 5 Cycle.
2.4 Symmetrical RMS 8 Cycle.
2.1 Asymmetrical (Total) RMS 2 Cycle.
2.2 Asymmetrical (Total) RMS 3 Cycle.
2.3 Asymmetrical (Total) RMS 5 Cycle.
2.2 Asymmetrical (Total) RMS 8 Cycle.
In the same TCC, you could have several relay-CB combinations that have different types of momentary and interrupting ratings depending on the date of manufacturing.
So two questions arise:
1. Which one of all these short-circuits should be shown in the TCC for coordination purposes?
2. Also, these short-circuit currents will limit the display of the relay curve on the right side of the curve. The coordination time interval (CTI) might be greater or smaller depending on the current we choose (if the relay curves separate or get further apart as current increases).
If we use the ANSI C37 standard for calculation, we have many types of short-circuits to show in the TCC. Some of these are:
1. Momentary: Calculated using the first-cycle network
1.1 Asymmetrical (Total) RMS.
1.2 Asymmetrical (Total) Peak.
2. Interrupting: Calculated using the interrupting network
2.1 Symmetrical RMS 2 Cycle.
2.2 Symmetrical RMS 3 Cycle.
2.3 Symmetrical RMS 5 Cycle.
2.4 Symmetrical RMS 8 Cycle.
2.1 Asymmetrical (Total) RMS 2 Cycle.
2.2 Asymmetrical (Total) RMS 3 Cycle.
2.3 Asymmetrical (Total) RMS 5 Cycle.
2.2 Asymmetrical (Total) RMS 8 Cycle.
In the same TCC, you could have several relay-CB combinations that have different types of momentary and interrupting ratings depending on the date of manufacturing.
So two questions arise:
1. Which one of all these short-circuits should be shown in the TCC for coordination purposes?
2. Also, these short-circuit currents will limit the display of the relay curve on the right side of the curve. The coordination time interval (CTI) might be greater or smaller depending on the current we choose (if the relay curves separate or get further apart as current increases).






RE: Which short-circuit current to use when checking coordination time intervals (CTI)
There is not really a simple answer to your question. If in doubt, use the momentary 1/2 cycle asymmetrical current.
RE: Which short-circuit current to use when checking coordination time intervals (CTI)
But if you have two relay TCC's growing apart from each other as current increases, the CTI using this larger momentary asymmetrical duties will be larger than the CTI obtained with the interrupting duties calculated at the appropriate cycles(depending of the CB).
So again, which is the proper CTI, the higher one using the larger momentary duties or the smaller CTI using the interrupting duties?
Example, assume that using the higher momentary duty, the CTI is 0.4 and with the lower interrupting duty, the CTI is 0.15. This is because the curves are not parallel and diverge as current increases. One satisfies the suggested IEEE Buff book values and the other doesn't.
I believe we just can't pick the duty that gives as the CTI that meets the IEEE suggestions and neglect the other.
Is there a suggestions from a reputable source that suggests choosing the worst case CTI?
RE: Which short-circuit current to use when checking coordination time intervals (CTI)