IEEE Over Current Relaying Standard

[stretched2thin]

Can anyone tell me which IEEE Standard gives guidelines on overcurrent relaying? I'm thinking it is C37.90-1989 (R1994), but I'm not sure.

I'm looking for the standard minimum coordination time buffer between time/current curves. I"m thinking it is 0.3 seconds, but I'm not sure.

 

[gordonl]

IEEE Std. 242 (Buff Book) gives a suggested grading margin of .3 seconds (Sect. 14.2.2) but suggests tight systems could are sometimes decreased to .25 or increased upto .4 seconds for mechanical relaying systems.

 

[busbar]

Gordon has nailed it. IEEE Std 141-1993 §5.7 is another reference.

[C37.90 seems to deal more with surge-withstand capability in relays.]

 

[jbartos]

Suggestion: Reference:
1. IEEE Std 242-2001 Buff Book
states in Chapter 15 on page 602:
When coordinating overcurrent relays with one another or with any other device, considering the location of the relays with respect to other devices is necessary. The CTI desired between the characteristic curves is usually 0.3 s to 0.4 s when coordinating induction disk overcurrent relays in series with one another. This interval is measured between the relay characteristic curves at the lesser value of either the instantaneous pickup setting of the downstream device or the maximum fault-current level that can be experienced simultaneously by both relays (see Figure 15-12).

 

[jbartos]

Clarification: CTI is Coordination Time Interval