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APPENDIX F
F.4 INTERNATIONAL STANDARD
F.4INTERNATIONAL STANDARD
Standard Reference: IEC 255-3 2nd Edition 1989-05
There is a General Section (page 7) in which we have several definitions. The main concepts for us are: the setting value of
the characteristic quantity (Gs) and the threshold value of the characteristic quantity (Gt).
According to this definition (and with Current as the characteristic quantity), Gs will be the setting value of the pickup and
Gt the lowest value for the relay to trip.
The two criteria that may apply for the trip, according to the IEC Standard, are:
1)Gs ≤ Gt ≤ 1.3*Gs
Therefore, there is a 30% tolerance.
In GE MIFII Relays, this is the followed criterion, with a 5% tolerance.
2)Gt = Gs
Not really recommended, in order to avoid undesired trips.
In relation to the timing accuracy for dependent time relays, Section 4.3.2 (page 13), the reference limiting error is defined
by an error (declared by the manufacturer), which may be multiplied by coefficients for different values within the effective
range of the characteristic quantity.
For relays with decreasing time function, this range lies between 2 and 20 times Gs.
F
This reference limiting error may be defined as:
a)A theoretical curve:
GEK-106237P
MIFII Digital Feeder Relay
F-5
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