Handbook of Electrical Installation Practice - BeKnowledge

604 Handbook of Electrical Installation Practice
Fig. 22.5 Standard idmtl curve.
Numeric overcurrent and earth-fault relays generally have less error and provide
choice of characteristics. This is an additional setting which complicates the grading
procedure but is a great advantage when relays have to be provided in advance of
finalisation of the power system data.
Effect of impedance
The magnitude of the fault current is determined by the impedances of the power
system feeding the fault and in the simplest form can be represented by source
impedance (Zs) and the impedance between the two relays being considered (ZL),
as shown in Fig. 22.6. Assuming that Zs is much greater than ZL, the fault current at
each relay position is approximately the same, i.e. IF = V/Zs = V/(ZS + ZL). If, in
addition, the load current and CT ratio are the same for each relay position, discrimination
could be obtained by time multiplier setting only.
The consideration of load current is important because the plug setting of overcurrent
relays, which have a current setting range of typically 50–200% of rating,
determine the basic pick-up and reset levels of the relay. Thus for discrimination
with load conditions, the relay current setting must be related to the CT ratio and
the maximum possible load current to ensure that the relay resets when clearance
of a fault may leave the circuit with increased loading due to an outage.