346 HANDBOOK OF ELECTRICAL ENGINEERING
12.9.1 Time-delayed overcurrent
A time-delayed overcurrent (51) relay would normally be used for a static load. The choice of the
characteristic would depend to some extent on the nature of the load. A standard inverse characteristic
would normally be adequate. Its pick-up current would be set at between 105% and 115% of the
rated current of the load.
12.9.2 Instantaneous or high-set overcurrent
Instantaneous overcurrent protection would detect short circuits in the load and along its feeder cable.
It would usually be practical to set the instantaneous elements of a moulded case circuit breaker to
their lowest value e.g. five times the nominal current. If the protection is provided by a set of fuses
then the fusing factor would be marginally above unity, the nearest fuse rating above the load current
would be chosen. The protection must fully cover theI^2 tcapacity of the feeder cable.
12.9.3 Core balance earth fault
The theoretical requirements for applying core balance earth fault protection are the same as those
for low voltage motors. Some additional requirements often apply.
The requirement for a sensitivity of 30 mA should generally apply to final sub-circuits; see
BS7671: 1992 Sections 412, 413 and 471 for further guidance.
In some situations the sensitivity may need to be reduced and a higher tripping current used
e.g. 100 mA or 300 mA. Fluorescent lighting systems and welding socket feeders are subject to
a poor quality of current waveform due to non-linear characteristics of their loads. The distortion
superimposed on the fundamental current may be sufficient to cause spurious tripping of a fast-acting
30 mA relay.
Inverse Relays 12.10 Mathematical Equations for Representing Standard, Very and Extremely
VERY AND EXTREMELY INVERSE RELAYS
Since 1976 many relays have generally followed the recommendations of the IEC255-4, Clause 3.5.2,
regarding the shape of their time–current curves. The general function recommended has the form:-
t=k
(
Ia
In)
−useconds ( 12. 1 )Where t=theoretical operating time, seconds.
I=relay current in pu or amps.
In=nominal current in pu or amps.
a=exponential constant.
k=constant for the particular relay.
u=constant for a particular relay determined from the time asymptote in the region of the
rated currentIn.It usually has the value close to 1.0, in the range of 0.95 to 1.3. For