Handbook of Electrical Engineering

330 HANDBOOK OF ELECTRICAL ENGINEERING

Very and extremely inverse relays are used in systems where the fault level downstream is low
when compared with that available at the point of main supply, e.g. a main generator switchboard.
They are also used where coordination with upstream or downstream fuses is necessary.

Extremely inverse relays have an inverse square law characteristic, which predominates at
high fault currents. It therefore closely matches theI^2 tcharacteristics of cables, motors, transformers,
NERs etc.

12.4.1.1 Comparison of inverse time curves

Before electronic relays were developed the standard inverse characteristic was taken as the reference
e.g. in BS142 for UK practice. A point on the characteristic was chosen for the comparison with
others, e.g. extremely inverse. The reference point was 10 times the nominal relay current and an
operating time of 3 seconds.

Most literature for modern relays, and the IEC60255, do not compare the characteristics in
this manner. Instead they use a standardised formula for each relay,

Inverse time

t=

0. 14

(

I

In

) 0. 02

− 1

seconds

Very inverse

t=

13. 5

(

I

In

) 1. 0

− 1

seconds

Extremely inverse

t=

80

(

I

In

) 2

− 1

seconds

Where the numerator is a constant that falls within the range of the time multiplier of the relay.
If the numerator is ‘temporarily’ modified then the characteristics can be compared in a similar manner
to the older method of BS142. A good pictorial comparison can be made by choosing the common
point to be at 5 times nominal current and 5 seconds operating time. The modified numerators are
0.1636, 20.0 and 120.0 respectively. The three characteristics are shown in Figure 12.12. It is feasible
with modern electronic relays to use any value for the exponent in the denominator. Figure 12.13
shows a family of curves in which intermediate values of the exponent are included, i.e. 0.5, 1.5
and 3.0.

12.4.2 High-set or instantaneous current

12.4.2.1 Basic considerations

The use of high-set or instantaneous current protection for the primary winding, or the secondary
winding, is determined by several factors, which differ for either winding. Consider the primary