65
64
Application Data
7
What Is Discrimination?
Discrimination, also called selectivity, is the co-ordination of protective devices such that a
fault is cleared by the protective device installed immediately upstream of the fault, and by
that device alone.
Total discrimination
Discrimination is said to be total if the downstream circuit breaker opens and the upstream
circuit breaker remains closed. This ensures maximum availability of the system.
Partial discrimination
Discrimination is partial if the above condition is not fulfiled up to the prospective short-
circuit current, but to a lesser value, termed the selectivity limit current (
I
s).
Above this value both circuit breakers could open, resulting in loss of selectivity
upstream
circuit
breaker
downstream
circuit
breaker
fault
current
X
X
How To Read The Discrimination Tables
Boxes containing the letter “T” indicate total discrimination between the relevant upstream and downstream circuit-breakers. Total
discrimination applies for all fault levels up to the breaking capacity of the upstream or the downstream circuit breaker, whichever is
the lesser.
For the other boxes, discrimination is either partial or there is no discrimination.
Worked Examples
Q (1) A main switchboard requires a 1600A ACB feeding a 400A MCCB.The fault level is 65kA. What combination of
protective devices would provide total discrimination?
A (1) A
TemPower2 ACB
AR216S feeding a
TemBreak2
S400GJ would provide total discrimination up to 65kA. See page 65
Note: Discrimination would be total whether the
TemPower2 ACB
had an integral or external protection relay because
I
cw
(1s) =
I
cs. Most other ACBs have
I
cw(1s) <
I
cs
.
Discrimination Table
Upsream:
TemPower2 ACB
with or without Integral Protection Relay
Downstream:
TemBreak2 MCCB
