3.8.6 - Discrimination
Most installations include a number of
protective devices in series, and they must operate correctly
relative to each other if healthy circuits are not to be
disconnect ed. Discrimination occurs when the protective
device nearest to the fault operates, leaving all other
circuits working normally.
Fig 3.26 System layout to explain discrimination
{Figure 3.26} shows an installation with
a 100 A main fuse and a 30 A submain fuse feeding a distribution
board containing 10 A fuses. If a fault occurs at point
Z, the 100 A fuse will operate and the whole installation
will be disconnected. If the fault is at X, the 10 A fuse
should operate and not the 30 A or 100 A fuses. A fault
at Y should operate the 30 A, and not the 100 A fuse. If
this happens, the system has discriminated properly.
Lack of discrimination would occur if a
fault at X caused operation of the 30 A or 100 A fuses,
but not the 10 A fuse. This sounds impossible until we remember
the time/current fuse characteristics explained in
{3.6.3}.
For example, {Fig 3.27} shows the superimposed characteristics
of a 5 A semi-enclosed fuse and a 10 A miniature circuit
breaker which we shall assume are connected in series.
If a fault current of 50 A flows, the fuse
will operate in 0.56 s whilst the circuit breaker would
take 24 s to open. Clearly the fuse will operate first and
the devices have discriminated. However, if the fault current
is 180 A, the circuit breaker will open in 0.016 s, well
before the fuse would operate, which would take 0.12 s.
In this case, there has been no discrimination.
To ensure discrimination is a very complicated
matter, particularly where an installation includes a mixture
of types of fuse, or of fuses and circuit breakers. Manufacturers'
operating characteristics must be studied to ensure discrimination.
As a rule of thumb where fuses or circuit breakers all of
the same type are used, there should be a doubling of the
rating as each step towards the supply is taken.
When fault current is high enough to result
in operation of the protective device within 40 ms (two
cycles of a 50 Hz supply), the simple consideration of characteristics
as shown in Fig. 3.27 may not always result in correct discrimination
and device manufacturers should be consulted.
When RCDs are connected in series, discrimination
between them is also important, the rule here being that
a trebling in rating applies with each step towards the
supply (see
also {8.6.3}).
Fig 3.27 To illustrate
a lack of discrimination
