Compléments techniques
Complementary technical
information
Discrimination
between protective devices
IAL
NoT
E
ly
D
n
I
F nal use
CON
inter
In a low voltage electrical installation, discrimination between protective
devices is a basic element that must be taken into account right from the
design phase to ensure the highest possible availability of electrical power
to users.
D1
D2
Installations without discrimination run a series of risks of varying
importance:
production deadline overruns;
interruptions in manufacturing entailing:
drop in production or finished product losses,
risk of damage to production machines in continuous processes,
restarting of machines, one by one, following a general power outage;
shutdown of vital safety equipment such as lubrication pumps, smoke
extractors, etc.
What is discrimination?
Discrimination, also known as selectivity, is the coordination of automatic
protective devices in such a manner that a fault appearing at a given point
in an installation is cleared by the protective device installed immediately
upstream of the fault, and by that device alone.
Discrimination is said to be total if the circuit breaker immediately upstream
of the fault (D1) opens and the circuit breaker further upstream (D1)
remains closed for all fault currents, from overloads on up to a bolted shortcircuit.
Discrimination is said to be partial if the above condition is not satisfied up
to the full short-circuit current, but only up to a lower value referred to as
the discrimination limit current.
Natural discrimination
with Compact circuit breakers
Thanks to the Roto-Active breaking function of Compact NS devices,
Merlin Gerin circuit breakers offer an exceptional degree of discrimination
between protective devices.
This performance is achieved by combining and optimising three principles:
current discrimination,
time discrimination,
energy discrimination.
Blokset • chapitre 6
page 13
Compléments techniques
Complementary technical
information
Discrimination
between protective devices
IAL
NoT
E
ly
D
n
I
F nal use
CON
inter
Protection against overloads: current
discrimination
Discrimination is ensured between two distribution circuit breakers if the
ratio of their current settings is greater than 1.5.
Protection against low short-circuits:
time discrimination
Tripping of the upstream device is slightly delayed to give the faster
downstream circuit breaker sufficient time to clear the fault by itself.
Discrimination is ensured between the two circuit breakers if the ratio of
their short-circuit protection settings is greater than 1.5.
Protection against high short-circuits:
energy discrimination
This system combines the exceptional current-limiting capability of
Compact NS devices with the advantages of reflex tripping, a technique
that is sensitive to the energy dissipated in the device by the short-circuit.
In the event of a high short-circuit detected by two devices, the
downstream device greatly limits the fault current. The energy dissipated in
the upstream device is not sufficient to cause it to trip: discrimination is
achieved regardless of the value of the short-circuit.
Discrimination is ensured if the ratio between the circuit breaker ratings is
greater than 2.
NS100 NS250
100A 250A
10000
1000
100
10
t (s)
1
.1
.01
.001
.5
1
10
100
300
x 100A
Blokset • chapitre 6
page 14