Chapter A General rules of electrical installation design

N - Characteristics of particular sources and loads
5 Asynchronous motors
Example of application:
Motors driving pumps for “sprinklers” fire-protection systems or irrigation pumps for
seasonal operation.
A Vigilohm SN21 (Merlin Gerin) monitors the insulation of a motor, and signals
audibly and visually any abnormal reduction of the insulation resistance level.
Furthermore, this relay can prevent any attempt to start the motor, if necessary
(see Fig. N69).
Schneider Electric - Electrical installation guide 2008
SM21
MERLIN GERIN
Fig. N69 : Preventive protection of stationary motors
Fig. N70 : Example using relay RH99M
RH99M
SM20
M E R L IN G E R IN
IN OUT
Limitative protections
Residual current diffential protective devices (RCDs) can be very sensitive and
detect low values of leakage current which occur when the insulation to earth of an
installation deteriorates (by physical damage, contamination, excessive humidity,
and so on). Some versions of RCDs, with dry contacts, specially designed for such
applications, provide the following:
b To avoid the destruction of a motor (by perforation and short-circuiting of the
laminations of the stator) caused by an eventual arcing fault to earth. This protection
can detect incipient fault conditions by operating at leakage currents in the range of
300 mA to 30 A, according to the size of the motor (approx sensitivity: 5% In)
b To reduce the risk of fire: sensitivity y 500 mA
For example, RH99M relay (Merlin Gerin) provides (see Fig. N70):
b 5 sensitivities (0.3; 1; 3; 10; 30 A)
b Possibility of discrimination or to take account of particular operation by virtue of 3
possible time delays (0, 90, 250 ms)
b Automatic breaking if the circuit from the current transformer to the relay is broken
b Protection against unwanted trippings
b Protection against DC leakage currents (type A RCD)
N51
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