Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 144)

indicate the maximum size of short-circuit current that can be handled. Standard values under
IEC 60898 are 1’500, 3’000, 4’500, 6’000, 10’000, 20’000 and 25’000 A.
When selecting a MCB to protect cables and conductors, the permissible let-through-I
2
·t values
for conductors must be respected. They may not be exceeded during clearing a short-circuit.
Therefore the I
2 ·t values in relation to the prospective short-circuit current are important
characteristic of MCB’s.
In some countries, miniature circuit breakers are classified according to the permissible I
2
·t
values. According to the “Technical Connection Conditions” (TAB) of the German power utilities
(EVU) for example only MCB’s with a rated switching capacity of at least 6’000 A and the
energy limitation Class 3 may be used for selectivity reasons in distribution boards of domestic
and utility buildings behind the meter. For industrial applications a switching capacity of
10’000 A is usually required.
4.2.3.3 Installation of Miniature Circuit Breakers, safety clearances
See also Section 4.2.2.5 .
MCB’s as components of installation systems are usually designed so that compliance with
safety clearance requirements is assured when arranged conform to the system structure.
4.2.4 Motor protection relays (overload relays)
Overload relays are used to protect electrical equipment, such as 3-phase AC motors and
transformers, against excessive temperature rise and measure the current to determine the
temperature-rise and danger to the object to be protected. Protective shutdown is performed via
the motor switchgear – usually a contactor.
4.2.4.1 Thermal motor protection relays
Principle of operation
Thermal motor protection relays contain three bimetal strips together with a trip mechanism in a
housing made of insulating material. The bimetal strips are heated by the motor current, causing
them to bend and activating the trip mechanism after a certain travel which depends on the
current-setting of the relay. The release mechanism actuates an auxiliary switch that breaks the
coil circuit of the motor contactor (
Fig. 4.2-10 ). A switching position indicator signals the
condition “tripped”.
B
A
E
D
C
NO NC
Fig. 4.2-10
Principle of operation of a three pole thermally delayed bimetal motor protection relay with temperature
compensation
A = Indirectly heated bimetal strips
B = Trip slide
C = Trip lever
D = Contact lever
E = Compensation bimetal strip
The bimetal strips may be heated directly or indirectly. In the first case, the current flows directly
through the bimetal, in the second through an insulated heating winding around the strip. The
insulation causes some delay of the heat-flow so that the inertia of indirectly heated thermal
relays is greater at higher currents than with their directly heated counterparts. Often both
principles are combined. For motor rated currents over approx. 100 A, the motor current is
conducted via current transformers. The thermal overload relay is then heated by the secondary
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