Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 66)

If both short-circuits occur on the load-side of the circuit breaker, the breaking work is shared
between two contacts and the required breaking capacity corresponds to the normal 3-phase
values.
If the location of one short-circuit is on the supply-side of the circuit breaker and the second
short-circuit on the load-side, then one contact only of the circuit breaker has to perform the total
breaking work and this at phase-to-phase voltage. In this case, the significantly lower single
pole breaking capacity of the circuit breaker at the phase-to-phase voltage is critical. If the
values cannot be obtained from the device documents, an inquiry should be made. If the short-
circuit current at the installation site exceeds the single pole switching capacity of the circuit
breaker, then a back-up fuse is required.
For three-pole short-circuits there is no difference between IT supplies and other supply types.
The ultimate short-circuit breaking capacity I
cu
and the service short-circuit breaking capacity I
cs
continue to apply.
Circuit breakers under IEC 60947-2 are suitable for use in IT supplies, if they are not marked
with the symbol . Testing is in accordance with Annex H.
IT
2.4.6 Switchgear for safety applications
The safety of machines, systems and processes with respect to the protection of persons and
property from damage of any kind is the primary objective of the legislation and of regulations
and norms for technical measures and solutions. Also low-voltage switchgear is being applied in
safety applications. In Section
4 “Protection” the dangers that result from electrical energy
directly are dealt-with in more detail.
In safety applications it is very important to receive reliable feedback about the main contacts
position so that for example it can be excluded that an auxiliary contact reports open main
contacts but while in fact – for example because of contact welding – they are closed. In this
context the term “mirror contacts” for power contactors is of importance. In a similar way, the
methods of “positive guidance” or “mechanical link” of contacts of control contactors ensure that
the position of NO and NC contacts cannot be mutually contradictory.
In modular systems, mirror and mechanically linked contacts must be mechanically fixed to the
base unit, so that they cannot separate.
2.4.6.1 Mechanically linked contacts
Appendix L of IEC 60947-5-1 defines the criteria for the mechanical linking of contacts. The
standard defines mechanically linked contact elements as a “combination of n Make contact
element(s) and m Break contact element(s) designed in such a way that they cannot be in
closed position simultaneously …”. The standard also defines the test conditions: With welding
of a contact – for example of a make-contact – the break-contacts of the contactor when
dropped out must still have an opening clearance of 0.5 mm or withstand an impulse voltage of
2.5 kV. The same applies with welding of a break-contact.
LVSAM-WP001A-EN-P - April 2009 2-38