Applications
3.4 Use of 3VA1 molded case circuit breakers as switch disconnectors
3VA molded case circuit breakers with IEC certificate
Manual, 03/2019, A5E03603177010-03
167
Making capacity
Switch disconnectors have a predefined rated making and rated breaking capacity. As a
result, loads are reliably switched on and off up to the specified breaking capacity.
Features
Switch disconnectors are primarily designed to conduct uninterrupted current up to the
magnitude of the permissible rated uninterrupted current I
u
.
Another important feature of these devices is their rated short-circuit making capacity I
cm
, as
this defines the capability of the switch disconnector to withstand dynamic and thermal
current loads. Current loads can reach very high values when the switch disconnector makes
on a short circuit. Switch disconnectors are designed to withstand these short-circuit currents
without sustaining damage up to the value of their rated short-circuit making capacity.
Utilization categories for switch disconnectors
Utilization categories are differentiated according to the following criteria:
●
The relationship between operational current and rated operational current
●
The relationship between operational voltage and rated operational voltage
●
Power factor (p.f.)
●
Time constant
The utilization category is also identified by a letter:
A - for frequent operation
B - for infrequent operation, e.g. disconnectors which are used only to isolate parts of
electrical installations for maintenance purposes.
It is important to note, however, that the term "frequent operation" does not mean that the
switch disconnector may be used to start up, accelerate or shut down individual motors in
normal operation.
For example, a switch disconnector with a rated operational current I
e
of 160 A and a rated
operational voltage U
e
of 400 V AC may be used for frequent switching of a mixture of
resistive and inductive loads including moderate overload only if it can conduct a making
current I of 480 A (3 x I
e
) and withstand a making voltage U of 420 V AC (1.05 x U
e
) with a
power factor of 0.65. This capability corresponds to utilization category AC-22A.
