DX581-S ABB distributed control system industrial DCS module card

Description

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Protection on the secondary side
Due to the transformer’s high insertion current, the
circuit-breaker on the primary winding may not guarantee
thermal protection for the transformer and its feeder line
on the primary side.
This is typical of modular circuit-breakers which must have
a higher rated current than the transformers. In such cases,
in the event of a single-phase short-circuit at the
transformer’s primary terminals (minimum Icc at end of line),
check that the circuit-breaker’s magnetic releaser is tripped.
In the normal application in distribution panels,
this condition is always satisfied provided that the length
of the feeder lines is reduced.
The transformer can be provided with thermal protection
by installing a circuit-breaker with a rated current less than
or equal to that of the transformer secondary winding
immediately downstream of the LV/LV transformer.
In lighting systems protection against overloads is not
necessary if the number of light points is clearly defined
(no overloads).
Moreover, the Standard in force for these systems
recommends the omission of protection against overloads
in circuits in which unwanted tripping may prove hazardous,
e.g.: circuits which supply fire-fighting equipment.