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Subject:
RE: Prolonged energisation of Arc Suppression Coils
Importance: High
At last I have unearthed a reference to the
time constrains on ASC’s.
This
reference is from a lecture to the institute of electrical engineers November
1960 by A.J. Ruddock.
Insulation
Levels
The
question is often raised whether any higher insulation levels are required for
cable and switchgear when operating on systems protected by continuously rated
arc suppression coils, especially as these may be subject to prolonged
operation at line voltage to earth.
Both
the electricity boards specification for cable and the B.S.S. for switchgear
adopt a common outlook, and standard equipment can be used for either of the
following working conditions:-
- The fault is automatically disconnected from the system
after a short delay (original practice in the Thameside
area).
- When non-automatic disconnection is used faults are
quickly cleared although they may be allowed to persist for several hours,
provided the total duration does not exceed 125 hours per annum with a maximum
of 8 hours in any 24.
It will
thus be appreciated that adequate operational facilities are offered for coli
working in modern systems. Further, in the older systems where coils are to be
introduced the insulation levels can usually be regarded as being if anything
more generous than those provided in present day specifications.
The above
would indicate that any time constraint on the operation of the ASC is for
reasons of insulation stress and not the ability of the coil to sustain the
load current.