Handbook of Electrical Installation Practice - BeKnowledge

392 Handbook of Electrical Installation Practice
Fig. 14.12 Circuit required to test for CT terminal polarity.
circuit a negative ‘flick’ is seen. Where CTs are mounted in transformer bushings it
is necessary to short-circuit the main transformer l.v. winding, thus reducing the
equivalent impedance of the transformer to give a good deflection of the instrument
needle. Voltage transformers may be checked in similar fashion.
Proving of protection
The operation of the various forms of protection which can be fitted to distribution
switchgear is covered in Chapter 22. Suffice it to say that the operation can be
simulated either by injecting heavy currents through the CT primaries or by the
injection of suitably chosen currents into the CT secondary circuits. In some
instances the CTs may be fitted with a special tertiary winding intended for test purposes
in this way. Whichever method is adopted, the intent is to inject currents that
represent fault conditions of various types, and to check that the relevant relays
operate correctly and that any auxiliary relays that are called upon to perform do
so also in a correct manner.
To adjust the current settings it is normal to use a variable autotransformer to
provide the supply. As the waveshape of the test current can have an influence on
the behaviour of the relay it is recommended that as much series resistance as
possible should be included in the injection circuit.
High-voltage tests
If called for in the contract documents h.v. tests can now be applied to the insulation.
The test values should be those be those laid down in the appropriate standard