Application Guide for GROUNDING OF INSTRUMENT TRANSFORMER SECONDARY CIRCUITS AND CASES
|Publication Date:||1 March 1951|
This Guide contains general and specific recommendations for grounding of the secondary circuits and cases of instrument, i.e., potential and current, transformers in all types of installations. These recommendations apply to all instrument transformers irrespective of the value of primary voltage or whether the primary windings of the transformers are connected to, or in, power circuits or whether these primary windings are connected in the secondary circuits of other instrument transformers.
Exceptions to grounding are permissible where the advantages obtained by not grounding in certain instances or in certain types of installations are considered to outweigh t'he safety or other advantages obtained by grounding. However, such exceptions should be made only
when the instrument transformers form part of a separate metering or network installation and not part of a switching station, substation or generating station, and
when the primary windings are connected to circuits of 600 volts or less.
Furthermore, for reasons of increased safety in those metering or network installations where the instrument transformers or live parts of the secondary circuit are accessible to other than authorized personnel,
the exceptions to the grounding of the secondary circuits should be made only if the voltage of the primary circuit is 300 volts or less to ground and
the exceptions to the grounding of the cases should be made only if the volt'age of the primary circuit is 150 volts or less to ground.