AIEE Application Guide for Grounding of Instrument Transformer Secondary Circuits and Cases

It has been common practice for a long time in this country to ground the secondary circuits and cases of most instrument transformers used in switching stations, substations and generating stations and in certain other types of installations for reasons of safety to personnel and equipment. The object of the grounding is to limit the voltage on the secondary circuits and cases due to the effects of (1) possible insulation failure or other accidental contact between the primary circuit and the secondary circuit and/or case, which can subject them to full primary voltage, and (2) the capacitive coupling! between the primary and secondary circuit and case, which may impart to the secondary circuit and/or case objectionable or dangerous voltages to ground. However, there has been no general agreement among manufacturers and users whether all such instrument transformers, including auxiliary transformers, irrespective of voltage, should be grounded or just how this grounding should be done. The object of this Guide, based to a large extent on the findings and conclusions contained in a report by the AIEE Relay Committee titled Grounding of Instrument Transformer Secondary Circuits'' published in the April 1947 issue of Electrical Engineering, is to give recommendations in this respect.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 irre-spective 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 the safety or other advantages obtained by grounding. However, such exceptions should be made only (1) 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 (2) 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, (1) 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 (2) the exceptions to the grounding of the cases should be made only if the voltage of the primary circuit is 150 volts or less to ground.