Single Point Grounding
By: Greg May, TSTM, Inc.

Single point grounding, relative to metering, is a technique where each system (i.e., CTs, VTs, meter can) has one and only one connection to ground. There is another grounding philosophy out there that preaches that the more grounds that you have in place, the better.

First, it is important to draw a distinction between a ground and a neutral. A neutral is a conductor designed to carry the unbalanced current in a system of wires. A ground conductor, on the other hand, is normally designed to carry current only if there is an unusual event, such as a lightning strike, equipment breakdown, etc. A ground is normally considered to be at earth potential. A neutral may or may not be connected to an earth ground.

The secondaries of all instrument transformers, under normal circumstances, should have a ground connection as a safety measure. A ground connection should always be present on the secondaries of instrument transformers applied on voltages greater than 600 volts. Very high voltages can be generated at the secondary of primary instrument transformers as the result of a capacitive coupling between the primary and secondary if a ground connection is absent or is removed. Normally, the ground connection is not required on the secondaries of instrument transformers for the metering to be accurate.

Multiple grounds are likely to establish closed loops which can create circulating currents in the vicinity of heavy current carrying conductors which occur in pad mounts, CT cabinets, etc. These circulating currents can cause measurement errors under certain circumstances. Multiple grounds also can cause equipment to be subjected to very high transient currents and/or voltages in the event of a fault or a lightning strike. Substantial voltage gradients can be present between ground points in a fault condition. If sensitive equipment (metering?) is connected in parallel with these ground points having substantial differences in potential, severe damage to that equipment can result.

It is acceptable, and many times advantageous, for equipment (i.e. meter cans, transformer cabinets, etc.) to have multiple grounds.

It should be stated that this discussion is not intended to be comprehensive. Remember to always follow any regulatory requirements, including your Company’s policies and procedures.