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Alert: Transformer Grounding

  • ISSN: 1195-5228
  • Issued: August 2007
  • Content last reviewed: June 2009

Hazard Summary:

An experienced power line maintainer suffered severe electrical burns to both hands during routine maintenance on a pole-mounted 75 kVa/14.4kV transformer with a multi-connection ground plate. Leads connected the pole-mounted plate to:

  • the surge arrestor on the primary lead into the transformer,
  • the transformer bushing of the primary neutral (or ground) line,
  • the secondary neutral or ground line,
  • and the transformer case itself.

All these were grounded only through a lead from the plate to a ground rod in the earth at the foot of the pole (see Figure 1 and 'A Brief Glossary').

At the time of the accident, the connection of this lead to the rod had broken off beneath a cover of ice and snow, so that neither the worker nor his partner operating the bucket truck could see that the plate was no longer a solid ground. When the worker disconnected the lead between the ground plate and the secondary neutral, he became a bridge between them. Wearing leather work gloves and holding uninsulated tools, he was unable to release his grip. Fortunately, his partner on the ground was able to operate the lower controls of the bucket truck and swing him away from the pole.

Figure 1. Old Ground System

Figure 1. Old Ground System

Locations and Sectors:

Power line maintenance; electrical sector.

Recommendations:

New transformer installations do not use a ground plate, and they provide two separate paths to ground:

  1. The primary bushing on the transformer is connected to the secondary bushing, which is then connected to the secondary neutral. The secondary neutral in turn is directly connected to the ground rods at regular pole intervals.
  2. he primary and secondary bushings are connected to the transformer case itself and through it by another lead to the secondary neutral.

When a worker breaks any one neutral connection a second path to ground remains intact—assuming everything else is in order. Since it is always possible there will be another break in the grounding system, caused by loose connections or faulty equipment, protective clothing, equipment, and procedures must still be used when working on a grounding system of this type.

For transformers using a ground plate with a single path to ground which remain in service, good maintenance is essential. This must include inspection and checking of all wiring, connections, ground rods and bonding. The maintenance crew must use appropriate protective equipment and procedures.

Transformers of this kind may be provided with additional grounding by removing the ground plate and installing a double ground system, as outlined above. During the upgrading, the equipment shall be de-energized.

Whenever work is performed on the ground (or neutral) system of transformers with ground plates:

  • The system shall be considered live, and appropriate protective clothing, equipment, devices and procedures shall be used to ensure the safety of workers. (The Electrical Utility Safety Rules published by EUSA, dated 2004, may be used for guidance.)
  • An adequately equipped second worker, trained in rescue operations including CPR, shall be readily available.

Figure 2. New Ground System

Figure 2. New Ground System

A Brief Glossary

"Primary" refers to the system which carries high-voltage power to the transformer.

"Secondary" refers to the system which takes power from the transformer, usually into a house or other building.

A "grounding system" is a safety system built into a larger electrical system to carry any excess voltage (for example, from lightning or power surges) safely to ground, i.e., into the earth, without traveling through and thus harming people or equipment.

A "ground line" or "neutral" is a line connected to the ground (if the system is working).

"Bushings" are the insulating sleeves around primary and secondary leads where they enter the transformer case. The bushings are connected to a ground line so that any excess voltage from the leads will be directed to the earth.

This Ministry of Labour Alert has no legal effect and does not constitute and is not a substitute for legal advice. If you require specific assistance with respect to the interpretation of a legislative provision and its potential application to you please contact your legal counsel.

Remember that while complying with occupational health and safety laws, you are also required to comply with applicable environmental laws.

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