TRANSFORMER GROUNDING


Produced by Professional and Specialized Services,
Ontario Ministry of Labour.
Disponible en fran¨ais.


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 ear at the foot of the pole (Figure 1*, and "A Brief Glossary", 
page 2).

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 away from the 
pole.

LOCATION 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.  The 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-generized.

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 
           Operations Rule Book" and "Ontario Hydro Corporate Safety Rules" 
           may be used for guidance.)

        -  A second worker trained in artificial respiration shall be 
           readily available.

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 travelling 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.


* To view Figures 1 and 2, contact Professional and Specialized 
  Services of the Ministry of Labour, (416) 326-7744 or toll-free in 
  Ontario 1-800-268-8013.