Depending on the manufacturer and style of disconnect switch, porcelain or fiberglass insulator inserts may be installed in the steel operating rod. However, these insulator sections should not be considered primary protection for the worker operating the switch. There is no assurance the insulting value is still effective after years of service.
Often a utility will install a disconnect switch in their T&D system with an insulated section located in the steel operating rod. The utility will then install a system ground on the structure bonding all parts of the switch and pole to the common neutral and the structure’s ground rod. The grounding system may include:
• Grounding the upper steel operating rod to the common neutral, above the insulated insert
• Installing a pole ground from the pole’s ground rod to the common neutral
• Grounding the disconnect switch handle to the pole ground
WAIT!!!
Think about the above steps the utility has taken to ground the switch. Their system grounding process is supposed to provide protection to everyone. But what the utility has done is effectively jumper out the operating rod’s insulated insert with pole ground attached to the ground rod, switch handle, upper operating rod and common neutral. The pole ground from the ground rod and switch handle jumpers out the insulating insert; if unsure draw it out.
Does your utility have switches installed and grounded in this manner?
When a disconnect switch is installed in this manner, the frame of the disconnect switch, the upper and lower steel operating rod and the switch handle are all bonded together and connected to the common neutral and the pole’s ground rod, effectively eliminating any insulating value of the insulated insert. The electrical worker operating the switch has no protection and could have as much as full system voltage from the worker’s hands on the switch handle to the worker’s feet.
The use of rated rubber gloves can eliminate touch potential if the switch were to fail and go to ground. But there is also the hazard of step potential for the worker operating the switch, and rated rubber gloves does nothing to eliminate step potential. Also, the maximum ASTM rating for rubber gloves is limited to 36 kV, eliminating worker protection from higher voltages.
Properly installed ground mats provide the best protection for workers operating disconnect switches while standing on the ground. My preferred choice is the temporary protective ground mat available from several industry suppliers. It is easy to spread out on the ground at the base of the switch where the worker will be standing and is easily attached with a short grounding jumper to the switch handle.
The temporary protective ground mat eliminates the need to dig up the ground around the pole to install an elaborate buried ground mat. It is easy to carry at about three pounds and stores in any truck compartment. It also requires very little maintenance compared with the need to inspect and maintain a permanently installed ground mat on a regular basis. And the temporary ground mat has a number of other uses besides the use of eliminating step and touch potential while operating disconnect switches.
One of my favorite questions that I receive on this subject is, “If I am standing on a ground mat, do I also wear rated rubber gloves when operating the disconnect switch?” It really does not matter if you do or don’t wear rated rubber gloves if you are standing on a ground mat properly connected to the switch handle.
Why? Because you will be in an “Equipotential Zone” with your hands and feet at the same potential.
How? If the disconnect switch were to fail and go to ground, the switch handle could be energized at potentially full system voltage, say 7,200 volts, energizing the switch handle at 7,200 volts, less the voltage drop in the grounding conductor from the switch handle to the ground mat (typically 20 to 25 volts).
But if the worker were wearing rated rubber gloves and standing on a ground mat attached to the switch handle, would they be safe? Yes!
If they were not wearing rated rubber gloves but still standing on a ground mat attached to the switch handle, would they be safe? Yes!
Why? When the worker wears rated rubber gloves while standing on a ground mat attached to the switch handle, the gloves are insulating the worker from the 20 to 25 volts developed across the ground mat and switch handle; well below any hazardous voltage. They are safe with or without rated rubber gloves if they are standing on a ground mat properly connected to the switch handle.
Check your disconnect switches and see how they are grounded, then review your work practices of operating energized disconnect switches.
About the Author: Brian Erga, president of ESCI Inc., has more than 36 years of electric utility expertise and holds a BSEE degree. An expert on safety practices and work methods related to the electric utility industry, he is a member of IEEE/ESMO, NSC, NFPA, ASTM F18 and a member of NESC Subcommittee 8, responsible for NESC Part 4 “Rules for the Operation of Electric Lines.”
