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You must be able to create an electrically safe work condition to work on high-current or high-voltage equipment. A critical step in that process is the application of safety grounds, and most of this equipment will have a ground bus or connection. When you ground the equipment you’re working on, you create a significant safety advantage.
We have seen facilities where vehicle battery cables have been used as
safety grounds; unfortunately, they would blow off instantly during a
fault. Safety grounds must meet ASTM F855-04 Standard Specifications
for Temporary Protective Grounds to Be Used on De-energized Electric
Power Lines and Equipment.
When purchasing safety grounds, you can choose between a cluster or
individual ground sets. Clusters have four cables tied to a crow’s foot
terminal block. The heel will have a cable connected to ground, and the
three toes will each be going to your phases: A, B and C. If you use
individual ground sets on a three phase system, you’ll have three
cables and six ends to manage, maintain and test. With a cluster, you
have only four ends plus the four connections of your terminal block.
Safety grounds should be sent for testing on a regular basis, and be
tested to F2249-03, “Standard Specification for In-Service Test Methods
for Temporary Grounding Jumper Assemblies Used on De-Energized Electric
Power Lines and Equipment”. They are typically insulated to 600 volts;
the reason being that the voltage dropped across them is far less than
that, so insulation voltage is not a major consideration. What is a
consideration, however, is the electrical resistance of the cable ends
and ground clamps, and connections between them.
One machine that performs this test is a microohmmeter, often referred
to as a ductor. A microohmmeter typically puts out 10 amps of current
and provides a digital read-out in microohms.
Again, there are standards for both clamps and cables for determining
how low the microohm resistance should be. The connection must be an
extremely low resistance. For instance, a 10,000-amp fault going
through 1Ω of resistance would allow a voltage drop of 10,000 volts
from the equipment-to-ground at the moment something is accidentally
energized. That would be lethal to the worker.
By ensuring the resistance of the grounding system is incredibly low, three objectives are accomplished:
• A high current is allowed to flow.
• The flow of this current will cause the protective devices to operate in their instantaneous regions.
• The voltage drop across the electrical worker will be negligent.
You need to perform a visual inspection prior to using the safety
grounds. They should be stored in a clean, dry location where they’re
not subject to damage. On a wall in the substation is a common
location, but within a custom storage box in a closet in a substation
is a better place. In the bottom of a job box, or bouncing around in
the back of a service truck, is a terrible place. Your electrical
safety program should be audited to determine the safe work practices
followed with regard to your safety grounds.
Once you have inspected your grounds, the next step is determining
where they should be placed. Prior to their placement, however, you
must ensure the absence of voltage! It is critical when placing grounds
to place them at exactly the same points where the voltage test was
done. If the voltage test was taken at Point A, then the grounds should
be applied to Point A.
There is a utility worker in a graveyard: several years ago he did his
voltage test at one part of the bus but applied his grounds at another
part of the bus without knowing there was an open breaker in between.
It was deenergized where he tested for voltage, but completely
energized in the location where he applied his ground. A miserable
death.
It is usually easy to find a place to apply your grounds, but sometimes
it is difficult; either way, always use a hot stick and wire brush your
clamps and grounding spots. The first connection to be made must always
be to ground itself, then to each of your phases—quickly and
definitively. ‘Tickling’ is highly dangerous!
It is critical to remove grounds in reverse. Take off each phase and
remove the ground connection last. One utility worker had A Phase
removed when his cell phone rang; he lost track of his tasks during the
conversation and accidentally lifted the ground. He was electrocuted
from induction. Another miserable death.
There’s a common misconception around the mantra: It isn’t safe until
it’s grounded. Even something that’s been grounded requires special
attention. For example, one utility worker applied his grounds properly
one day, then left them in place overnight. Earth resistance increased
due to drying from continuous inductive current driving up the voltage
drop. Although the worker received a shock the next day as he brushed
past the ground set, he did not realize the danger he was in. He was
electrocuted soon afterward when he made a solid contact. Yet another
miserable death.
Grounding looks simple, but it is far from. When done right, it will
save your life; when not done at all or done incorrectly, you may very
well endure a miserable death.
Until next time, be ready, be careful and be safe.
Dave Smith is president of CANADA TRAINING GROUP and has been providing consulting services to industry since 1980.
