- Prudently treat all lines and equipment as live – and as vulnerable to fault currents – unless s/he has verified that the equipment is correctly de-energized, tested for zero voltage, and grounded.
- Select protective ground equipment of appropriately high conductive capacity and low impedance, and verify that grounds are spaced at required minimum intervals.
- Safely connect and remove grounds (including specific scenarios for parallel and cable grounding).
First, let’s review. What is a “ground” and what is “grounding”? The Occupational Health & Safety Administration (OSHA) defines each term as follow:
- Ground refers to a conductive body, usually the earth.
- Grounding’ a tool or electrical system means intentionally creating a low-resistance path to the earth. When properly done, current from a short or from lightning follows this path, thus preventing the buildup of voltages that would otherwise result in electrical shock, injury and even death.
To work lines or equipment safely, as “de-energized,” OSHA normally requires that workers ground lines or equipment. The exceptions are where grounding is impractical, or would introduce greater hazards than working without grounds—such as near energized circuits. Even when workers have de-energized lines, they risk a fault current running through the lines. To protect themselves from the hazard, workers must anticipate the likely magnitude of the available fault current and its duration, so that they can install adequate grounds.
Common Causes of Hazardous Fault Currents
- Accidental Re-Energizing
- Physical Contact between Lines
- The Three Essentials for Safe De-Energized Work:
- Electrical workers must de-energize the lines and equipment.
- Electrical workers must test for zero voltage, using a proper meter.
- Electrical workers must install properly rated grounds.
Before a lineman installs a ground, they must test lines and equipment for nominal voltage. Use high-power tests to evaluate equipment, either at the equipment's rated voltage or at lower voltages. During testing, they may remove grounds. But first, they should inform co-workers and isolate themselves and others against exposure to energized parts and other hazards.
The protective grounding equipment that workers use must be able to conduct the maximum fault current that could flow through it, for long enough to clear the fault. It must also have low-enough impedance, which is defined as “the apparent opposition in an electrical circuit to the flow of an alternating current that is analogous to the actual electrical resistance to a direct current and that is the ratio of effective electromotive force to the effective current”, to operate immediately if lines or equipment accidentally get energized. OSHA requires all grounds to have an ampacity equal to at least number-two AWG cable.
Industry guidelines help the electrical workers go beyond OSHA’s minimum requirements, and choose grounding equipment adequate to the available fault current at the work location.
Grounds must be placed at different intervals for different types of work. Industry consensus standards, including the National Electrical Safety Code, specify these intervals.
Types of Grounds
There are two kinds of grounds, both are required by the OSHA construction standard.
- System or Service Ground: In this type of ground, a wire called "the neutral conductor" is grounded at the transformer, and again at the service entrance to the building. This is primarily designed to protect machines, tools, and insulation against damage.
- Equipment Ground: This is intended to offer enhanced protection to the workers themselves. If a malfunction causes the metal frame of a tool to become energized, the equipment ground provides another path for the current to flow through the tool to the ground.
Further, there is one disadvantage to grounding: a break in the grounding system may occur without the user's knowledge. Using a ground-fault circuit interrupter (GFCI) is one way of overcoming grounding deficiencies.
System Safety Grounds
For overhead transmission, System Safety Grounds must be located no further than four miles apart. They can be spaced more closely. If these grounds are located more than one mile apart, electrical workers must use Personal Protective Equipment (PPE), like a jumper, at the work location. For overhead distribution, System Safety Grounds for employee protection must be located no further than two miles apart. For underground distribution, System Safety Grounds should be connected to the neutral at the nearest termination point. The best practice is to ground both ends.
It’s not the amount of electricity that hurts the worker; it’s when they bridge a difference in electrical potential. So, to protect themselves and co-workers from accidentally bridging such differences, they want to place temporary protective grounds to create an “equipotential zone” that’s safe to work in. They can also use isolating or grounding mats, and insulated footwear, to reduce step and touch potential hazards near a ground’s connection point.