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National Safety Council statistics show that electrical accidents kill 75 to 100 persons and injure another 750 to 1,000 per year in the United States alone. Many of these accidents could have been prevented through proper grounding.
In the realm of switchgear maintenance, the purpose of grounding is to reduce the risk of arc flash or maintenance personnel electrocution during maintenance processes. By placing current-carrying switchgear parts at ground potential, incident current can be redirected to ground instead of through the human being.
When properly applied, maintenance grounding eliminates dangerous differences in electric potential, and can redirect harmful current during events such as unforeseen induced voltage from motors and generators, accidental bus contact with adjacent live parts, or human error that causes equipment to be re-energized.
To decrease risk of personal injury, workers should stay away from open energized switchgear panels prior to proper equipment grounding. Any time panels are removed and energized components are exposed, the chance for arc flash or electrocution increases.
An additional layer of safety: Integral grounding switches
Traditional grounding methods leave too much potential for human error during the many steps of the process. New integral grounding switches help to address this problem. They allow bus bars and feeder cables to be grounded without removing switchgear panels or covers. These switches connect current carrying parts of the switchgear to ground. This allows maintenance personnel to ensure the removal of electric energy from the potentially energized parts of switchgear during maintenance.
It is important to note the difference between internal grounding switches (the topic of this blog) and the steps involved in proper grounding for maintenance work (not discussed in this blog). The purpose of an internal grounding switch is to provide a process to ground MV switchgear before removing panel covers and exposing personnel.
Grounding switches offer the benefits of being simple to operate, easy to secure during lockout/tagout procedures, and provide the added protection of grounding the switchgear without exposing maintenance personnel to exposed energized parts. Personnel that operate switchgear with grounding switches have a higher degree of confidence in applying ground without having to open switchgear compartments first. By specifying switchgear equipment to the highest standards, owners send a message to their maintenance service teams that they are serious about personnel safety and equipment protection.
Proper precautions need to be taken when installing the appropriate grounding switches. Below is a list of important considerations:
- A properly sized grounding switch must be tested and approved for the available fault current of the distribution system. An undersized switch may not adequately reduce the risk to personnel.
- An effective bonding and grounding mechanism must be constructed to bond the switch to the ground bus of the switchgear.
- The switch must be of robust construction so as to withstand the mechanical forces experienced during a fault.
- There should be a source of voltage indication on the bus or cables being grounded. Live line indicators (LLIs) are an excellent option for addressing this requirement.
- An effective interlocking scheme should be implemented to prevent closing the grounding switch while the switchgear is under normal operation and voltage is present.
These are steps that are taken in order to eliminate the possibility of dangerous voltages which could destroy the equipment or cause severe injury.
Integral grounding switches are important tools that remove the need to expose personnel to energized equipment by grounding current carrying elements before panels or covers are removed. For more details, download the Schneider Electric white paper entitled “New Approaches for Maintenance Grounding in Medium-Voltage Switchgear”.
