So, you're about to embark on your plant outage and scheduled preventative maintenance initiatives. You are well into your tasks of determining all possible sources of electrical energy, properly interrupting the load current, opening the disconnecting devices for each source, releasing stored electrical energy, and generally working toward establishing and verifying an electrically safe work condition. Now, it is time to apply lockout/tagout devices in accordance with your documented and established set of procedures. Using an adequately rated portable test instrument, test each phase conductor or circuit part to verify it is de-energized. Well, are we good to go? Have we established electrically safe working conditions? What about grounding?
Voltage may appear at a worksite due to accidental energization, either through the isolating device or due to contact with another energized circuit. Voltages or currents may be present due to electric or magnetic induction from adjacent energized circuits or due to a direct or indirect lightning strike. Therefore, the NFPA 70E Standard for Electrical Safety in the Workplace states that where the possibility of induced voltages or stored electrical energy exists, we must ground the phase conductors or circuit parts before touching them. When it could be reasonably anticipated that the conductors or circuit parts being de-energized could contact other exposed energized conductors or circuit parts, we need to apply temporary protective grounding equipment.
Personal protective grounding should be applied to de-energized lines and equipment having a nominal voltage rating over 600 volts if the normally exposed current-carrying parts are to be contacted or approached within the minimum approach distances given in OSHA CFR 29 1910.269 Table R-6. Other nearby exposed parts of any electrical equipment rated over 600 volts that are not associated with the work but maybe approached within the minimum distance during the work activities will either be de-energized and grounded or suitably isolated to prevent contact.
Grounds must be placed close enough to protect workers, but not so close that they can strike them if the grounds become re-energized, especially due to fault-level currents. The current flowing through a ground cable can create a magnetic field strong enough to make the cable snap like a whip, possibly breaking bones or knocking workers off structures. The electromagnetic forces on grounding cables during a fault increase proportionally to the cable length. These forces can cause the cable to move violently during a fault and can be high enough to damage the cable or clamps and cause the cable to fail. Consequently, cable lengths should be as short as possible.
Temporary protective grounding equipment is capable of conducting the maximum fault current that could flow at the point of grounding for the time necessary to clear the fault. If the ampacity of any part of the ground set is inadequate (cable, ferrule or clamp) or if the connection has high impedance due to a poor connection or defect, the personal protective ground cluster could "fuse." That's a fancy way of saying it will melt. And finally, use the grounding clamp that is designed to fit the type of bus or connection you're connecting to. For example, don't try to use a clamp designed for a tubular bus on a flat bus. It probably won't hold if there's a fault.
Now, with the temporary protective grounds properly in place, we have established our electrically safe work condition and are ready for our outage. Remember, prior to return to service, tests and visual inspections need to be conducted to verify that all tools; mechanical restraints; and electrical jumpers, short circuits and temporary protective grounding equipment have been removed.
For more information, visit www.shermco.com/sto or call (888) SHERMCO [743-7626].
