If a person is following the appropriate safety guidelines, there should be no possibility of an electrical shock. OSHA and the NFPA 70E are devoted to worker safety and have provided all the necessary requirements to ensure that a worker goes home safely at the end of the day.
OSHA and the NFPA 70E are very clear on the fact that there are very few situations where it is permitted to work on energized equipment. The only time that energized work is permitted is when disconnecting power creates a greater hazard, or if it is infeasible due to equipment design and operational limitations.
The most important thing, that can’t be stressed enough, is DO NOT WORK ON ENERGIZED CIRCUITS! If all potential power supplies have been removed or discharged, there is no chance of get-ting an electrical shock. There are several important steps to follow to ensure that there is no potential voltage.
- Planning;
- Disconnect the power;
- Use the appropriate lockout-tagout procedures to ensure the power isn’t turned back on;
- Release any stored energy;
- Verify that there is no potential voltage, using the proper procedures (PPE, meter requirements);
- Ground conductors and equipment.
Planning
Every time a piece of equipment is going to be de-energized for ser-vice there is planning involved. For smaller jobs this may be as simple as figuring out where the switch is located, while others may require the use of schematics, wiring diagrams, or blueprints. It is also necessary to communicate with workers who may be affected by the power being removed.
Disconnect the Power
Disconnecting the power sounds easy enough, but there are several ways to accomplish this and a few scenarios to keep in mind. This chapter is on electrical shock so that is the focus, but don’t forget that there may be other forms of energy that may have to be locked out, such as hydraulics, air, gas, steam, etc.
Switch
When working on equipment such as lights, disconnecting the power may be a simple as shutting off a switch. What one has to keep in mind is that although the switch may remove power to the light, there may still be potential voltage in the box above the light. Another issue is the possibility of someone walking into the room and turning on the switch before they notice you are working on the light. There are lockout devices which are designed to prevent a toggle switch from being turned on. (Figure 1) Some areas have multiple switches feed a light: if this is the case, care must be taken to ensure that all of these switches are secure against being turned on. If there is any concern that a switch may be turned on, or that there may be power in the box even with the switch in the “off” position, it is better to be safe and disconnect the circuit at the electrical panel.
Figure 1. A lockout device to prevent a switch from being turned on.
Circuit Breaker
Many times the entire circuit needs to be shut down. This is done by shutting off the circuit breaker or removing a fuse. It is not safe to shut off a breaker or remove a fuse while it is under load. Contacts being opened with high current levels will cause excessive arcing which could damage the equipment, shorten its life, or even lead to a dangerous arcing incident. To eliminate this danger, the equipment that is fed from the circuit being shut down must be turned off before the breaker is operated or the fuse is removed. The electrical panel is often in a different room than the work being done, so it is imperative that the appropriate lockout-tagout procedures be followed to ensure that the circuit doesn’t get turned back on. (Figure 2)
Figure 2. A lockout device to prevent a circuit breaker from being turned on.
Disconnect
Most equipment will have a means of disconnection within sight of it. As with breakers and fuses, it is not safe to shut off a disconnect while it is under load. Contacts being opened with high current levels will cause excessive arcing which could damage the equipment, shorten its life, or lead to a dangerous arcing incident. To eliminate this dan-ger, the equipment fed from the disconnect being shut down must be turned off before the disconnect is operated. After shutting off the dis-connect switch, be sure to use the appropriate lockout-tagout procedures to ensure that it doesn’t get turned back on. (Figure 3)
Figure 3. A lockout device to prevent a disconnect switch from being turned on.
Cord and Plug
Some equipment is simply plugged into a receptacle. In this case all that is necessary to disconnect the equipment is to unplug it. Remember to shut the equipment off before removing the plug to prevent excessive arcing that could produce a dangerous situation or damage the equipment. There are caps that can be locked over the end of the cord to ensure that it doesn’t get plugged back in while you are working on it. (Figure 4)
Figure 4. A lockout device to prevent a cord from being plugged in.
Lockout-Tagout
Locking and tagging out equipment is as important as shutting it off ; this procedure prevents someone from turning the equipment back on. Specific lockout-tagout procedures will be included in a company’s safety plan. Locking out equipment provides a physical restraint that prevents the power from being turned back on, and tagging out equipment provides information on who is performing the work, the date and time, reason for work, etc.
Stored Energy
Some equipment may have electrical, kinetic, or potential energy that must be released or removed. Electrical energy stored may be in batteries that must be disconnected or capacitors that must be dis-charged. Be sure to follow the correct procedures for disconnecting batteries and discharging capacitors, which will include verifying that the power has been disconnected before discharging capacitors, and wearing the appropriate personal protective equipment, as the components may have a potential voltage. This chapter is on electrical shock so that is the focus, but don’t forget that equipment may have kinetic energy (spinning wheels, blades, etc.) or potential energy (springs, elevated weight, etc.) that must also be addressed.
Verify There Is No Potential Voltage
Before considering equipment de-energized, the absence of potential voltage must be verified with an electrical meter. (Figure 5) There are several reasons why there could be potential voltage even though you have just disconnected the power.
- It is possible that the wrong breaker or disconnect was shut off ;
- The power was turned back on;
- An induced voltage;
- A voltage backfed from other circuits or equipment;
- Stored energy (batteries or capacitors).
While performing this test, a person must wear the appropriate personal protective equipment and assume that the power is still on. Always verify that the meter is working properly by checking a known voltage before using it to check for the absence of voltage on the supposedly de-energized equipment; a faulty meter or meter lead could prove fatal. While checking for voltage, always check from phase to phase and from phase to ground on all conductors.
Figure 5. A voltmeter can be used to verify the absence of voltage.
Ground Conductors
If there is potential for an induced voltage or for a wire to become energized while working on the circuit, then all phase conductors and equipment must be grounded before work commences. Be sure the conductor used to ground the conductors and equipment is sized to handle the potential fault. (Figure 6)
Figure 6. Ground set.
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