The prevention of electrical shock can be achieved in two ways: Deenergizing the circuit or using safety equipment on the energized circuit. Both methods are fairly straightforward and consist of common sense thinking. Let's discuss them in detail.
Deenergizing the circuit
Obviously, if you work on deenergized equipment, the risk of personal injury is greatly reduced. Unfortunately, many people have been electrocuted by circuits they thought were "dead." To ensure that a circuit is safe and deenergized, these procedures should be precisely followed.
Preliminary planning. First, prior to system shutdown, you should review an updated single-line diagram of the power distribution system. You should determine the available power sources, grounding points, switching sequencing (if any), and any equipment affected by the proposed shutdown.
Second, you should verify that the nameplate callouts of affected equipment match those shown in the updated single-line diagram. Should any disconnects lack an identification label, you should prepare one and further update the single line diagram with this information.
Third, you should coordinate the shutdown by notifying all affected departments within the facility. Special equipment such as elevators, computers, life safety systems, etc. might require additional coordination to ensure facility operation.
Fourth, you should schedule the availability of necessary people, equipment, maintenance tools, and life safety equipment for the proposed work.
Fifth, you should write an operating procedure for the intended work and review it with pertinent facility personnel as required. This procedure should contain "as found" labeling information for all breakers, switches, and controls. Also included should be a sequence of breaker and/or switch operation, with the sequence starting with downstream loads and working up.
Shutdown. After the circuit has been deenergized, it should be tested with an appropriate test instrument having the proper voltage rating before the proposed work begins. This instrument should have been tested on a known energized circuit prior to this. The circuit should be tested phase-to-phase and phase-to-ground on all possible sources. The test instrument should be re-tested immediately on a known energized circuit.
Next, you should install safety grounds on the deenergized circuit. These grounds will protect against induced voltages from adjacent energized circuits; inadvertent circuit re-energization due to switching errors; any unusual conditions that might bring an energized conductor in contact with the deenergized circuit; and any stored charges (capacitors). When installing the grounds, you should connect to "ground" first and then to the phases.
A ground cable must be able to conduct the anticipated fault current and have a minimum conductance of a No. 2 AWG copper conductor (95A). The ground cable length should only be as long as necessary since excess cable could "whip" in the event of a fault, causing injury to nearby personnel. The ground cable's clamp should be sized for adequate current flow and designed for the particular application (bus, cable, tube, special grounding knobs, etc.).
Finally, you should lockout and tagout the circuit's switching device. Each person working on the circuit should have his or her own lock and tag. Only that person can remove his or her lock and tag. Laminated tags having a picture of the worker are ideal.
Using safety equipment on energized circuit
The primary function of safety equipment is to increase the dielectric between the energized source and you. This is usually done with hot sticks, rubber gloves, rubber blankets, or other insulating materials. However, these products can provide protection only if properly tested, maintained, and safely used. There are some very valuable guidelines that certainly pertain here.
Personal guidelines. Do not work on energized equipment with wet hands or while wearing wet clothing. Do not wear any clothing with exposed zippers, buttons, other metal fasteners. Do not wear loose or flapping clothing. Do not wear rings, wristwatches, bracelets, or other similar items when doing work on or within 4 ft of electrical equipment with exposed current-carrying parts.
Safety equipment guidelines. Do wear rubber gloves when working on energized equipment. They should be put on before coming within reach of the energized parts and removed when entirely clear of these parts. Leather protectors should always be worn over the rubber gloves and should not be used for any other purpose. Rubber gloves should never be rolled down or worn inside out. Never wear two pairs of gloves.
Rubber gloves and other rubber protective equipment should be tested at intervals not to exceed the following.
* Rubber gloves: every six months.
* Rubber blankets: every year.
* Rubber line hoses and hoods: every year.
The date of the testing should be marked on this protective equipment.
These insulating and protective devices should be stored flat in areas. The areas should be free of oils, chemicals, and other detrimental materials.
Rubber gloves should be "air-tested" before starting work on energized equipment or circuit by grasping the glove's cuff at opposite points and twirling the glove. With the open end held closed by one hand (so that the air inside can't escape), squeeze the glove with the other hand to force air into the thumb, fingers, and palm. Hold the glove to your ear and listen for escaping air, making a thorough inspection for holes or thin spots. You can also hold the inflated glove under water and check for air bubbles. If the glove is defective, tag it and return it for replacement.
If the work requires that one hand be free of covering, a rubber glove should still be worn on the other.
Hard hats should always be worn, especially when working directly under other workers. Standard hard hats are required to have a degree of insulation resistance so that personnel (other than electrical) are protected from accidental contacts with electrical circuits and equipment at relatively low voltages (less than 2200V). Electrical workers, especially those involved with transmission or distribution line installation and repair, should wear insulating safety helmets or all-purpose protective helmets that have been proof-tested to more than 20,000V.
On energized low voltage equipment (600V or less), hand tools with insulated handles should be used. Many makes are available with insulation levels up to 1000V. Hand tools with broken, nicked, or cracked insulated handles should be discarded and replaced.