The American University in cairo

Electrical Safety

Subpart "S"

Introduction:

Electricity has become an essential of modern life, both at home and on the job. Some employees work with electricity directly, as is the case with engineers, electricians, or people who do wiring, such as overhead lines, or circuit assemblies. Others such as office workers, work with it indirectly.

As a source of power, electricity is accepted without much thought to the hazards encountered. Perhaps because it has become such familiar part of our surroundings.

For 1989, the Bureau of Labor Statistics reported that 3,600 work-related deaths occurred in private sector workplaces employing 11 workers or more. Nine percent of the fatalities, or around 324 deaths, were the direct result of electrocutions at work. What makes these statistics more tragic is that, for the most part, these fatalities could have been easily avoided.

The objective of OSHA's electrical standards is to minimize the potential hazards of electricity by specifying design characteristics of safety in use of electrical equipment and systems.

OSHA's electrical standards were carefully developed to cover only those parts of any electrical system that an employee would normally use or contact.

The OSHA's electrical standards were based on the National Fire Protection Association's standard NFPA 70 E. (National Electrical Code)

The standards extracted from the NEC were those considered to most directly apply to employee safety.

General Requirements: 29 CFR 1910.303 :

1- The conductors and equipment required or permitted by this subpart shall be acceptable only if approved (U.L.).

2- Electrical equipment shall be free from recognized hazards that are likely to cause death or serious harm to employees.

3- Listed or labeled equipment shall be used or installed in accordance with any instructions included in the listing or labeling.

4- Parts of electric equipment which in ordinary operation produce arcs, sparks, flame, or molten metal shall be enclosed or separated and isolated from all combustible material.

5- Marking of electrical equipment is very important. Marking giving voltage, current, wattage, or other ratings shall be provided. Marking plates shall be installed in such a manner so the marking can be examined by the inspector without removing the installed equipment from a hard wired position.

6- Each disconnect switch or over-current device required for a service, feeder, or branch circuit must be clearly labeled to indicate the circuit's function, and the label or marking should be located at the point where the circuit originates.

7- All labels and marking must be durable enough to withstand weather, chemicals, heat, corrosion, or any other environment to which they may be exposed.

600 Volts, Nominal, or Less:

Working Space About Electrical Equipment:

1- A minimum working space of 30 inch (76 cm) width is required in front of electrical equipment operating at 600 volt or less. Distances shall be measured from the live parts if they are exposed, or from the enclosure front if the live parts are enclosed. This space permits sufficient room to avoid body contact or elbows from contacting live parts and metal parts at the same time while working on the equipment. Equipment doors and hinged panels must have at least a 90 degree opening provided in the workplace.

2- Working space is not required in back of electrical equipment where there are not any removable or adjustable parts such as circuit breakers, fuses, or switches mounted on the back of the equipment. All connections and services areas for maintenance must be accessible from other locations other than the back of the equipment.

3- Where the electrical equipment is installed in or on one wall with the wall on the other side being an insulated wall (Constructed from wood or metal studs with the wallboard consisting of sheetrock, wood-panels, and etc.) the minimum workspace distance shall be 36 inch (91 cm) between the equipment and the wall.

4- Where the electrical equipment is installed one wall with the wall on the other side being a conductive wall, the distance shall be minimum of 36 inch (91 cm) for voltage ranging from 0 to 150 V, and the distance shall be 42 inch (106 cm) for voltage ranging from 151 to 600 V.

5- Where the electrical equipment is installed in or on one wall with the wall on the other side having electrical equipment mounted or set on it. The distance shall be minimum 36 inch (91 cm) in case of voltage ranging from 0 to 150 V. and shall be minimum 48 inch (122 cm) in case of voltage ranging from 151 to 600 V.

6- Working space in front of electrical equipment must be free from storage of materials and etc. Mains and over-current protection devices are required to be accessible to the users in case of emergencies.

7- Easy and fast access to electrical devices is essential. Special consideration should be given to electrical equipment that is over 6 ft. (2m) wide with 1200 Amps or more of bus containing over-current devices, switching devices, or control devices. Such equipment must have a clearance of 24 inches (61 cm) wide and 6.5 ft. (2 m) high at each end for safe exit in case of a ground fault.

8- Service equipment, switchboards, panel-boards, and motor control centers installed indoors must be provided with adequate lighting for the safety of electrical workers servicing such equipment from the front or rear when live parts are accessible. Lighting fixtures can be incandescent or fluorescent as long as they provide the proper lighting for parts to be serviced.

Lighting fixtures must have a head room clearance of at least 6.5 ft. (2 m) to give personnel sufficient room to stand in front of electrical equipment without a treat of their head or head gear contacting metal etc.

9- A minimum headroom clearance of 6.5 ft. (2 m) must be maintained from the floor or platform up to the lighting fixture or any overhead obstruction.

10- The general rule of protecting live parts (energized) from accidental contact is by installing them in a complete enclosure which provides a dead front is preferred. Some times it is not practical to construct enclosures to house large control panels and etc. and in such cases if the apparatus is rated at 50 V or more, then suitable guards or isolation should be provided by one of the following rules:

a- The live parts in electrical equipment that is not mounted in a completely enclosed enclosure to be installed in a room, vault, or similar enclosure that is accessible only to qualified personnel.

b- By separating live parts by permanent partitions or screens so located that only qualified persons will have access and reach of the live parts.

c- By locating the exposed live parts on a suitable balcony, gallery, or platform that is high enough or designed in such a manner to keep unqualified personnel out.

d- Permits the live electrical parts to be elevated at least 8 ft. (2.5m) above the floor or other working surface.

29 CFR 1910.304 Wiring Design and Protection:

Conductors used in electrical wiring systems must be identified properly to protect personnel working on such systems. It is essential to know which conductor by color represent the ungrounded hot phase conductor (live), the grounded neutral conductor (neutral), and equipment grounded conductor (earth) in electrical circuit. Because it is by color coding that conductors are connected to color coded terminals of equipment.

The grounded neutral conductor of a branch circuit shall be identified by a continuous white or natural gray color. It is an energized circuit conductor that is connected to the earth through the system ground.

The equipment grounding conductor of a branch circuit shall be identified by continuous green color or a continuous green color with one or more yellow stripes unless it is bare. It is not an energized conductor under normal conditions.

The ungrounded phase conductors can be identified with any color other than those used for the grounded neutral conductors or equipment conductors ( it could be Black, Blue, or Red)

Reversed Polarity:

Many pieces of equipment will operate properly even though the supply wires are not connected in the order designated by design or the manufacturer. Improper connection of these conductors is most prevalent on the smaller branch circuit typically associated with standard 120 volt receptacle outlets, lighting fixtures and cord - and plug - connected equipment.

When plugs, receptacles, and connectors are used in an electrical branch circuit, correct polarity between the ungrounded (hot) conductor, the grounded (neutral) conductor, and the grounding conductor must be maintained.

Reversed polarity is a condition when the identified circuit conductor (the grounded conductor or neutral) is incorrectly connected to the ungrounded or (hot) terminal of a plug, receptacle, or other type of connector.

For example: if the hot lead by accident was connected to the white lead (neutral) of a light socket, the shell of the socket would be energized with a hot conductor. An employee changing a light bulb could receive a fatal electrical shock.

The figure below shows an extremely dangerous situation. In this example, the black (ungrounded) and green (grounding) conductors have been reversed. The metal case of the equipment is at 120 volts with reference to the surroundings. As soon as a person picks up the equipment and touches a conductive surface in their surrounding, he will receive a serious, or even deadly, shock.

Although the equipment will not work with this wiring error, it would not be unusual for a person to pick up the equipment before realizing this. The person may even attempt to troubleshoot the problem before unplugging the power cord.

Ground-Fault Circuit-Interrupters (GFCI's)

A ground-fault circuit-interrupter is not an over-current device like a fuse or circuit breaker. GFCI's are designed to sense an imbalance in current flow over the normal path.

The GFCI contains a special sensor that monitors the strength of the magnetic field around each wire in the circuit when current is flowing.

The magnetic field around a wire is directly proportional to the amount of current flow, thus the circuitry can accurately translate the magnetic information into current flow.

If the current flowing in the black (ungrounded) wire is within 5 () milliamperes of the current flowing in the white (grounded) wire at any given instant, the circuitry considers the situation normal. All the current is flowing in the normal path. If, however, the current flow in the two wires differs by more than 5 mA, the GFCI will quickly open the circuit. This is illustrated in the following figure

Note that the GFCI will open the circuit if 5 mA or more of current returns to the service entrance by any path other than the intended white (grounded) conductor. If the equipment grounding conductor is properly installed and maintained, this will happen as soon as the faulty tool is plugged in.

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