Electrical Safety Standards Chapter 44 Electrical Safety Standards National Electrical Codeâ (NECâ) • Hazardous Locations Classifications • Electrical Protections • Electrical Enclosures • Hazardous Area Wiring • Intrinsically Safe Systems

The NEC® divides hazardous locations into classes, divisions, and groups based on the chemicals handled in that area. The classification system evaluates the various types of hazardous substances and groups them according to their type and potential for hazard. Electrical classifications for hazardous locations are identified by class, division, and group. See Figure 44-1. It is the responsibility of knowledgeable personnel at each manufacturing facility to specify the appropriate hazardous zone, location, size, and classifications. Each area has specific wiring requirements.

NEMA classifies electrical enclosures according to intended use. An enclosure is a case or housing for equipment or other apparatus that provides protection for control-lers, motor drives, or other devices. See Figure 44-2. The National Electrical Manufacturers Association (NEMA) classifies enclosures according to their purpose and intended resistance to the ingress of moisture and hazardous materials. In the United States, equipment is tested to the NEMA 250 standard. Some enclosures are designed to include cooling units to remove heat generated by the enclosure contents. The most common types of enclosures are discussed below. There are other, specialized NEMA classifications that are not discussed here.

Purged enclosures are kept at a positive pressure to prevent flammable gases from entering the enclosure. A purged enclosure is a nonhazardous enclosure that is pressurized and purged with air to allow it to be used in hazardous areas. Some enclosures cannot be made explosionproof. This may be because of the required size or because of non-explosionproof instruments that must be mounted in the face of the cabinet. However, such an enclosure must be located in a hazardous area. See Figure 44-3.

Enclosure temperature codes are used to determine which devices are allowed in classified areas. A temperature code is a designation that specifies the maximum surface skin temperature obtained by an enclosure during testing by approval agencies. See Figure 44-4. Any equipment that does not exceed 185°F (65°C) is not required to be marked with the temperature code. The allowed surface temperature limit is determined by the degree of difficulty in igniting a flammable mixture and the amount of heat capacity in the enclosure that prevents a temperature rise and a resulting internal explosion.

Class I, Division 1 wiring requirements include seals to prevent flammable gases from flowing through conduit. All enclosures, even those only containing terminal strips, must be of explosionproof design. All fittings must be explosionproof. Flexible conduit can be used only if it is an explosionproof design. Conduit can become a passageway for explosive gases to move from one area to another. Therefore, seals must be used in every conduit within 18″ of any instrument or junction box, and all conduits must be sealed at the boundary between hazardous and nonhazardous areas. See Figure 44-5.

Class I, Division 2 wiring requirements are slightly less stringent than Division 1 requirements. An EYS seal must be used in every conduit within 18² of any instrument or explosionproof enclosure. All fittings between the instrument and the EYS seal are to be explosionproof. Flexible conduit, if used, can be general-purpose with provision for a continuous ground. All conduits must be sealed at the boundary between hazardous and nonhazardous areas. See Figure 44-6.

Intrinsic barriers prevent electrical hazards from entering a classified area. An intrinsic barrier is a specially designed electronic circuit containing resistors and diodes that is used to prevent any electrical ignition energy from being carried into a hazardous area along power, signal, or control wiring. Intrinsic barriers are available to protect low-level signals such as from thermocouples and RTDs and high-level systems such as 115 VAC power or signals. In addition, barriers are used to send power to transmitters and solenoids. See Figure 44-7.