The more common types of surge arresters used with electronic equipment are the Metal oxide varistor (MOV), avalanche diodes, and spark gap arresters. The type and electrical rating of the surge arrester is generally determined by the requirements of the circuit being protected, and by the amplitude and duration of the expected surge. (See ANSI/IEEE C62.11-1986.)

Metal oxide varistors and avalanche diodes are voltage-dependent devices whose impedance changes from a near-open circuit to a highly conductive level when subjected to transient voltages above their rated voltages. A MOV is considered "sacrificial" in that a portion of its material is literally burned-off each time such a surge is encountered. The response time of a MOV is limited to approximately 500 picoseconds while avalanche diodes can respond in approximately 50 picoseconds. Lead lengths can greatly increase the response times of these devices. The normal failure mode of both devices is a short circuit although sustained voltages well beyond the rating of the MOV can cause the device to rupture and result in an open circuit. When used at a point on a circuit, a surge arrester should be connected between each ungrounded conductor and ground.

For power line applications, MOV manufacturers recommend a varistor be used with a fuse that limits the current below the level that MOV package damage could occur. In general, circuit breakers are not recommended for this application since circuit breaker tripping is too slow to prevent excessive fault energy.

Consult the manufacturer's application data sheets for more information.

9.6.2 FUSES

Fuses are temperature-sensitive, current-sensing elements that are generally used as short circuit protective devices in individual electrical chassis. The fusing characteristic, or opening time versus current, must be within the safe time/temperature characteristic of the device being protected.

Designers must carefully consider the load requirements in the fuse selection process, particularly when high surge currents may be encountered during initial turn-on. Operating time/current characteristics of the various types available can usually be found in fuse manufacturers catalogs. A fuse's interrupting current capacity must also be considered when connected to a power distribution system having a significant fault current capacity.

The voltage rating on a fuse shall be equal to or greater than the device's operating voltage.

In general, cartridge fuses should have a disconnecting means on the supply side, (NEC Section 24040), and shall not be connected in parallel unless factory assembled and listed as a unit (NEC Section 240-8).

9.6.3 CIRCUIT BREAKERS

A chassis or cabinet shall not employ circuit breakers as "on/off' switches unless rated for the application by the manufacturer.

9.6.4 POWER INTERLOCK DEVICES

Cabinets and equipment having potentially dangerous currents and/or voltages present should have a means of controlling access, or a power interlock device designed to interrupt the power to the cabinet. Provisions shall also be made to discharge any stored energy, such as in capacitors or inductors, to less than 50 volts within 10 seconds when the safety interlock is opened. Interlocks may not be used as a substitute for lockout/tagout. (29 CFR1910.333(c) and ANSI/ISA S82.01-1988, Section. 9.3.5.2)

9.7 DISCONNECTING MEANS

All enclosed electrical/electronic equipment shall be provided with a means for disconnecting it from each external or internal operating energy source. This disconnecting means shall disconnect all current carrying conductors.

9.7.1 GENERAL

Interlock systems are not a recommended disconnecting means for cabinets and equipment having potentially dangerous currents and/or voltages present. (See Section 9.6.4)

Permanently connected equipment and multi-phase equipment should employ a switch or circuit breaker as means for disconnection.

All cord-connected equipment should have one of the following as a disconnecting device:

1. A switch or circuit breaker,

2. Plug that can be disconnected without the use of a tool, or

3. A separable plug, without a locking device, to mate with a socket-outlet in the building.

Where equipment is connected to the source of supply by flexible cords having either an attachment or appliance plug, the attachment or appliance plug receptacle may serve as the disconnect (ANSI/ ISA - S82.01 - 1988 - Section 13.10.1 and NEC Section 422-22).

Where a switch is not part of a motor, motor circuit or controller, the disconnecting means should be within 50 feet and in sight of the operator and marked as the disconnection device for the equipment.

Where a disconnecting means is not part of the equipment, the disconnecting means should be near the equipment, within easy reach of the operator during normal operation of the equipment, and marked as the disconnection device for the equipment.

If a disconnecting device is part of the equipment, locate it as close as practical to the input power source.