Terminal/Live Parts

DOE-HDBK-1092-2004

ac power, a warning label should be affixed next to the connector stating: "WARNING - POWER

MUST BE REMOVED BEFORE CONNECTING/DISCONNECTING."

9.5.1.3 TERMINALS/LIVE PARTS

All terminals/live parts with a potential of 50 volts or greater shall be guarded to protect from

accidental contact or bringing conductive objects in contact with them (NEC 110.27). Consult

ANSI/ISA-S82.01-1988, Table 9-1 for spacing information regarding live parts.

All energized switching and control parts shall be enclosed in effectively grounded metal

enclosures and shall be secured so that only authorized and qualified persons can have access.

9.5.2

DC POWER DISTRIBUTION

Guidelines for dc power distribution include:

1. The metal chassis or cabinet should not be used as a return path.

2. High-current analog or switching do power supplies should use separate return paths from

digital circuits.

3. All of the guidelines pertaining to ac power such as grounding, proper wire size, high

voltage, etc. should apply to do circuits as well.

An accessible terminal charged by an internal capacitor should be below 50 volts within 5

seconds after interruption of the supply.

As with ac power, avoid contacting dc parts when working on a live chassis. The use of the

appropriate class gloves should be considered when performing this type of work.

9.6

PROTECTIVE DEVICES FOR ENCLOSED ELECTRICAL/

ELECTRONIC EQUIPMENT

This section deals with the various protective devices commonly found in electrical/electronic

equipment not discussed elsewhere.

9.6.1

SURGE ARRESTERS

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 C62.11-1987.)

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. An 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 an

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.

9-10