DC Power Distribution

DOE-HDBK-1092-98

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 dc 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 dc circuits as well.

An accessible terminal charged by an internal capacitor should be below 50 volts within 10 seconds

after interruption of the supply. (ANSI/ISA-S82.01-1988 - Section 9.3.5.2)

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 C62.11-1986.)

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.

9-11