9.1 PURPOSE

This section provides guidelines to

1. complement existing electrical codes and recommend industry standards,

2. improve electrical safety in the work environment for personnel within the DOE complex.

3. eliminate the ambiguity and misunderstanding in design, construction and implementation requirements for electrical/electronic equipment, and

4. assist the Authority Having Jurisdiction in providing information for acceptance of equipment within the scope of this document.

9.2 SCOPE

This section addresses enclosed electrical/electronic equipment electrical safety guidelines which are not specifically addressed elsewhere in the Electrical Safety Handbook.

These types of equipment include: instrumentation and test consoles; enclosed electrical/electronic equipment; other laboratory diagnostic electrical/electronic equipment (stationary or mobile) mounted in or on an enclosure or chassis; and special electrical/electronic equipment facility requirements.

9.3 GROUNDING AND BONDING

Many ground system types exist within electrical equipment. All metal parts of electrical equipment enclosures and chassis shall be bonded and grounded as per the NEC. The methods chosen to avoid ground loops and reduce noise shall meet the requirements of the NEC (See Section 4).

9.3.1 OBJECTIONAL CURRENT OVER GROUNDING CONDUCTORS

Enclosed Electrical/Electronic equipment has both power and signal conductors entering and leaving these enclosures. Objectional currents and noise may be the result of the design or installation of conductors and equipment and their grounding locations. NEC Section 250-21 addresses these objectional currents and noise (See Section 10.9.2.1).

NEC Section 250-21 must be used with care because it seems to give blanket authority to do whatever is necessary to stop objectionable currents from flowing in the grounding system. This is not the intent. This article specifically excludes modifying grounding systems in order to overcome noise that may be causing problems in sensitive electronic equipment.

This NEC Section principally deals with objectionable currents that can flow over grounding conductors due to severely unbalanced loads or improper installation practices. Because of the complexity and number of interconnections of most grounding systems, the NEC allows modifications of the grounding system and connections in order to address such problems. Those permitted:

1) Arrangement to prevent objectionable current. Grounding of electric systems, circuit conductors, surge arresters, and conductive noncurrent-carrying materials and equipment shall be installed and arranged in a manner that will prevent an objectionable flow of current over the grounding conductors or grounding paths.

Use of a single-point grounding system, as well as meeting the other requirements of Article 250, will usually overcome problems.

2) Alterations to stop objectionable current. If the use of multiple grounding connections results in an objectionable flow of current, one or more of the following alterations are permitted to be made, provided that the requirements of NEC Section 250-51, are met. Such permitted alterations are:

1. Discontinue one or more, but not all, of the grounding connections;

2. Change the locations of the grounding connections;

3. Interrupt the continuity of the conductor or conductive path interconnecting the grounding connections; and/or

4. Take other suitable remedial action satisfactory to the authority having jurisdiction.

3) Temporary currents resulting from accidental conditions, such as ground-fault currents, that occur only while the grounding conductors are performing their intended protective functions shall not be classified as objectionable. This does not prohibit changes in the system to correct excessive current during a fault condition.

4) Limitations of permissible alterations. The intent of NEC Section 250-21 is not that of permitting electronic equipment to be operated on AC systems or branch circuits that are not grounded as required by Article 250. Currents that introduce noise or data errors in electronic equipment are not considered to be the objectionable currents addressed in this Section.

Voltage differences do exist because impedances to ground are not equal throughout a grounding system due to variations of the resistance of the earth, improper connections, or other problems.

Even though voltage differences allow unwanted currents to flow in the grounding conductors, and induced noise may travel over this path, it is not to be used as a reason to disconnect the grounding connections.

9.3.2 EQUIPMENT GROUNDING CONDUCTOR

The equipment grounding conductor of a power-supply cord or interconnecting cable should be at least the size of the largest circuit conductor in the power-supply cord or interconnecting cable. (ANSI/ISA-S82.01-1988 - Section 9.5.4.2)

9.3.3 ENCLOSURE GROUNDING AND BONDING

Enclosure grounding and bonding should comply with the following requirements: (See Figs. 9-1 thru 9-3)

1) Have a common grounding or bonding bus (normally a cabinet rail).

Figure 9-1

Figure 9-2

2) When the enclosure contains more than one bay, bond all grounding or bonding busses together.

3) All mounted chassis within rack cabinets shall have a grounding or bonding conductor attached to the common grounding or bonding bus when the chassis is not grounded or bonded through the power cord.

4) The grounding or bonding conductor shall be permanent and continuous.

5) Subassemblies mounted in other types of enclosures should be bonded by adequate preparation of the mounting surfaces or by the use of a bonding conductor.

6) To provide protection against grounding or bonding conductor breakage, conductors between the common grounding or bonding bus and moveable chassis should be braided cable or stranded wire.

All grounding or bonding points should be tight for good continuity, identified by green color, permanently labeled, and properly prepared by cleaning metal surfaces to bare metal or by the use of serrated bushings. Anodized aluminum must be cleaned to bare metal.

The resistance across the bonding point should not be more than 0.1 ohm. If a measurement is required, the method of measurement is to be determined by the user. (See ANSI/ISA - S82.01-1988 - Section 9.5.2)