Grounding - hdbk10920212

DOE-HDBK-1092-98

10.9.2.1

GROUNDING

10.9.2.1.1 HAZARDS

The lack of proper grounding can cause electrical shock and/or burns to personnel. The NEC and

NESC define legally-required grounding. To mitigate potential hazards, grounding shall be provided

in accordance with the NEC and NESC.

10.9.2.1.2 DESIGN AND CONSTRUCTION

NEC, Article 250, "Grounding" notes that grounds also provide:

1. Voltage limitation in case of lightning, line surges, or unintentional contact with higher voltage

lines

2. Stability of voltage to ground under normal operation

3. Facilitated overcurrent device operation in case of ground faults

4. A path to conductive objects that will limit the voltage to ground.

In R&D work there is one additional function for grounds: a common reference plane or system

ground return for electronic devices, circuits, and systems. (See Section 9.3) It is recognized that such

grounds are essential in some cases to control:

1. Noise associated with the primary power system:

a. Incoming on the line

b. Outgoing from local equipment

2. Ground wire noise

3. Circuit coupling

a. Ground loop (shared circuit return)

b. Magnetic, capacitive, or electro-magnetic.

If system return impedances are low enough, then simple radio-frequency chokes can be used to limit

this noise with no effect on the safety function.

A 50-microhenry choke will add 1/50 of an ohm at 60 Hz, but will look like 2 ohms at 7.5 kHz and

30 ohms at 100 kHz. Such an RF choke will serve to discriminate against noise on the ground circuit.

An inexpensive RF choke may be installed in the safety ground by:

1. Pulling the green ground wire 20 feet longer than required.

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