Noise Coupling Mechanisms

DOE-HDBK-1092-2004

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.

2. Coiling the extra length on a 6-inch diameter (about 12 turns).

3. Securing it tightly wound with cable ties.

4. Connecting it into the circuit.

These actions satisfy the NEC requirement for a continuous ground and noise isolation is also

enhanced.

Whatever scheme is used, the ground of experimental equipment shall be connected to the

same ground as the facilities' electrical system to ensure equal potential.

For practices involving hazardous materials, such as explosives, the grounding shall also

comply with the requirements of Section 5.0, Special Occupancies.

10.9.2.1.3 NOISE COUPLING MECHANISMS.

Grounding can reduce the interference in the five types of coupling mechanisms listed here.

1. Conductive Coupling. (Source and load wired together) It is sometimes practical to provide a

separate return path for both the source and the load. If the system layout allows this, then

conductive coupling cannot occur between these two, as is shown in Figures 10-2 and 10-3.

2. Capacitive Coupling. (High-impedance proximity coupling) The technique for increasing

resistance to capacitive coupling among cables is to ground one end of the shield to produce

the shortest, most direct shunt path back to the source of the coupled current as is shown in

Figures 10-4 and 10-5.

Caution: It is possible to inadvertently increase coupling between source and load if the shield

ground does not properly shunt the current coupled onto the shield.

10-16