Capacitive Coupling Situation

DOE-HDBK-1092-2004

Figure 10-4. Capacitance between

Figure 10-5. A shield has been

circuit 1 and circuit 2 is allowing

interposed between circuit 1 and circuit

current to be transferred into circuit

2. The coupling will be reduced as it

2 via an electric field. This current

shunts the coupled current around Z2

flows through Z2, the impedance of

instead of through it. The interfering

circuit 2, and develops an

voltage could be increased instead of

interfering voltage.

decreased if not properly shunted.

3. Inductive Coupling. (Near-field, low-impedance loop-to-loop coupling) The technique for

increasing resistance to magnetic coupling in shielded cables is to ground both ends of the

shield to an effective signal return ground as is shown in Figures 10-6 and 10-7.

4. System Signal Returns. Each installation will require individual analysis and treatment. A

single ground poses no problem, but multiple grounds can result in a ground loop. These can

upset the proper functioning of instruments. A signal isolator offers a way of overcoming the

problem.

5. Instrumentation Grounding2. Equipment that is used to implement a control instrumentation

strategy (see Figure 10-8) makes use of a common signal ground as a reference for analog

signals. Any additional grounds that are introduced into the control circuit will almost

certainly cause ground loops to occur.

A typical process instrumentation loop is shown in Figure 10-8. It is a DC system that operates

at a specific voltage (24 volts in this case) to a master ground reference called a signal ground.

The instrumentation signal varies within the range of 4-20 mA, depending upon the value of the

variable (pressure, temperature, etc.) seen by the sensor. A precisely calibrated circuit takes

this mA signal and converts it into a form that can be used by a process-control computer, PLC,

The information in this section and figures 10-8 and 10-9 is reprinted with permission from the September

10-18