Capacitive Coupling Situation

DOE-HDBK-1092-98

Circuit 1

Circuit 2

IC1

V2 = IC1Z2

CAPACITIVE COUPLING SITUATION

CAPACITIVE COUPLING SHIELD

Figure 10-4. Capacitance between circuit 1 and

Figure 10-5. A shield has been interposed between

circuit 2 is allowing current to be transferred into

circuit 1 and circuit 2. The coupling will be reduced

circuit 2 via an electric field. This current flows

as it shunts the coupled current around Z2 instead

through Z2, the impedance of circuit 2, and develops

of through it. The interfering voltage could be

an interfering voltage.

increased instead of decreased if not properly

shunted.

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, dedicated

instrument, or whatever controller that supervises the system. In this example, the mA signal is

converted to a 1-5 V signal for a chart recorder. At 4 mA, the voltage measured by the recorder is

250 x .004 = 1 V. At 20 mA, the measured voltage is 5 V. Normally, the recorder scale is calibrated

so the voltage reads directly in 0F, psi, etc.

In order to minimize the danger of introducing ground loops into this complicated network of

sensitive equipment, a dedicated instrumentation system ground bus is usually employed. This bus

ultimately receives grounds from the signal common, the dc power supply common, the cabinet

ground, and the instrumentation ac power ground. The bus is tied to earth via the building ground

and the plant ground grid. Figure 10-9 shows the typical way in which interconnection of these

various grounds is accomplished.

The cabinet ground is a safety ground that protects equipment and personnel from accidental shock

hazards while providing a direct drain line for any static charges or electromagnetic interference

10-17