Inductive Coupling Situation - hdbk109220040182

DOE-HDBK-1092-2004

Figure 10-6. The pulse power supply, its cable, load, and return form a transmitting loop

which couples into the loop formed by the controller, its multiconductor cabling, 1/0 and

return. Note that in actual installations these loops can be very large and very close.

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 F, 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 do 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 (EMI) that may affect the cabinets. The cabinet ground remains separate from the do

signal ground until it terminates at the master ground bus.

10-19