Ground loop

Presumably, each chassis is fairly well grounded, and the heater will not affect the controller, except via the signal from the thermocouple. However, some of the power going through the heater can quickly go to the controller through the shield, before the heat gets a chance to build up and operate the thermocouple. This would be a false kind of signal which causes the system to malfunction. 

The experimenter should build the circuit of Fig. 6.4. At this point, things are getting a bit complicated. The result of closing and opening the make-believe switch should be similar to the result of the previous experiment, except that there is slightly less effect. If this is not what happens, possibly go back to the previous circuit, try it again, and then re-attach the 180. (In Fig. 6.3, there was a dashed wire placed around the 150, and of course this must be removed.) 

The disadvantage of the ground loop, in real factory or laboratory equipment, is that the shielding (or other coimection) is carrying stray voltage over to the controller. Therefore, two solutions to the problem are evident. 

One is to make sure the shield (or any other inter-chassis connection) is disconnected, as in Fig. 6.5. The result is nearly obvious, but the experimenter 

1 In real systems, these might be long wires, and therefore poor quality ground connections. 

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The rule of thumb to determine the ground loop impedance is to consider the ground fault current as one and a halftimes that of the overcurrent setting of the circuit breaker for breaker-controlled systems (a fault condition for a breaker) or three times the rating of the fuses, for fuse-protected systems (an overcurrent condition for the fuses). Based on this rule. Table 21.2 suggests the optimum values of ground loop impedances for circuits of different... 

Table 21.2 Maximum impedances of ground loop, when protected by overcurrent releases of circuit breakers or fuses...

The leakage current at hazardous locations such as refineries, petrochemical plants and mines should not exceed 15% of the rated current of the circuit or 5 A, whichever is greater. Table 21.3 indicates the maximum permissible ground leakage currents for such areas at 15% of the rated current and the recommended maximum ground loop impedances. 

Table 21.3 Maximum impedances of ground loop lor protection by ground leakage relays in hazardous areas...

Current rating of circuit Amp Maximum permissible ground leakage current C Amp Recommended maximum ground loop impedance on a 240 V phase to ground circuit 240... 

The vibration indieator as well as the taehometer should read zero. If these instruments are reading a higher number, a diseontinuity or short eireuit may exist in the eleetrieal system. Sometimes, a ground loop on the eleetronie system ean ereate a high level reading on the taehometer, RTD, or vibration monitor. 

Onshore, grounding is generally provided by installing a ground loop, made of bare copper conductors, below the finished grade of the facility. 

Offshore, the equipment and system ground conductors are connected to the facility s metal deck, usually by welding. The metal deck serves the function of the ground loop and is connected to ground by virtue of solid metal-to-metal contact with the platform jacket. 

Central multiple heat pumps with ground loop... 

The first cost is expected to be less than a central heat pump system using ground loop. 

Lower first cost than the central heat pump systems using ground loop. Disadvantages ... 

WLHP with Ground Loop Heat Exchanger... 

CENTRAL HEAT PUMP (CHILLER) WITH GROUND LOOP... 

A central heat pump is used for providing heating water for space heating. The heat source is the ground loop. A stand-by boiler is provided to meet... 

The central heat pump is a reversing type. During summer, the central heat pump acts as a chiller supplying chilled water for cooling and rejecting heat to the ground loop. When there is a demand for simultaneous heating... 

A cooling tower is provided in series with the ground loop to assist during peak loads in extreme weather conditions. The cooling requirement of the... 

Ground loop must be sized to meet the heating and cooling loads during extreme weather conditions. 

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