In order to ensure the control cable in the event of insulation breakdown, mechanical damage or fire, reduce the scope of the spread, the national standard GB50217-91 "Power Engineering Cable Design Specification" specifies: dual protection current, voltage and DC power supply and trip control loop, etc. Two systems that require enhanced reliability should use separate control cables.

After the control cable is put into operation, there is a problem of electrical interference between the different cores of the same cable and between the cables laid in parallel, and the main causes of electrical interference are:

(1) Electrostatic interference generated by the externally applied voltage on-line capacitive coupling between cores;

(2) Electromagnetic induction interference due to energization current. In general, when there is a high-voltage, high-current interference source nearby, the electrical interference is more serious. Because the distance between the cores of the same cable is small, the degree of interference is much greater than that of the parallel cable. For example, the control circuit of a phase-division operated circuit breaker of an ultra-high voltage substation, a three-phase combined cable, has had such an accident, the pulse of the phase separation operation causes the thyristors of other phases to be triggered, and the three-phase linkage is caused by mistake. With separate cables, no accidents occurred. Another example is the computer monitoring system of a power plant. Since a four-core cable is used to mix the analog low-level signal line with the power line of the transmitter, it has caused a 70V interference voltage on the signal line, which is in millivolts. The low level signal loop will obviously affect normal operation.

Regarding the grounding method of the shielding layer, attention should be paid to the following points:

(1) The control cable shielding layer of the analog signal loop of the computer control system should be grounded at a centralized point. The reason is based on the requirement to ensure the normal operation of the computer monitoring system, because even the interference voltage of only about 1V may cause the logic judgment to be delayed, and the grounding circulation can be avoided by concentrating a little grounding;

(2) Except that the control cable shield of the computer monitoring system is only allowed to be grounded at one point, the other control cable shielding layer should be grounded at two points when the electromagnetic induction interference is large, and the interference caused by static induction is large. Then use a little grounding;

(3) The inner shield of the double shield or composite shield should be grounded at one point, and the outer shield can be grounded at two points;

(4) When selecting two points of grounding, it should also be considered that under the action of transient current, the shielding layer will not be burned.