SUBSTATION SHIELDING AND GROUNDING PRACTICES BASIC INFORMATION

The following are recommendation based on IEEE STD 525-1992

Shielding practices
a) The cable for computer or high-speed data logging applications, using low-level analog signals, should be made up of twisted and shielded pairs. For noncomputer type applications, such as annunciators, shielding may not be required.
b) Twisting and shielding requirements for both digital input and digital output signals vary among different manufacturers of computerized instrumentation systems. Separation of digital input cables and digital output cables from each other and from power cables may be required.

Where digital inputs originate in proximity to each other, twisted pair multiple conductor cables with overall shield should be used or multiple conductor cable with common return may be permitted, and overall shielding may not be required.

Digital output cables of similar constructions may also be permitted. Individual twisted and shielded pairs should be considered for pulse-type circuits.
c) Cable shields should be electrically continuous except when specific reasons otherwise dictate. When two lengths of shielded cable are connected together at a terminal block, an insulated point on the terminal block should be used for connecting the shields.
d) Shields should be isolated and insulated except at their selected grounding point to prevent stray and multiple grounds to the shield.
e) At the point of termination, the shield should not be stripped back any further than necessary from the terminal block.
f) The shield should not be used as an electrical conductor except for neutralizing transformer excitation.
g) For signal circuits, the shield must not be part of the signal circuit. Furthermore, the use of shielded, twisted pairs into balanced terminations greatly improves transient suppression. It is never acceptable to use a common line return both for a low-voltage signal and a power circuit.
Grounding practices
a) All shields should be grounded in accordance with provisions above.
b) Signal circuits, if grounded, should be grounded at only one point.
c) Digital signal circuits should be grounded only at the power supply.
d) The shields of all grounded junction thermocouple circuits and the shields of thermocouple circuits intentionally grounded at the thermocouple should be grounded at or near the thermocouple well.
e) Multipair cables used with thermocouples should have twisted pairs with individually insulated shields so that each shield may be maintained at the particular thermocouple ground potential.
f) Each resistance temperature detector (RTD) system consisting of one power supply and one or more ungrounded RTDs should be grounded only at the power supply.
g) Each grounded RTD should be on a separate ungrounded power supply except as follows:
h) Groups of RTDs embedded in the windings of transformers and rotating machines should be grounded at the frame of the respective equipment for safety. A separate ungrounded power supply should be furnished for the group of RTDs installed in each piece of equipment.
i) When a signal circuit is grounded, the low or negative voltage lead and the shield should be grounded at the same point.

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