scope:

This application guide was developed to identify instrumentation and control (I & C) equipment grounding methods to achieve both a suitable level of protection for personnel and equipment, and to provide suitable electric noise immunity for signal ground references in generating stations. Both ideal theoretical methods and accepted practices in the electric utility industry are presented. Special applications relating to advanced energy storage and conversion technologies (photovoltaics, fuel cells, etc.) have not been considered.

This guide is intended to provide information about grounding methods for generating station I & C equipment. Grounding design is normally based on the concept of two separate grounding systems: the equipment ground and the signal reference ground. The grounding of instrument chassis, racks, cable sheaths, or cable shields and signal pairs requires special care to ensure that personnel working on equipment are adequately protected against electrical shock hazards during both normal and abnormal conditions, as well as to ensure that interference signals are not inadvertently coupled into signal circuits.

The basic theory and guidelines to be understood before designing I & C grounding are presented in clause 4. Clause 5 provides guidance for grounding of equipment associated with generating station I & C systems and presents various approaches to providing a signal ground system. Clause 6 presents accepted practices in grounding the shields of I & C cables, while clause 7 covers the testing of I & C grounding systems.

Purpose

The typical environment in a generating station provides many sources of electrical noise, such as the switching of large inductive loads, high fault currents, static switching, and high-energy, high-frequency transients associated with switching at the generator or transmission voltage level. The increasing use of solid-state equipment, computer- or microprocessor-based control, and signal multiplexing systems in these applications introduces a number of specific concerns with respect to electrical noise control. This guide discusses methods for the grounding of I & C equipment and their associated circuits in generating stations.

It is intended to provide guidance for the design of grounding systems for I & C equipment specific to the generating station. The low-level electrical signals transmitted from various I & C equipment in a generating station through long cables may undergo signal distortion as they travel to the receiving end. This distortion is typically caused by noise pickup either at the signal source or along the cable run. The level of noise on the received signal can cause errors in measurement and control functions (and in extreme cases, damage to equipment), which in turn may result in costly unit downtime. The use of proper grounding along with proper shielding techniques can solve a large percentage of noise problems. It should be recognized that there are numerous accepted grounding techniques and that the actual installation of a ground system should be made in consultation with I & C equipment manufacturers.

The grounding methods in this guide are intended to minimize degradation of I & C signals in generating stations. By contrast, the station grounding system is mainly oriented toward meeting the requirements of various safety codes that aim to establish a grounding system that will provide a low-impedance path to ground. This low-impedance path helps to ensure that high voltages are not developed on equipment or structures as a consequence of lighting surges, electrical faults, circulating currents, or static charges. This guide is complementary to IEEE Std 665-1995.¹

¹Information about references can be found in clause 2.