Improper grounding of the electrical switchgear, motor control centers, and machinery has been a source of electrical noise for reactor instrumentation A ground bus with a rating equal to the rating of the largest circuit breaker in the structure should extend throughout the largest of the switchgear assemblies. Each enclosure should be grounded directly to the ground bus The frame of each circuit-breaker unit should be connected to the ground bus through a separate ground contact device except when the primary disconnecting devices are separated a safe distance.

All other equipment requiring ground connections should be connected to the main ground bus by a copper bar or stranded copper cable The terminal fittings should be pressure-type solderless connections. All contact surfaces at splices should be silver-plated. The ground bus should have the same rating throughout the length of the cabinet and switchgear assemblies Tapered ground buses should not be used At each end of the switchgear assembly, cabinets and panels, and motor control centers, provisions should be made for connecting the ground bus to the station grounding system, these connections should consist of silvered sections of the bus

10- 5.4 Grounding of Instrument Panels and Cabinets

Grounding the instrument panels and cabinets does not normally involve the massive amounts of metal required in grounding power systems. Nevertheless, the principles are the same, and all equipment cabinets, racks, etc., must be electrically bonded together and connected to a common ground point These connections should be either welded or bra/ed, particularly the connections to the ground bus from the equipment.

Each instrument-rack structure must be equipped with an electrical grounding bus This bus is generally a 1 25-in. copper bar (Fig. 10.4) mounted in the lower section of the structure. The bus is provided with a means for connecting it to the plant grounding system. Maximum resistance measured from the grounding bus to the building ground should be less than 1 ohm. Where electrical grounding of equipment is required, the structure frame must not be used as a ground path A conductor from the equipment to the ground bus must be provided.

So that electrical noise will not be induced in the instrument circuits, the circuit components must not use the equipment frame ground conductor as a ground. Currents in an equipment ground conductor can cause a voltage drop along the ground conductor, thus changing the point of reference for any circuit using the conductor.

Figure 10.16 shows the grounding system for a series of instrument racks and cabinets scattered throughout the reactor plant, including an instrument panel located in an adjacent building

The independent grounding system for reactor instru­mentation shown in Figs 10 15 and 10.16 terminates at an instrument grounding well and is isolated from the building grounds. The most widely accepted method employs a single-point grounding system for reactor instrumentation that is terminated at one point to the building ground. This latter method provides a ground at the amplifier cabinets in the control room, however, in some nuclear stations the grounding may terminate at the reactor near the neutron detectors.

If an independent grounding system is used, it should be entirely separate from the power grounding system. Several separate areas or zones of instrumentation, divided as to type and location, should be provided with individual grounding buses or conductors that can be connected to the independent grounding system or left floating, as operating experience dictates

An independent ground system requires that all instru­mentation be constructed with the signal grounds insulated from frames, chassis, power-supply grounds, etc. Separation of the two grounding systems involves some difficult practical problems For example, any sensor, amplifier, or other component normally grounded to its housing requires special construction, therefore the type of grounding system used must be decided in advance.

Because of inevitable ground-loop currents over any appreciable length of building ground bus and for added reliability, relays for control circuits should always be operated with one twisted pair of wires per relay. In addition, neither of the relay control wires should be grounded except at the controlling location since to do so may’ introduce circulating currents in the grounded wire as well as m the ground bus.

It is good engineering practice to install suppression devices on control relays to attenuate interference from relay operation. Such devices as diodes, thyristors, or capacitor—resistor networks can be used for this purpose.