10- 5.1 Electrical Noise Problem

Establishing a common ground may be the goal of electrical machinery design, but it creates problems in data measuring systems Ground-loop currents between pieces of equipment that are gounded at separate points to

a common ground introduce voltages that can affect measurements. Differences in potential between various points in a grounding system are not uncommon. These differences are caused by stray currents of any origin m the system, such as faults or transients on electric-power equipment If low-signal-level instrumentation has multiple connections to ground, either bv intention or accident, these potential differences can result in ground-loop cur­rents.

Difficulties attributable to grounding have been experi­enced in several nuclear-power-reactor installations and are difficult to locate For this reason a single-point grounding system is used in nuclear-power-plant reactor instrumenta­tion systems An independent grounding system, isolated from the building grounds, has been installed The concept of an independent grounding system has advantages, how­ever, it will not eliminate capacitive coupling or leakage resistance to ground, which also results in ground-loop currents.

The final design of instrumentation grounding depends on several factors, particularly the types of reactor instru­mentation to be used and the nature of the reactor-building grounding system. Intricate reactor instrumentation sy s­tems almost always require some extensive modifications of grounding connections after the equipment is installed to obtain satisfactory operation

10- 5.2 Grounding System for Reactor Building

The grounding system for a nuclear power station must provide for (1) instrumentation-system grounding, (2) ground connections for grounded neutral power sys­tems, (3) a discharge path for lightning arrestors,

(4) grounding of equipment frames and housings to protect equipment and personnel from dangerous electrical poten­tials caused by faults, and (5) communication and fire — alarm-system grounding

Figure 10.15 shows a system using a grounding grid for a nuclear power station. A properly installed grounding grid with its associated grounding rods or grounding wells should have a total resistance across the entire grid of less than 0.2 ohm. Some nuclear power stations may use means other than a grounding grid for grounding between build­ings, containment spheres, and other major systems. The essential requirement is that all major systems, subsystems, and equipment be thoroughly grounded with ample size grounding conductors and proper grounding connections.

The grounding of a reactor building should be well established All grounding connections to stainless-steel equipment and piping should be made to stainless-steel stringers or saddles welded to the equipment with, if possible, thermite welds Grounding connections should be accessible Two or more grounding connections are recom-

mended for large equipment An equipment ground con­ductor should be included for all power circuits entering the containment. Penetrations of the independent instru­mentation grounding conductors must be insulated from the containment