Wiring and Grounding

The National Electrical Code (NEC) and other important standards provide the minimum standards for wiring and grounding. It is often necessary to go beyond the requirements of these standards to achieve a system which also minimizes the impact of power quality variations (harmonics, transients, noise) on connected equipment.

Typical Wiring and Grounding Problems

1. Problems With Conductors and Connections.
2. Missing Safety Ground
3. Multiple Neutral-to-Ground Connections
4. Ungrounded Equipment
5. Additional Ground Rods
6. Ground Loops
7. Insufficient Neutral Conductor

Solutions to Wiring and Grounding Problems

1. Proper Grounding Practices
2. Ground Electrode Resistance
3. Service Entrance Connections
4. No Neutral-to-Ground Connection at Panelboard
5. Isolated Grounds
6. Separately Derived Systems
7. Signal Reference Grounds

Summary of Wiring and Grounding Solutions

1. There should never be load currents flowing in the grounding system under normal operating conditions. There are likely to be low currents in the grounding system due to the connection of protective devices and coupling between line and ground (in fact, if the ground current is actually zero, there is probably an open ground connection). However, these currents should be negligible compared with load currents.
2. There should be, as near as possible, an equipotential reference for all devices and locations in the system.
3. To avoid excessive touch potential safety risks, all equipment and enclosures should be connected to the equipotential grounding system.

The most important implications resulting from these objectives are:

1. There can only be one neutral-to-ground bond for any sub-system. A separately derived system may be created with a transformer, allowing establishment of a new neutral-to-ground bond.
2. There must be sufficient interconnections in the equipotential plane to achieve a low impedance over a wide frequency range.
3. All equipment and enclosures should be grounded.

Grounding Definitions

1. Definition: Grounding - (IEEE Dictionary Standard 100-1992) A conducting connection whether intentional or accidental, by which an electric circuit or equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth. It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it; and for conducting ground current to and from the earth (or the conducting body).
2. Definition: Grounded System - (Green Book IEEE Standard 142-1991) A system of conductors in which at least one conductor or point (usually the middle wire or neutral point of transformer or generator windings) is intentionally grounded, either solidly or through an impedance.
3. Definition: Grounded Effectively - (Green Book) Grounded through a sufficiently low impedance such that for all system conditions the ratio of zero sequence reactance to positive sequence reactance (X0/X1) is positive and less that 3, and the ratio of zero sequence resistance is positive sequence reactance (R0/X1) is positive and less than 1.
4. Definition: Grounding Electrode System - (Green Book) Defined in NEC Section 250-81 as including: (a) metal underground water pipe; (b) metal frame of the building; © concrete encased electrode; and (d) ground ring. When these elements are available, they are required to be bonded together to form the grounding electrode system. Where a metal underground water pipe is the only grounding electrode available, it must be supplemented by one of the grounding electrodes in Section 250-81 or 250-83.
5. Definition: Ufer ground - (Green Book) A method of grounding or connection to the earth in which the reinforcing steel (rebar) of the building, especially at the ground floor; serves as a grounding electrode.
6. Definition: Equipment Grounding Conductor - The conductor used to connect the non-current- carrying parts of conduits, raceways, and equipment enclosures to the grounded conductor (neutral) and the grounding electrode at the service equipment (main panel) or secondary of a separately derived system (e.g. Isolation transformer) NFPA 70-1990 Section 100.
7. Definition: Ground Loop - A potentially detrimental loop formed when two or more points in an electrical system that are normally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential.
8. Definition: Ground Window - The area through which all grounding conductors, including metallic raceways, enter a specific area. It is often used in communications systems through which the building grounding system is connected to an area that would otherwise have no grounding connection.
9. Definition: Ground Grid - A system of interconnected bare conductors arranged in a pattern over a specified area and on or buried below the surface of the earth. The primary purpose of the ground grid is to provide safety for workers by limiting potential differences within its perimeter to safe levels in case of high currents which could flow if the circuit being worked became energized for any reason or if an adjacent energized circuit faulted. Metallic surface mats and grating are sometimes utilized for the same purpose. This is not necessarily the same as a signal reference grid.
10. Definition: Shield - As normally applied to instrumentation cables, shield refers to a conductive sheath (usually metallic) applied, over the insulation of a conductor or conductors, for the purpose of providing means to reduce coupling between the conductors so shielded and other conductors which may be susceptible to, or which may be generating, unwanted electrostatic or electromagnetic fields (noise).
11. Definition: Coupling - Circuit element or elements, or network, that may be considered common to the input mesh and output mesh and through which energy may be transferred from one to another.
12. Definition: Noise - Unwanted electrical signals which produce undesirable effects in the circuits of the sensitive equipment control systems in which they occur.
13. Definition: CBEMA Curve - A set of curves representing the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance. Developed by the Computer Business Equipment Manufacturers Association (CBEMA), it has become a de facto standard for measuring the performance of all types of equipment and power systems, and is commonly referred to by this name. (Note: We typically employ the curve only form 0.1 cycles and higher due to limitations in power quality monitoring instruments and differences in opinion over defining the magnitude values in the subcycle time frame.)

Note: Early history of the power quality movement has created many colorful phrases to entice potential customers to buy products and services. Unfortunately, many of these terms are ambiguous and cannot be used for technical definitions. The following words have a varied history of usage, and some have specific definitions for other applications. The use of these words for describing power quality phenomena is discouraged unless specifically qualified by descriptive text.

Blackout	Blink	Brownout	Bump
Clean Ground	Dirty Ground	Outage	Interruption
Clean Power	Dirty Power	Raw Power	Glitch
Power Surge	Surge	Spike	Wink

Additional information and site survey information can be obtained by contacting

POC: Mike Helms mike@lightningmike.com