5.13 Grounding in Facilities: a Brief Review

The term grounding has many meanings. In utility power, the term means a connection to earth or its equivalent. In an aircraft or an automobile, the metal framework is the equivalent of earth. In a building, one of the power conductors at the service entrance must be earthed to provide a path for lightning. If lightning should hit the utility wiring, the intent is to provide an earth path for current flow outside of the facility. For uniformity, these grounding rules are followed even if the power is brought into a facility underground.

The grounding electrode system in a building includes such diverse items such as building steel, plumbing, gas lines, conduit, metal floor mats, boilers, motor frames, air ducts, and all equipment grounding conductors. In effect, all of the conductors in a building that could contact a power conductor must provide a fault path back to the service entrance ground. There can only be one grounding electrode system in a facility. The grounding electrode system can be multiply earthed. The grounding electrode system cannot be used as a path for power current.

Equipment grounding conductors (safety conductors, bare wires, or green wires) are used for fault protection and for shock avoidance inside a facility. Equipment grounding conductors are a part of the grounding electrode system. Equipment grounding conductors must be installed to carry any fault currents back to the grounding point at the service entrance. To keep the inductance in the fault path low, the equipment grounding conductors must parallel the path taken by the power conductors. Equipment grounding conductors must be run inside the same conduit as the power conductors. All metal enclosures that could contact a power conductor must be connected to an equipment grounding conductor. A fault to this conductor must trip a breaker. In very special cases, a temporary fault condition may simply cause an alarm rather than disrupting the power. Examples might be in electric furnaces or on assembly lines. This approach is permitted if an electrician is on duty at all times.

A facility can have several sources of separately derived power. In these systems, there can be separate neutrals, feeders, and equipment grounding conductors. Examples are auxiliary generators, special distribution transformers, or battery backup systems. Separately derived power sources are treated like a new service entrance. Each separately derived neutral is connected once to the nearest point on the grounded electrode system of the facility. Equipment grounds associated with this power are returned to this same point.

A connection to earth is rarely below 10 ohm. If there were two such connections, the total resistance might be 20 ohm. Assume that a facility has two earth grounds (isolated from each other). If a hot lead somehow connects to earth, the earth resistance is placed across the power line. This would allow 6 A to flow. In most branch circuits, this current is not high enough to trip a breaker. This means that somewhere in a facility, there could be “grounded” conductors that are physically close together but that are 120 V apart. A person touching these grounds could be electrocuted.

The impedance between all grounded (power) conductors must be in the milliohms at power frequencies. It is preferable to bring separate sources of power into a facility in the same physical area.