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Wiring and Grounding

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# Power quality problems are related to wiring and grounding.

# Many power quality problems are solved just by tightening a loose connection or replacing a corroded conductor.

# Therefore, the study on wiring and grounding is necessary to evaluate power quality problems.

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Summary of US Practices for Building Wiring and Grounding

• National Electric Code (NEC) published by the National Fire Protection Association (NFPA) as ANSI/NFPA Standard 70.– A circuit conductor that is intentionally grounded is a

grounded conductor.– A conductor that connects equipment or a grounded

circuit to the grounding electrodes is a grounding electrode conductor.

– Examples of grounding electrodes are grounded building steel, grounded water pipes and driven ground rods.

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Transformer

N

Service Entrance

WhiteRed

Black

Green or barewire or metal conduit

Hot wire insulation colors are Red and Black on 120/240 V single phase system.Neutral always has white insulation and equipment grounding conductor is either bare or has green insulation.

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N

Service Entrance

WhiteRed

Black

Green or Bare

Grounding electrodeconductor

Bonding jumper

120 V240 V

Equipment grounding conductor

Building grounding electrode

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Discussion• Neutral will carry normal currents on 120 V

circuits, but equipment grounding conductor will not carry normal currents

• Hot wires carry normal load currents on 240 V circuits

• Equipment grounding conductor provides a safety ground

• Equipment grounding conductor may be metallic conduit or separate bare or green wire

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Transformer

N

Service Entrance

WhiteGreen or Bare

Three-phase system, note that grounded conductor (neutral) and equipment grounding conductor (green) are bonded at service entrance.

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Reasons for grounding

1.Personnel safety-Dangerous touch potential

2. Grounding to assure protective device operation

3. Noise control-Equipotential ground system

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Typical Wiring and Grounding Problems

• Problems with conductors and connectors

• Missing safety ground

• Multiple neutral-to-ground connections

• Ungrounded equipment

• Additional ground rods

• Ground loops

• Insufficient neutral conductor

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Service Entrance

Note that grounded conductor (neutral) and equipment grounding conductor (green) are bonded at service entrance, not at panel boards.

Panel Board

N

G

Feeder Branch Circuit

Solutions to Wiring and Grounding Problems

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Service Entrance Panel Board

N

G

Branch CircuitReceptacle

Load

Load current return path may include the neutral but not the grounding conductors

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Service Entrance Panel Board

N

G

Branch CircuitIsolated Ground Receptacle

Load

Special case: Isolated ground receptacle (orange color) can be used if induced noise is a problem

IG

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Transformer

N

Service Entrance

Special case: separately derived system grounded to building steel or metallic cold water pipe

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Comments• Never install a separate ground rod for the

“isolated ground” but bond to ground at the service entrance or at the transformer of a separately-derived system

• If noise is really a problem, use an isolation transformer (creating a separately derived system) nearby, ground to building steel

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Service Entrance

Building steel

Isolated Ground Receptacle

Load

Isolationtransformer

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Summary of Wiring and Grounding Solutions

The grounding system should be designed to accomplish the following minimum objectives:

• There should never be currents flowing in the grounding system under normal operating conditions. One can likely measure very small currents in the grounding system due to inductive coupling, capacitive coupling, and the connection of surge suppressors and the like. In fact, if the ground current is exactly zero, there is probably an open ground connection. However, these currents should be only a tiny fraction of the load currents.

• There should be, as near as possible, an equipotential reference for all devices and locations in the system.

• To avoid excessive touch potential safety risks, the housings of all equipment and enclosures should be connected to the equipotential grounding system.

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The most important implications resulting from those objectives are:

• There can only be one neutral-to-ground bond for any subsystem. A separately derived system may be created with a transformer, which establishes a new neutral-to-ground bond.

• There must be sufficient interconnections in the equipotential plane to achieve a low impedance over a wide frequency range.

• All equipment and enclosures should be grounded.