wiring and grounding issues

7/27/2019 Wiring and Grounding Issues

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2/21

Typical Wiring and Grounding Problems

Missing Ground Connections

Multiple Ground Connections

Ground Loops

Neutral Sizing

Standards and Solutions


3/21

80 % off all power quality issues are wiring and

grounding issues with in the facility.

National Electric Code (NCE) provides minimumrequirements for wiring.

Often necessary to go beyond standards to limit

impact of variations.


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IEEE Dictionary (Standard 100)

IEEE Green Book (Standard 142)

National Electric Code (NCE)

Definitions: p438-441


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Personal Safety:

To prevent the possibility of high touch voltage

Grounding to assure protective device operation

Proper operation of circuit breakers and fuses


6/21

Problems with Conductors and connectors


7/21

Missing safety ground:

Multiple neutral-to-ground connections

Ungrounded equipment

Additional Ground rods Ground loops

Insufficient neutral conductor


8/21

Missing a ground connection can result in line voltage

present on the exposed surface, no circuit breaker will trip,

very hazardous.


9/21

Only one neutral to ground connection at service entrance

Ground and neutral should be separate at all panels and

junction boxes.

If downstream neutral ground connections, then parallelreturn path through the ground contact.

This can lead to misoperation of protective devices.


10/21

Important in data processing and communication equipment.

When different equipment have different grounds, but

connected through a communications layer. If the grounds

are at slightly different potentials a ground loop occurs.

Introduces major noise issues. Can be avoided by using

optical fiber in the communication layer.


11/21

Switch mode power supplies and fluorescent with electronicballasts introduce third harmonic.

Third harmonic current will add in the neutral instead ofcancelling out as in the main 60 Hz current

Will impact the sizing of the neutral conductor. Can be up to

170% of the fundamental frequency phase current.


12/21

Proper grounding practices:


13/21

Ground Electrode: Electrode Resistance: resistance from the ground wiring

to the rode.

Rod-earth: resistance between soil and rod.

Ground resistance: resistance of the soil

Important due to the voltage across the resistance

during faults or lighting strike, that can rise the

ground potential for the whole facility


14/21

Service entrance connection: This is the only point where neutral ground and

grounding electrode are connected.

Can be accomplished by using the following for a

grounding electrode: Underground water pipe

Building steel

Ground Ring

Concrete encased electrode

Ground rod


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Isolated ground: Improve the noise performance of sensitive loads.


16/21

Separately derived Systems: Can be accomplished using a delta-wye isolation

transformer.

Secondary ground produces new ground reference that

can reduce the noise levels for sensitive loads


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18/21

Must have low impedance over range offrequencies


19/21

When the ground conductor is longer than1/20 of the wavelength, it is no longereffective.


20/21

Power sensitive equipment with individual branchcircuits

Conduit should never be the sole source of

grounding

Green-wire grounds should be the same size as

current carrying conductors

Use building steel whenever available, provides

an excellent low impedance ground.


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There should never be current flowing through theground system under normal conditions.

An equipotential reference should always exist forall device on the system

All metallic system housing should be connectedto ground.

There should only be one neutral to ground bondin a system

There should be sufficient interconnects in theground plane to provide a low path impedanceover a range of frequencies.

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