Ignition or Shock

Is Grounding the Culprit

Marcus O. Durham, PhD, PERobert A. Durham, PhD, PE

ThewayCorp.com

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M.O. DurhamTheway Corp.

Why Ground ?

What is the purpose of grounding? Can grounding issues cause ignition or shock? Where is a ground or return path used? What is the difference between ground, bond, & neutral?

Grounding Complex Topic Critical for electrical safety

BiologicProperty

NEC - > 28 pages NESC – Specific Requirements IEEE – Multiple Standards NFPA – Myriad Publications

Significance Grounding is major element of every

electrical system, but rarely understood “Hot” cause shock or ignition only with

return path – complete circuit Either neutral (controlled) or ground

(uncontrolled) provides return Electrical activity on neutral or ground –

fault involving ground system Ground is code and legal issue

Grounding System Three components Grounding Electrode

Contact point with earth NEC <25Ω or additional electrode required NESC - < 25Ω. IEEE 142 - 25Ω not satisfactory – 1-5Ω

Grounding Electrode Conductor Connects grounding electrode to systemMust handle fault currents – NEC size

120 620

V VZI A

Grounding system Bonding

Connects metal surfaces to grounding systemRequired between grounding electrodesNEC FPN –bond all metal even if not specified

Without bond – Potential difference exists between metalsPotential difference causes current flow

Vd

I

Neutral Current carrying conductor Controlled return path for 1Φ & DC Single point connection to ground If multiple points –

current flows through grounding system If poor connection –

circuit current can flow through ground

ServiceGround

Water & OtherMetal

UtilityGround

Power H

Neutral

Ground

Power H

Neutral

Ground Power H

Transformer Entrance Load

Stray Current Unintentional current flow through earth Three causes

Neutral grounded at multiple pointsFault of “hot” wire to groundDifference in potential of ground connections

Multi grounded neutral allows ~60% return current in earth

Allows V between metals and between metal and earth

Multi Point Neutral Portion of In flows through earth Disturbances cause I flow in alternate

routes Risk of shock or equipment damage

In

Transients Short duration disturbances Should be shunted to ground Causes

LightningUtility OperationsEvery On/Off of electrical circuit

Transients - Lightning Lightning is natural phenomena Uncontrolled – damage Can be controlled Industry standards and practices Controlled – no or limited damage

Energization Issues Three issues

Energized GroundFloating NeutralEnergized Neutral

Energized GroundI from another circuitCaused by poor cnxns and poor ground ZMelted insulation on ground wire or jacket

Energization Issues Floating Neutral

Poor N connection Allows return current on alternate pathUncontrolled V levels

Energized NeutralI returns on gnd (water pipe, etc.)Melted neutral or ground insulationUncontrolled I flow

Faults Three forms

Loss of InsulationHigh Impedance ConnectionBreaking faults

Loss of InsulationV breakdown of dielectric Heat, ejecta, loss of materialEasily recognized

Faults High Z connection

Most common faultMisaligned contactsPoor connectionsPartially damaged insulationNot recognized by standard breakersHeat exceeds ignition

Faults Breaking Faults

SwitchingPulling apart cableHigh frequency componentSudden increase in voltageCauses damage to insulation particularly at

high inductance points (bends)

Why Fail Now? A failure may not result in immediate

catastrophe Catastrophic failures are the result of multiple

improper conditions.

Seldom do systems have serious consequences when only one component is improper.

Grounding Purpose To ensure that all systems

(facility and service) are operating at the same POTENTIAL reference

To prevent circulating CURRENTS from developing in ground system

To allow building and service protection systems to operate effectively

and as designed in TIME

Recap Improper grounding and bonding is

frequent problem

Electrical ignition and shock that is “undetermined” is often grounding

Codes and standards are minimum requirements. Not following creates hazards

Grounding issues are code, then legal issues.

Questions?