Transcript Ignition or Shock - Theway Corp Home

Is Grounding the Culprit
Marcus O. Durham, PhD, PE
Robert A. Durham, PhD, PE
ThewayCorp.com
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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?
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Why Ground ?
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M.O. Durham
Theway Corp.
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Grounding
Complex Topic
 Critical for electrical safety
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 Biologic
 Property
NEC - > 28 pages
 NESC – Specific Requirements
 IEEE – Multiple Standards
 NFPA – Myriad Publications
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Significance
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Grounding is major element of every
electrical system, but rarely understood
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“Hot” cause shock or ignition only with
return path – complete circuit
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Either neutral (controlled) or ground
(uncontrolled) provides return
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Electrical activity on neutral or ground –
fault involving ground system
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Ground is code and legal issue
Grounding System
Three components
 Grounding Electrode
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 Contact point with earth
○ NEC <25Ω or additional electrode required
○ NESC - < 25Ω.
○ IEEE 142 - 25Ω not satisfactory – 1-5Ω
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Grounding Electrode Conductor
 Connects grounding electrode to system
 Must handle fault currents – NEC size
Z
V 120V
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 6
I
20 A
Grounding system
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Bonding
 Connects metal surfaces to grounding system
 Required between grounding electrodes
 NEC FPN –bond all metal even if not specified
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Without bond –
 Potential difference exists between metals
 Potential difference causes
current flow
I
Vd
Neutral
Current carrying conductor
 Controlled return path for 1Φ & DC
 Single point connection to ground
 If multiple points –
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 current flows through grounding system
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If poor connection –
 circuit current can flow
Transformer
through ground
Entrance
Load
Power H
Power H
Neutral
Neutral
Power H
Ground
Ground
Utility
Ground
Service
Ground
Water &
Other
Metal
Stray Current
Unintentional current flow through earth
 Three causes
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 Neutral grounded at multiple points
 Fault of “hot” wire to ground
 Difference in potential of ground connections
Multi grounded neutral allows ~60% return
current in earth
 Allows V between metals and
between metal and earth
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Multi Point Neutral
Portion of In flows through earth
 Disturbances cause I flow in alternate
routes
 Risk of shock or equipment damage
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In
Transients
Short duration disturbances
 Should be shunted to ground
 Causes
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 Lightning
 Utility Operations
 Every On/Off of electrical circuit
Transients - Lightning
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Lightning is natural phenomena
Uncontrolled – damage
Can be controlled
Industry standards and practices
Controlled – no or limited damage
Energization Issues
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Three issues
 Energized Ground
 Floating Neutral
 Energized Neutral
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Energized Ground
 I from another circuit
 Caused by poor cnxns and poor ground Z
 Melted insulation on ground wire or jacket
Energization Issues
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Floating Neutral
 Poor N connection
 Allows return current on alternate path
 Uncontrolled V levels
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Energized Neutral
 I returns on gnd (water pipe, etc.)
 Melted neutral or ground insulation
 Uncontrolled I flow
Faults
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Three forms
 Loss of Insulation
 High Impedance Connection
 Breaking faults
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Loss of Insulation
 V breakdown of dielectric
 Heat, ejecta, loss of material
 Easily recognized
Faults
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High Z connection
 Most common fault
 Misaligned contacts
 Poor connections
 Partially damaged insulation
 Not recognized by standard breakers
 Heat exceeds ignition
Faults
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Breaking Faults
 Switching
 Pulling apart cable
 High frequency component
 Sudden increase in voltage
 Causes 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.
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Seldom do systems have serious
consequences when only one
component is improper.
Grounding Purpose
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To ensure that all systems
(facility and service) are operating at the
same POTENTIAL reference
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To prevent circulating CURRENTS from
developing in ground system
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To allow building and service protection
systems to operate effectively
and as designed in TIME
Recap
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Improper grounding and bonding is
frequent problem
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Electrical ignition and shock that is
“undetermined” is often grounding
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Codes and standards are minimum
requirements. Not following creates
hazards
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Grounding issues are code, then legal
issues.
Questions?