02/20/2015
Advanced Topics
in Stray Voltage ‐
Why is that voltage there?
Paul Ortmann, P.E.
Senior Electrical Engineer
Idaho Power Company
portmann@idahopower.com
Imagine…
You are “the stray voltage person” at work.
Your phone rings
…
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02/20/2015
Five phone calls…
1. Blown transformer fuse, voltage normal.
2. Customer shocked from aluminum siding,
where is the fault?
3. Worker shocked on grounded pivot sprinkler.
4. Gas company tech finds about 60V AC
between gas pipeline and earth.
5. High frequency NEV and contact voltage.
Phone call 1: Blown transformer fuse
Routine neutral-to-earth voltage check: neutralto-earth voltage “seems a little high”
Three-phase transformer bank has one fuse
blown.
120/240V three-phase service has normal
voltage; dairy is running.
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02/20/2015
We find this:
Are these really any different?
A
B
C
N
Open Fuse
a
b
n
c
Grounded-Wye Delta
(with blown fuse on one
power transformer)
Open-Wye – Open-Delta
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02/20/2015
Grounded‐Wye – Delta
• Contributes to primary system ground fau s – hence
blown fuse.
• Used temporarily on ungrounded wye-delta banks.
• Sometimes installed accidentally.
• Contributes primary neutral current.
lt
Ungrounded‐Wye ‐ Delta
• Primary neutral point is deliberately floated.
• No primary neutral current due to transformer.
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02/20/2015
Open‐Wye – Open‐Delta
• Three-phase service from two p mary phases
• Less prone to ferroresonance than some other
connections
• Significant primary neutral current contribution
ri
Primary Neutral Current Comparison
Open W ye Open Delta
Grounded W ye-Delta
Ungrounded Wye-Delta
0
2
4
6
8
10
12
Load:
60kVA 3-phase (balanced)
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02/20/2015
Primary Neutral Current Comparison
Open W ye Open Delta
Grounded W ye-Delta
Ungrounded Wye-Delta
0
2
4
6
8
10
12
Load:
60kVA 3-phase
25kVA 240V single-phase
Ungrounding a grounded‐wye‐delta transformer
72-hour NEV at Transformer Pole
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Grounded W ye-Delta
Ungrounded Wye-Delta
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02/20/2015
Closing an Open‐Delta transformer
Primary Profile NEV
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1
2
3
4
5
6
Open Delta NEV
7
8
9
10
11
12
13
Closed Delta NEV
An NEV model
Two important laws:
Kirchhoff’s Current Law: Current In = Current Out
Ohm’s Law:
Voltage = Current x Impedance
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02/20/2015
Open, Grounded, and Ungrounded‐Wye‐Delta
Primary System NEV
9
8
7
6
5
4
3
2
1
0
1
11
21
31
41
Ungrounded W ye-Delta
Grounded W ye-Delta
51
61
71
81
91
Open W ye-Delta
Lessons Learned
Check Delta transformer connections carefully.
Close open wye-open delta banks if possible.
Adding balanced 3-phase load to an ungrounded
wye-delta transformer bank should not change
NEV. If it does, investigate further.
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02/20/2015
Intelligent troubleshooting: Compare
conditions to expectations: Example
expectations:
“If this transformer is connected properly and we
add load, NEV should…”
“If there is a good neutral-to-ground bond, then
voltage between these two points should be…”
“If the neutral connections between these two
panels are in good condition then I should
measure…”
In short, if you don’t know what “normal” is, you
won’t recognize “abnormal”.
Phone call 2: Shock from aluminum siding
• Customer touched aluminum siding on garage
and got shocked
• No breakers have tripped
• Customer’s equipment appears to operate
normally
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02/20/2015
Ground faults and effective current paths:
From the 2014 NEC: (paraphrased)
• Ground Fault. An unintentional connection
between an ungrounded conductor and the
normally non–current-carrying conductors,
metallic enclosures, metallic raceways, metallic
equipment, or earth.
• Effective Ground-Fault Current Path. An
intentional, low-impedance path to carry current
under ground-fault conditions and facilitates the
operation of the overcurrent protective device or
ground-fault detectors.
Earth faults:
The earth shall not be
considered an effective
ground-fault current
path. (2014 NEC-250.4)
But what if the earth is
the only path for ground‐
fault current?
We’ll call this an “Earth
fault"
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02/20/2015
Observations:
• Separate garage has a short 3-wire 120/240V
overhead service from a breaker in the house
panel.
• Garage has a subpanel with a main breaker and
several branch circuit breakers.
• No GFCI circuit breakers or receptacles are
installed in garage.
Faults – What is supposed to happen?
Low-impedance fault current
path ensures that plenty of
current flows to quickly trip
the circuit breaker
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02/20/2015
A few cycles after the fault starts…
Circuit breaker quickly trips.
But the breaker did not trip…
Earth faults rarely produce enough
current to trip a non‐GFCI circuit
breaker or blow a fuse.
If someone reports being shocked when
they touch something grounded, you may
have an earth fault to find.
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02/20/2015
M easurements:
•Voltage measured between a
screwdriver in the earth and the
garage s aluminum siding is
approximately
110-volts, even
’
with meter on “Lo-Z” setting.
•Net current on overhead service
to garage is about 3-Amps.
•Neutral current on overhead
service to garage is negligible.
Tests:
•Open main breaker in garage
• Contact voltage disappeared
• Fault current is going through main breaker
•Close main breaker and open individual
breakers one at a time
• Opening one branch circuit breaker eliminates
the contact voltage
• Fault is on that circuit
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02/20/2015
Investigate:
• Open electrical boxes on that circuit
• Found line-to-ground fault in metal receptacle box
• Why didn t the breaker trip?!
’
One missing screw…
No
bond
Earth fault current is too
low; breaker won’t trip.
Earth is only fault current path
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02/20/2015
Lessons Learned:
If a “ground fault” doesn’t cause a circuit breaker
to trip or a fuse to blow, it may actually be an
“Earth fault.”
Earth faults are a combination of two problems:
Problem 1: The ground fault
Problem 2: A poor fault current path
Both problems must be found!
Normal neutral and ground current
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02/20/2015
Primary NEV with an earth fault at a service
Primary System NEV
6
5
4
3
2
1
0
1
11
21
31
41
Normal
51
61
71
81
91
Earth Fault
Primary and service current with an earth fault
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02/20/2015
Voltage gradients –
where current enters or leaves the earth
Altitude gradient
Voltage gradient
Touch and step voltages during an earth fault
• Touch and step voltage downstream of the
break in the grounding path can be very high –
even lethal.
For a single isolated vertical ground rod:
~88% of the voltage drop to remote
earth is within one rod-length
For a 10’ rod, the voltage from 1’ to 3.5’
away will be about 20% of the faulted
line voltage
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02/20/2015
Finding both parts of the earth fault
•The ground fault:
1 . M onitor the fault safely while opening
circuit breakers, fuses, or disconnects.
• M ultimeter or voltage detector can be used.
• Can measure contact voltage, gradient, or NEV.
2. Do not assume that first upstream device
will interrupt fault current.
3 . W hen opening a disconnect de-energizes
the fault, the ground fault is downstream of
that disconnect and upstream of other
disconnects further downstream.
Finding the break in the
fault current path
1 . M easure voltage across
connections; should be
nearly zero
2. For underground or
overhead, measure NEV;
Downstream of break, NEV
will be high, and may
approach normal line-toground voltage
3. Upstream of break, NEV will
be elevated but not normally
to more than 10-20V.
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02/20/2015
An Example:
NEV at panels 2 and 3 is close to line voltage
NEV at panel 1 is 14 Volts
Opening disconnect 1 or 2 eliminates high NEV readings
Opening disconnect 3 does not eliminate high NEV readings
W here is the ground
fault?
W here is the break in the
effective ground fault
current path?
Phone call 3: The shocking pivot
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02/20/2015
Report and Observations:
• Worker receives painful shocks when working
from a ladder on the pivot to replace sprinklers
• Pivot is parked parallel and next to a 345kV
transmission line for service
M easurements:
• Voltage from earth to pivot is 2V or less along
the pivot’s entire length
• Not a pivot grounding problem
• Voltage between a person standing on the
ground in dry shoes and the wheeled pivot
towers is also less than 2V.
• Person-to-pivot voltage increases to over 600V
across a 10MΩ digital voltmeter as the person
climbs an insulated ladder in the middle of a
pivot span.
• A person-grounding problem!
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02/20/2015
Electric Field Calculations:
• Open-circuit person-to-pivot AC voltage with
person on insulated ladder: Over 10kV!
• No wonder it hurts…
• The pivot is not shocking the worker. The
worker is shocking the pivot! The effect is very
similar to a fencer pulse.
• Steady-state short circuit current for person at
working height is less than 0.1mA
• Not a steady-state problem if person can be kept
grounded.
The situation:
Person touching pivot is
shorting the capacitance
between them and the
earth/pivot.
That capacitance is
small, but may be
charged to over
10kV at the time
of contact.
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02/20/2015
Typical electric field shape
(horizontal conductors)
V/m
0
- 100
- 50
0
50
100
You can use a voltage detector to find the
peak of the field near a transmission line
The issue: Series capacitance
•Electric field redistributes
charge
•Large voltage can appear
across capacitances
•Shorting the capacitance
results in a capacitive
discharge – like a fencer
pulse
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02/20/2015
Solution:
• Anti-Static wrist strap with
large alligator clip.
• Clipped to pivot while
worker is on the ground.
• 1MΩ wrist strap prevents
voltage from building up in
capacitance between
worker and pivot/earth.
• keeps discharge currents
low while allowing
capacitance between
person and pivot to
discharge.
Phone call 4: Pipeline voltage
• Pipeline technician measured ~ 60V AC between earth
and piping at cathodic protection test point.
These are all Inductors
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02/20/2015
A pipeline parallel to transmission or
distribution conductors
Current
Earth
Pipe
Voltage
profile
But is it induction?
• Induced voltage will be highest at the ends of
insulated, parallel sections and lowest in the
middle of a section.
• Resistively coupled voltage, will be relatively
uniform along the entire pipeline section.
• Software can help you predict voltages
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02/20/2015
Software simulation:
Low-FI (Low Frequency Induction)
software from:
Sigmapower.com.au
M itigating induced voltages:
AC, but not DC grounding
Before (55.0V M ax)
After (8.4V M ax)
AC grounding near the ends of
sections or where the model
predicts excessive voltage
Decouplers usually required
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02/20/2015
Phone call 5: High frequency issues
• Call starts as a “routine” stray voltage call
• Spot-measured NEV at service is surprisingly
high (>5V).
• SVM-10 stray voltage recorder doesn’t record
NEV that high.
When instruments don’t agree:
• Check the frequency – push the “Hz” button
• Is a high (>1kHz) frequency displayed?
• Different bandwidths may explain different readings
Homework: The Fluke 87V and
some other DMMs have a built‐
in low pass filter. Find out how
to use it and try it out!
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02/20/2015
The SVM ‐10’s frequency response
measurements
Freq.
60.4Hz
601Hz
1.995kHz
3.019 kHz
6kHz
10.01kHz
SVM-10 Ch1
V
1.010
1.003
1.000
1.000
1.005
1.009
1.006
0.994
0.897
0.715
0.120
0.013
Ch2
Ch3
Ch4
1.008
0.996
0.911
0.728
0.117
0.013
1.010
0.997
0.917
0.734
0.120
0.012
1.009
0.996
0.916
0.740
0.140
0.022
The Fluke 87V low-pass filter cuts off at 1kHz.
If the measured frequency was several kHz, then
using the low-pass setting should get the D M M and
the SVM-10 recorder to agree reasonably well.
Voltage (Zero to Peak) 500 Ohm Cow Contact
Cow frequency response
Behavioral response for 5% most sensitive cows, sine waves
muzzle to hooves exposure
10000
1 cycle Biphasic
1 cycle Monophasic
1000
Multi Cycle
100
10
1
0.1
1
10
100
1000
10000
Phase Duration (microseconds) = time between zero crossings
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02/20/2015
Using this information in an investigation
Behavioral response for 5% most sensitive cows, sine waves
muzzle to hooves exposure
10000
1 cycle Biphasic
1 cycle Monophasic
Multi Cycle
Recorded transient
Voltage (Zero to Peak) 500 Ohm Cow Contact
1000
100
10
1
0.1
Transient plotted on
sensitivity graph
0.01
1
10
100
1000
10000
Phase Duration (microseconds) = time between zero crossings
Where is the high frequency coming from?
High frequency “noise” on the grounding system is nearly
always associated with nearby variable frequency drives.
Voltage pulses from the VFD cause “ringing” current
which travels across the internal stray capacitance of the
motor to the case and into the grounding system.
M otor grounding current with a VFD
Plot from: “Modeling of M otor Bearing Currents in P WM Inverter Drives”, S. Chen, T. Lipo, and
D. Fitzgerald; IEEE Transactions on Industry Applications Vol. 32, No. 6 Nov/Dec 1996.
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02/20/2015
Specialized cabling for VFDs is designed to
minimize noise coupling to other conductors
VFD-to-motor cable:
• Symmetrical grounds
• Overall shield
• Shield bonded to enclosures
at both ends
Finding signal sources: Timing
For transients, use oscilloscope or recorder to
measure time between transients, from seconds
to minutes
Find devices that operate at the same interval
Useful for fencers, refrigerators, float switches,
etc.
58
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02/20/2015
Finding signal sources
The time‐of‐arrival method
Capture the signals at two
A
separate locations on the same
neutral-grounding system
simultaneously
Travel direction can be
determined from “time of
arrival”
B
A
B
Oscilloscope
In this case, the transient
originated closer to location
A than to location B.
59
In conclusion:
People are often surprised by “normal” voltage
measurements.
The more you learn about how electrical systems
behave, the better you will be at recognizing and
resolving truly abnormal conditions.
Paul Ortmann, P.E.
Senior Electrical Engineer
Idaho Power Company
portmann@idahopower.com
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