“Stray Voltages and Public-Exposed Lighting
Installations”
Massimo Mitolo, Ph.D.
IEEE Senior Member
Outdoor lighting installations
Lighting fixtures along with their supply
circuits, including:
transformers, breakers, reclosers,
switches, manholes, and whatever is
functional to the installation
performance.
All publicly exposed!
Stray Voltages
“Stray”: Element or occurrence not desired in
theory, but unavoidable in a practical realization.
Term coined in the 1970s: Stray Voltages In Dairies
“Elevated metal object-to-ground or neutral-to-ground
voltages”
Current, and not Voltage, is the proper criterion of
shock intensity
3
Stray Currents
Permanent current
other than momentary fault current
 circulating over the earth
 objectionable and undesirable


Concerns for pedestrians and their
animals!
4
TT earthing system per IEC
T T
T= direct connection of
one point of the
power system to
ground.
Utility
T= direct electrical connection of
exposed-conductive-parts to
ground, independently of the
grounding of any point of the
power system
Customer
5
TT earthing system
To 480V, 3 Phase Utility Transformer
The intensity of the ground fault current is limited
by the series resistance RL and RN of the grounds.
RCD is crucial!
Drawbacks with the RCDs
Nuisance trips
e.g. transient (10 μs) overvoltages during
thunderstorms
RCD trips the breaker in 10-40 ms: it
operates when the impulse has already
expired.
RCD with intentional tripping delay is
allowed, better if with high immunity to
disturbances.
RCD with automatic reclosure
capability?
Not required, so far, by IEC standards
because of the risk of repetitive shock to
persons due to a persistent fault.
Professional Engineers must decide on an
individual case basis!
TT Equivalent fault circuit
Person’s body resistance:
@125V, 700 Ω @220 V, etc.
788Ω
VOLTAGE DIVIDER!
Person’s resistance to ground in the
absence of floor: 2ρ.
Person assumed shoeless as per:
IEC 60479-1; 1994-09, 3rd Ed.,
“Effects of current on human beings
and livestock - Part 1: General
aspects”.
Persons in contact with the energized pole, and standing in
its proximity, are in an area at a potential other than zero.
This circumstance limits the source touch voltage VST!
Independently grounded poles in TT Systems
What if the
neutral
conductor is
faulted?
the RCD is desensitized!
Independently grounded poles in TT Systems
No, thank you!
TN-C-S Earthing System
T
N
T= direct connection of
one point to ground.
N= direct electrical connection of
the ECPs to the grounded
point of the power system.
-C
-S
S= protective function
provided by a conductor
separate from the grounded
conductor
C= neutral and protective
functions combined in a single
conductor
12
No-Fault Condition
Stray current as a result of unbalanced loads:
Permanent!
13
Fault Condition
The fault current’s return path to the power-supply winding
TN-C-S Earthing systems
B
V B V
IF 
ph

Z eq
Z eq  Z ph
B
V ph
Z Loop
B
 Ia
Ia
is the current causing the automatic operation of the
overcurrent protective device within the safe time ta
Vph (V)
120
230
277
400
>400
Disconnecting time ta (s)
0.8
0.4
0.4
0.2
0.1
DOSE!
Extraneous-conductive-part (EXCP)
Conductive part:

not forming part of the electrical system

liable to introduce a “zero” potential
(local/remote earth potential) or

an arbitrary potential
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Whole potential between the faulted pole and the fence
Should we bond?
EXCP
EXCP
Bonding Jumper between pole and fence (only if ExtraneousConductive-Part)
Drawback: Ground Potential Rises are tranferred!
Class II equipment
Double insulated (or reinforced insulated)
equipment:
incorporates a supplementary insulation, in
addition to the basic one.
The two insulations are physically separated
(and tested)
they cannot be subject to the same
deteriorating factors (e.g. temperature,
contaminants, etc.) to the same degree.
Class II equipment
It is an equivalent protection against indirect
contacts if applied to the entire outdoor
installation (i.e. light fixtures, conductors, splices,
and terminal strip).
According to IEC, class II equipment is not
permitted to be grounded.
According to NEC, double insulated equipment is
not required to be grounded.
Measuring stray voltages
Persons are sensitive to
currents, and not to voltages.
Stray voltages, thus, cannot
point out, per se, dangerous
situations.
The human body resistance RB
is variable with the touch
voltage.
It is important to assess the
capability of the stray voltage
to impress a dangerous current
and not the magnitude of the
voltage itself.
Touch-voltage must be measured with reference to a
standard human body resistance value: 1 kΩ as per IEC
Is the faulted pole found?
Poles’ enclosures are connected together.
The presence of stray voltage at the pole
under investigation does not necessarily
mean a fault on it!
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Conclusions
Analysis of the fault-loops is crucial to understand the causes of
stray voltages.

TT and TN-C-S distribution systems have fault loops of different
natures, the first one comprising the actual earth.

Both distribution systems require an effective partnership between
the protective device and the grounding system, in order to protect
persons by automatic disconnection of supply.

Class II installations are an efficient solution to protect persons
from electrocution and preserve the continuity of the service,
especially in areas at high pedestrian and/or vehicular circulation.

“Stray Voltages and Publicly Exposed Lighting
Installations”
References
1.
M. Mitolo, “On Outdoor Lighting Installations Grounding Systems”, IEEE-
2.
IEC 60364-1; 2002-06, 2nd Edition, “Electrical installations of
buildings”.
M. Mitolo, “Protective bonding conductors: an IEC point of view”, I&CPS
3.
4.
5.
6.
7.
8.
9.
IAS “Industry Application Society 41st Annual Meeting”, Tampa, Fl, October 2006.
Technical Conference 2005, Saratoga Springs, NY, Proceedings.
IEC 60479-1; 1994-09, 3rd Edition, “Effects of current on human
beings and livestock - Part 1: General aspects”.
IEC 60364-7-717; 1996-04, 1st Edition, “Electrical installations of
buildings”, part 7. “Requirements for special installations or locations Section 714: External lighting installations”.
NFPA 70, National Electrical Code 2005, National Fire Protection
Association, Quincy, Massachusetts.
IEEE Std. 142-1991, “IEEE Recommended Practice for Grounding of
Industrial and Commercial Power Systems”.
British Standard BS 7671; 2001, 16th Edition “IEE Wiring Regulations”.
“Stray Voltages, Concerns, Analysis and Mitigation”, NEETRAC Project,
Number 00-092, September 2001.
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