Content uploaded by Massimo Mitolo
Author content
All content in this area was uploaded by Massimo Mitolo on Mar 10, 2015
Content may be subject to copyright.
IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY2014 33
On Outdoor Lighting Installations
Grounding Systems
Massimo Mitolo, Senior Member, IEEE
Abstract—Outdoor lighting installations are publicly accessible
electrical pieces of equipment. Upon faults-to-ground, the appear-
ance of dangerous potentials on the metal parts of such equipment
exposes persons to shock hazards. This unsafe situation poses se-
rious problems to utilities, or municipalities, or whoever operates
the lighting installations. This paper, through the analysis of the
fault loops, seeks to clarify the reasons of the manifestation of stray
voltages on public-exposed non-current-carrying metal parts, in
light of different earthing systems as defined in IEC standards.
Index Terms—Earth, exposed conductive parts (ECPs),
extraneous conductive part (EXCP), ground, ground fault circuit
interrupter (GFCI), ground potential rise, neutral, single-phase
line-to-ground (SLG) fault.
CONVENTIONS
Designations for system grounding will use the capital letter
T as the first letter to indicate the relationship of the power
system to ground or as the second letter (relationship of the
exposed conductive parts (ECPs) of the installation to ground)
and the capital letter N as the second letter (relationship of the
ECPs of the installation to ground):
T first letter direct connection of one point to ground;
T second letter direct electrical connection of ECPs to
ground, independently of the grounding of
any point of the power system;
N direct electrical connection of the ECPs to
the grounded point of the power system (e.g.,
neutral point).
Subsequent letter(s) (if any) (arrangement of neutral and
protective conductors):
S protective function provided by a conductor
separate from the grounded conductor;
C neutral and protective functions combined in a
single conductor.
I. INTRODUCTION
OUTDOOR lighting installations are the lighting fixtures
along with their supply circuits, including transformers,
Manuscript received May 29, 2012; revised October 29, 2012 and March 16,
2013; accepted April 19, 2013. Date of publication June 12, 2013; date of
current version January 16, 2014. Paper 2012-PSPC-339.R2, presented at the
2006 IEEE Industry Applications Society Annual Meeting, Tampa, FL, USA,
October 8–12, and approved for publication in the IEE E TRANSACTIONS ON
INDUSTRY APPLICATIONS by the Power Systems Protection Committee of the
IEEE Industry Applications Society.
The author is with Eaton Corporation, Irvine, CA 92618 USA (e-mail:
mitolo@ieee.org).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIA.2013.2267792
Fig. 1. TT system supplying outdoor lighting installations.
breakers, reclosers, switches, splices, manholes, and whatever
is functional to the performance of the system.
Reference [1] respectively defines TT and TN systems as
electrical systems:
whose polyphase/single-phase supply system has one point
directly connected to ground: the common point for
polyphase systems (i.e., the neutral), or a phase conductor,
if that is not available;
whose ECPs, as defined in [2], (e.g., conductive enclosures
containing live parts) are connected to ground through
electrodes electrically independent of the supply grounded
point (TT system) or connected to the system grounded
point by protective conductors, referred to as PE (TN
system).
The above defined earthing systems imply different ground
fault loops and different safety issues should a single-phase
line-to-ground (SLG) fault occur along the lighting installation.
II. TT EARTHING SYSTEMS
Fig. 1 shows a TT system: The supply transformer is
grounded with an independent earth electrode (of resistance
RN) from the metal poles’ ground rods. The contribution of
each actual pole rod is shown as lumped into one equivalent
resistance RG. At the occurrence of an SLG fault on a pole,
the fault current’s return path to the power-supply winding
circulates through the actual earth since no other means of
reclosing back to the source is available. The magnitude of this
fault current is greatly limited by the series resistance RGand
RNof the grounds. As a consequence, the SLG current might
not be sufficient to cause the protective time-inverse overcurrent
device to promptly trip. The continuous leakage to ground, not
0093-9994 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
34 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY2014
Fig. 2. Equivalent fault circuit showing the touch voltage exposure to a person
standing in proximity of the pole’s electrodes.
cleared by any circuit breaker, energizes the pole, imposing a
stray voltage VGon it, whose magnitude is given by
VG=RG·IG(1)
where IGis the fault current to ground.
For the above reasons, in TT systems, the presence of resid-
ual current devices (RCDs) is mandatory. Persons who come
into contact with the energized pole and stand in proximity
of its grounding electrode may be in an area at a potential
other than zero. Consequently, open-circuit touch voltage VST,
also defined as source touch voltage, may be less than VG,as
pictured in the equivalent circuit in Fig. 2.
Vph is the source phase voltage, RNis the ground resistance
of the source, Ziis the source impedance, Zph is the phase
conductor impedance, and VTis the actual touch voltage the
person is exposed to.
RBindicates the human body resistance, which is not a
constant value, but varies, among other factors, with the touch
voltage [3]. RBG symbolizes the person’s resistance to ground.
In the absence of a floor, this can be calculated considering the
feet as two circular electrodes in parallel, with a conventional
radius of 10 cm, placed on the soil [4]. Calculations yield for
such electrodes a resistance approximately equal to RBG =2ρ,
where ρis the superficial soil resistivity.
The resistance of footwear, which has a considerable favor-
able impact on increasing the body resistance to ground, is not
considered by [3], which assumes the person, conservatively,
shoeless.
Should a person be in contact with the faulted pole and an
extraneous conductive part (EXCP) (e.g., a metal fence, a fire
hydrant, and a water pipe), as defined in [2] as a metal object
with resistance to ground less than 1000 Ω, though, the person
would be subject to the whole ground potential.
In general, this hazardous condition can be avoided if the
EXCPs, which can be touched, are bonded to the pole enclosure
with a jumper conductor. In some cases though, this bonding
connection just transfers the hazard to another area. In Fig. 3
this is shown as the touch voltage is transferred from A to B
along the entire length of a metal fence.
The fault potential appearing on the pole can in fact travel
for miles and shock persons touching the energized fence and
another EXCP, for example. The engineer, then, must decide
on an individual case basis when it is the case to bond poles
Fig. 3. Fault potential is transferred from the faulted pole to the metal fence
because of the bonding jumper.
to metal fences that have an inherent good connection to earth
through their foundations; the decision if touch potential issues
are of greater concern than transferred potential must be made.
III. GROUND FAULT PROTECTION IN
TT EARTHING SYSTEMS
A fundamental requisite necessary to provide effective pro-
tection to persons in TT distribution systems is the residual
current device (RCD) (also referred to as ground fault circuit
interrupter). The RCD, associated with the earthing system,
forms an efficient protection against indirect contact.
This device detects the zero-sequence current leaking to
ground and instantaneously interrupts the supply. Its operation,
in fact, does not depend on the magnitude of the fault current
and is triggered as long as it exceeds its residual current thresh-
old (e.g., 30 mA). On the other hand, protective overcurrent
devices, due to the high resistance of the fault loop, might not
trip, leaving the pole energized.
Despite the high level of safety offered by the partnership
RCD–grounding system, risk of nuisance tripping is indeed
present due to surge voltages. These surge voltages can be
the result of switching transients or induced by lightning.
During thunderstorms, in fact, momentary current leakages to
ground, due to transient overvoltages on power lines, can cause
unwanted tripping of the RCD. It is important, then, to organize
the supply circuits of the lighting installation in more than one
branch, each one with its own RCD. RCDs with high immunity
to disturbances should be employed [5]. International standards
have not considered, thus far, RCDs with automatic reclosure
capability. These devices try a few times to restore the power,
rearming their contacts after they have tripped. If the fault is not
permanent (e.g., transient arcs), the RCDs can restore the sup-
ply, restoring illumination and safety to areas of the pedestrian/
vehicular circulation. On the other hand, though, there is no
MITOLO: ON OUTDOOR LIGHTING INSTALLATIONS GROUNDING SYSTEMS 35
Fig. 4. Lighting poles grounded through independent electrodes (rods).
guarantee that persons touching faulted poles are not repeatedly
shocked by the automatic reclosure if the fault is persistent.
RCD unwanted tripping is a typical maintenance issue for
outdoor lighting systems. It is important to note that outdoor
lighting systems are themselves a safety device, whose outage
may present hazards worse than electrical hazard.
As to the lightning protection of light poles, international
and European standards deem the probability that a person
touches a metal pole during the occurrence of a lightning strike
statistically insignificant; hence, no specific lightning protec-
tion requirements for safety are provided in [10]. However,
the engineer may decide to perform a risk assessment of the
lighting poles against lighting, as well as select protection
measures, by using [14]. Such risk assessment is defined as the
value of probable average annual loss (humans and goods) due
to lightning, relative to the total value (humans and goods) of
the object to be protected.
IV. INDEPENDENTLY GROUNDED POLES
IN TT SYSTEMS
Connecting metal lighting poles, protected by the same RCD,
to independent grounds (e.g., one rod for each pole) (see Fig. 4)
can lead to a dangerous situation and, therefore, should be
avoided.
If the insulation of the neutral wire fails and this conductor
is in contact with the metal pole (or if during installation the
neutral wire has been mistaken for the PE conductor and,
therefore, bonded to the pole), no appreciable current-to-ground
flows and no protective device can trip. Therefore, this situation
can indefinitely persist.
At the occurrence of a ground fault on phase L (see Fig. 5),
a current I2will circulate to ground, but part of it (i.e., I1) will
return to the source through the ground rod RG1and the faulted
neutral conductor.
Fig. 5. Equivalent fault loop in case of simultaneous failure of neutral and
phase conductors for independently grounded poles.
Fig. 6. TN-C-S system supplying outdoor lighting installations.
This desensitizes the RCD, which will only sense as a resid-
ual current the difference I2−I1. Such a residual current might
not be enough to trip the RCD and would cause a permanent
stray voltage on the pole.
The magnitude of such stray voltage depends on the respec-
tive values of the resistances included in the fault loop, as shown
in Fig. 5.
V. TN-C-S EARTHING SYSTEMS
Fig. 6 shows a typical TN-C-S system supplying outdoor
lighting installations.
The supply transformer ground electrode (of resistance RN),
the user ground (of resistance RU), and the poles’ ground
(lumped in RL) are not independent as they are connected
together due to the main bonding jumper (MBJ).
At the occurrence of an SLG fault on pole B, the fault
current’s return path to the power-supply winding circulates
through a low-impedance metal path, constituted by the PE.
The actual earth is only minimally involved in the fault
loop; therefore, the ground fault current is sufficient to operate
overcurrent devices. The fault loop is pictured in Fig. 7.
Upon a fault-to-ground at pole B, all the poles toward the
source will become energized at different and decreasing poten-
tials, whereas all the poles downstream of the faulted pole will
remain at their fault potential. It is therefore apparent that in TN
systems, potential differences will arise between poles. These
potentials vary as a function of the phase conductor impedance
Zn
ph, the protective conductor impedance Zn
PE (as both seen at
36 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY 2014
Fig. 7. Fault loop in a TN-C-S distribution system.
TAB L E I
MAXIMUM DISCONNECTING TIMES AS A FUNCTION
OF THE SYSTEM VOLTAGE
the generic point of fault “n”), and the impedance of the PEN
conductor ZPEN.
In reference to Fig. 6, the prospective touch voltage in B
will be
VST =Vph ·ZPEN +ZPE
ZU
ph +Zph +ZPEN +ZPE
(2)
where ZPE is the impedance of the PE from the service panel
to the point of fault in pole B, ZPEN is the impedance of
the protective conductor from the service panel to the source,
ZU
ph is the impedance of the phase conductor external to the
installation, and Zph is the impedance of the phase conductor
within the installation up the point of fault. The values ZU
ph and
ZPEN can be determined based on the nameplate of the supply
transformer and the main service feeder cable if these data are
available; a pessimistic value of Ze=ZU
ph +ZPEN likelytobe
communicated by utilities in compliance with [6] per TN-C-S
systems is 0.35 Ωmaximum [7].
The designer must verify the following inequality relating
the fault current IFto the fault-loop impedance ZLoop, which
comprises the source, the live conductor up to the point of
the fault, and the equipment grounding conductor between the
point of the fault and the source:
IF=Vph
ZLoop
≥Ia(3)
where Vph is the nominal voltage to ground, and Iais the
current causing the automatic operation of the overcurrent
protective device within the maximum permissible time taas
a function of the nominal voltage, as listed in Table I [8].
If the previous inequality (3) is fulfilled, the automatic
disconnection of the supply will occur within a safe time. If
the fault loop impedance, though, is excessive and (3) is not
satisfied, the installation of RCDs or Class II poles become
necessary.
Should the user lose the MBJ, the TN-C-S system would
evolve into a TT system, which is unsafe if no RCD is present.
In such situation, the system might be affected by permanent
stray voltages appearing over the poles [9].
VI. CLASS II EQUIPMENT
A protection against indirect contact equivalent to the part-
nership of the protective device and the grounding connection,
called for by [7], is the double, or reinforced, insulation of the
entire outdoor installation, both in TN and TT systems. Such
solution is defined as Class II equipment. All the components
of the pole, i.e., light fixtures, cables, splices, and terminal
strip, have the basic insulation and a supplementary layer of
insulating material so that, should the basic insulation fail, no
voltage would appear on any accessible part. Reference [4]
defines as Class II the metal poles whose handhole cover on the
lighting column is separated from wires by insulating materials
(e.g., sleeves or tubes).
For example, Class II cables are defined as those cables
with a nonmetallic sheath (i.e., first layer), and the voltage
rating of the basic insulation (i.e., second layer) is one standard
step greater than the nominal voltage of the electrical system.
However, in outdoor installations, it would be preferable to have
cables with an insulation at least two steps greater than the
nominal voltage of the system.
Earthing Class II poles is not allowed by [10]. Thus, neither
protective conductors nor RCDs are provided in such systems;
this arrangement reduces the chance of a protective disconnec-
tion of the supply in the case of faults-to-ground, which could
cause unsafe traffic conditions.
The reason of this prohibition lies in the risk of the energiza-
tion of the double insulated pole by transferred potentials. Other
equipment sharing the same earthing system as Class II, in fact,
may fail, causing the energization of the earthing system; the
probability that the Class II pole becomes energized for this
reason is deemed greater than the probability of failure of its
double insulation [11].
The drawback of this solution is that vehicular collisions may
damage the double insulation, which might go unnoticed and
cause the system to degrade to a TT system, without its own
safety requirements.
This prohibition, valid in the context of one single grounding
system serving all the Class II poles, would be meaningless if
each Class II pole had its own earthing electrode; in such case,
RCDs (one per pole) and protective conductors could be em-
ployed without causing the issue of transferred potentials and
greatly limiting the risk of nuisance trippings while increasing
the safety of the lighting installation.
As an alternative solution, to mitigate the chronic problem
of stray voltages appearing on metal poles, which have caused
several fatalities, nonconductive composite covers on utility
service boxes are used. In addition, transformers are installed
within the poles. Such transformers allow the galvanic separa-
tion of the lighting circuit from the earth, thereby preventing
the circulation of currents in the case of touch, if the basic
insulation of the circuit, or of other components within the pole,
fails [12].
MITOLO: ON OUTDOOR LIGHTING INSTALLATIONS GROUNDING SYSTEMS 37
Fig. 8. Test circuit for the stray voltage measurement.
The use of Class II poles would be an effective safety solution
in existing Class I lighting poles lacking in protective con-
ductors. This may be the case of lighting systems installed by
utilities in the USA as per [15], which do not employ equipment
grounding conductors; such lighting systems constitute a great
risk of electric shock for pedestrians, and the adoption of
Class II poles may harmonize the requirements of [15] with the
safeguard of the general public on roadways.
VII. MEASURING STRAY VOLTAGES
Persons are sensitive to currents and not to voltages; as
a consequence, stray voltages cannot be evidence per se of
dangerous situations. In addition, the human body resistance
RBdepends on the voltage of the energized object, which
the person is exposed to. Thus, two different touch potentials
may correspond to two different body resistance values RBbut
induce same body currents.
As a result, the capability of the voltage source to impress a
dangerous current should be assessed. Touch voltages will be
considered unsafe only if they cause circulation of a current
through the human body greater than the threshold considered
dangerous [13]. Touch voltage must be measured, then, with
reference to a standard human body resistance value. IEC
standards consider such value as 1 kΩ.
If the floor is the reference ground contact for a test on a pole,
a pair of 200-cm2metal plates can be used as electrodes. Each
plate should weigh at least 250 N and be 1 m apart from the pole
being tested. In addition, voltage measurement should be taken
with a 1-kΩresistance connected in parallel to the voltmeter
leads in order to simulate the resistance of the standard person
(see Fig. 8).
The test measures the voltage across the human body as if
the person had zero resistance-to-ground (unlikely in outdoor
locations) and does not account for the actual context that
may be found in reality. As a result, the measurement gives
conservative values.
It should be also noted that, as previously shown, the poles’
enclosures are connected together through the PE; therefore, the
assessment of stray voltage at the pole under investigation does
not necessarily mean a fault on it.
Voltage measurements should be made by using true RMS
voltmeters or digital multimeters. Minimum requirements for
such instruments are capability to detect voltages in the range
of 8–600 V, input impedance of at least 10 kΩ/V, and minimum
accuracy of 0.25%.
The voltmeter should be waterproof rated (e.g., IP67), as well
as CE marked.
Should the pole result energized, the exact form of the
waveform of the stray voltage should be ascertained, and dif-
ferential oscilloscope could be used. This might be necessary to
discriminate the sinusoidal waveform from “foreign” induced
signals on the equipment being tested.
Whenever stray voltages have been determined, the leak-
age current circulating in the equipment grounding conductor
should be measured with clamp-on ammeters, without being
necessary to open the fault loop.
VIII. CONCLUSION
It has been substantiated that, in order to ascertain the
magnitude of stray voltages appearing on publicly exposed
enclosures of outdoor installations, the analysis of the fault
loops is necessary. The nature of such loops depends on the
earthing system employed.
TT and TN distribution systems have fault loops of different
natures, the first one comprising the actual earth. Both earthing
systems require an effective partnership between the protective
device (i.e., overcurrent and/or RCD) and the grounding system
in order to protect persons and livestock from shock hazard by
automatic disconnection of supply.
Class II installations seem to be an effective solution to
protect persons from electrocution and preserve the continuity
of the service, particularly in areas at high pedestrian and/or
vehicular circulation. The safety offered by Class II equipment
could be enhanced with dedicated RCDs and rods associated to
each pole in order to disconnect the supply in case of failure of
the double insulation.
ACKNOWLEDGMENT
The author would like to dedicate this effort to his daughters
Alessandra and Giorgia, who light up his life.
REFERENCES
[1] Low-Voltage Electrical Installations—Part 1: Fundamental Principles,
Assessment of General Characteristics, Definitions, IEC Std. 60364-1,
2005.
[2] M. Mitolo, “Protective bonding conductors: An IEC point of view,” IEEE
Trans. Ind. Appl., vol. 44, no. 5, pp. 1317–1321, Sep./Oct. 2008.
[3] Effects of Current on Human Beings and Livestock—Part 1: General
Aspects, IEC Std. 60479-1, 2005.
[4] IEEE Guide for Safety in AC Substation Grounding, IEEE Std. 80-2000,
2000.
38 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY 2014
[5] Residual Current Operated Circuit-Breakers Without Integral Overcur-
rent Protection for Household and Similar Uses (RCCB’s)—Part 1:
General Rules, EN Std. 61008-1:2004, 2004.
[6] 17th Edition “IEE Wiring Regulations,” British Std. BS 7671, 2008.
[7] B. Scaddan, 17th Edition IEE Wiring Regulations: Explained and Illus-
trated, 9th ed. Oxford, U.K.: Newnes, 2011.
[8] Low-Voltage Electrical Installations - Protection for Safety - Protection
Against Electric Shock, IEC Std. 60364-4-41, 2005.
[9] “Stray Voltages, Concerns, Analysis and Mitigation,” Nat. Elect. Energy
Testing Res. and Appl. Center, Forest Park, GA, USA, NEETRAC Proj.
No. 00-092, Sep. 2001.
[10] Electrical Installations of Buildings, Part 7. Requirements for Special
Installations or Locations—Section 714: External Lighting Installations,
IEC Std. 60364-7-714, 2011.
[11] M. Mitolo, “Is it possible to calculate safety: Safety and risk analysis
of standard protective measures against electric shock,” IEEE Ind. Appl.
Mag., vol. 15, no. 3, pp. 31–35, May/Jun. 2009.
[12] M. Mitolo, Electrical Safety of Low-Voltage Systems. New York, NY,
USA: McGraw-Hill, 2009.
[13] Effects of Current on Human Beings and Livestock—Part 1: General
Aspects, IEC Std. 60479-1, 2005, 4th ed.
[14] Protection Against Lightning—Part 1: General Principles,IECStd.
62305-1, 2012.
[15] National Electrical Safety Code (NESC), IEEE, Piscataway, NJ, USA,
2012.
Massimo Mitolo (SM’03) received the Doctoral de-
gree in electrical engineering from the University of
Naples “Federico II,” Naples, Italy.
He is currently the Advisory Engineer at Eaton
Corporation, Irvine, CA, USA. He has authored more
than 50 journal papers, as well as the books Elec-
trical Safety of Low-voltage Systems (McGraw-Hill,
2009) and Laboratory Manual for Introduction to
Electronics: A Basic Approach (Pearson Prentice-
Hall, 2013). His field of research is the analysis and
grounding of power systems.
Dr. Mitolo is a Registered Professional Engineer in Italy. He is active within
the IEEE Industry Applications Society Industrial and Commercial Power
Systems Department, where he is currently the Chair of the Power Systems
Engineering (PSE) Committee, the Power Systems Analysis Subcommittee,
and the Grounding Subcommittee; he is the Cochair of the working group on
the dot standard 3002.1 “Load Flow” and a member of the working group on dot
standards 3002.2 “Short Circuit,” 3003.1, and 3001.2 on bonding and ground-
ing. He also serves as an Associate Editor of the IEEE PSE and Energy Systems
Committees with ScholarOne Manuscripts. He was also the recipient of the
IEEE Power Engineering/Industry Applications Society, Orange County Sec-
tion 2012 Outstanding Engineer Award for “the development of new technical
concepts and designs for the advancement of the electrical safety engineering of
low-voltage systems;” the 2012 I&CPS Ralph H. Lee Department Prize Paper
Award; and the PSE Committee 2012 Prize Paper Award.
