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Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
Matthew Caie
B.E. Hons (Elect), M.B.A
High-energy over-current and over-voltage transients induced by direct lightning strikes or conducted
into a site via power, signalling or communication lines cause millions of dollars damage each year.
No single technology or action can prevent damage for all possible mechanisms, therefore a coordinated protection approach is required. Commencing with, for sites exposed to direct lightning, an
effective means to capture, conduct and then safely dissipate the energy in direct lightning strikes to
earth. For sites with incoming power, communication or signalling services, a means of diverting
transients arriving at the site via co-ordinated surge protection and isolation. In addition to surge
protection, the need to provide a low impedance earth plane throughout the site is required. The
required features of such configurations for Rail Facility applications are discussed. This paper will
concentrate on addressing co-ordinated earthing with effective techniques for protection against the
effects of lightning for rail facilities, focusing on the elements of the earthing system from the ground
up to the equipment being protected. It is acknowledged that damage to electrical equipment may be
derived from sources other than lightning, the principles discussed however may also be applicable as
a design foundation in protection with additional consideration.
Lightning and over-voltage transients cause
millions of dollars damage to low voltage
installations each year. Damage to equipment
in the US for example is estimated to be at
least US$1.2 billion annually, before including
the loss of productivity from industrial and
business downtime [1].
High-energy over-voltage transients may be
derived from direct lightning strikes to
structures or they may be conducted on power,
signalling or communication cables entering
facilities. Induced transient over-voltages may
also originate from near strikes due to
capacitive or inductive coupling.
Some of the characteristics of the dangers
• Peak currents can exceed 200kA with
10/350µs wave-shapes (IEC 62305-1 [2]
• Current rise times vary from 0.1 to 100 µs.
• Multiple pulse surges are experienced in
over 70% of lightning strike cases. This is
a naturally occurring phenomenon where
up to 20 restrikes may follow the path of
the main discharge.
• Continuing currents of 200-500A lasting 12 seconds may also occur.
There is no single technology that can
eliminate the risk of lightning and subsequent
transients that occur. A holistic co-ordinated
systems approach is required, and can be
applied to the majority of applications. This
systems approach is documented in a number
of national and international standards, notably
the IEC62305 [2] series of standards for
protection against the effects of lightning, and
can be broken down into a co-ordinated
approach as referenced by ERICO as the Six
Point Protection Plan.
The ERICO Six Point Plan recommends:
1. Capture the direct lightning strike at a
preferred point, creating a zone of
protection for the structure to be protected;
2. Conduct the lightning current to ground
safely, minimising the dangers of side-flash
to people, equipment or structures;
3. Dissipate the energy into the ground with
minimal rise in earth potential through a low
impedance earthing system;
4. Eliminate earth loops and differentials by
creating an equipotential earth plane under
transient conditions;
5. Protect equipment from surges and
transients on power lines; and
6. Protect equipment from surges and
transients on communications and signal
lines to prevent equipment damage and
costly operational downtime.
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
This paper will focus only on the points 3 to 6
of the Six Point Plan.
Co-ordinated earthing for protection of railway
signalling and communication systems
The IEC 62561-7 set out the test and
performance requirements for such materials
which include:
Regardless of the path of the lightning energy,
either a direct strike, conducted surge, induced
transients or through earth potential rise, once
the energy is conducted to ground level, a low
impedance earth is essential to dissipate the
lightning energy into the earth mass as
effectively as possible. The earthing systems
for dedicated lightning protection terminals,
tower or mast footings and electronic
equipment rooms or control centres are critical
design elements.
Attributes of an ideal earthing arrangement:
• Each earthing system (lightning, electrical,
communications, and equipment room)
must be individually of high integrity, as well
as being considered a component of an
overall earthing network. Where separate
earths exist, they should be bonded
Bonding of all earthing
systems is also typically required by code,
with some exceptions not covered in the
scope of this paper.
• Because lightning is a multiple frequency
event, it is the high frequency “impedance”
that is the critical design element, not the
D.C. resistance.
• An earth loop conductor should surround
sensitive electronic equipment rooms or
facilities. This will reduce the risk of
potential gradients across the facility.
• There should be a “single” point
connection to the ground network from all
equipment within a facility.
The use of earth enhancing materials to
improve soil conditions and therefore achieve a
lower resistance connection to ground is
becoming more common practice around the
world. Until a few years ago, there was no
requirements of such materials. Now into its
second committee cycle, the relatively new
Enhancing Compounds) [3] standard sets out
to define the product testing and marking
requirements for such products.
1. Leaching tests – important to consider the
longevity of the material in the ground over
2. Sulfur content tests – lower quality materials
can include bi-products or additives that act to
corrode the earthing conductors.
3. Determination of resistivity – an important
measure to determine the impact on the earth
system resistance the material will have.
4. Corrosion tests – to determine the corrosion
rate on copper or other conductors by the
material, used to predict the life of the earthing
system when the material is used.
5. Marking and indications – to ensure that
users of the material can contrast and compare
different standardised materials.
This standard illustrates the key factors
important to consider when considering the
use of an earth enhancing compound.
The earthing conductor and interconnection of
earthing conductors are also important factors
in the longevity of the earthing system. In this
section a review of the basic metals used for
earthing conductors are reviewed.
A set of basic requirements (desirable
features) is defined for earthing conductors.
These basic requirements are detailed below:
Conductive (reduce voltage differences).
The more conductive the conductor the more
beneficial it will be to the application. In
addition to being conductive, it is important for
the conductor to exhibit a low contact
resistance, meaning good electrical connection
either with the soil (ground) or with other
conductors through connectors. Certain
materials may have good conductivity,
however exhibit poor contact resistance,
especially following oxidization.
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
Resist fusing and remain mechanically
sound when exposed to electrical fault
The ability of a conductor to conduct high
electrical fault currents is a function of the
materials conductivity and thermal properties.
Electrical fault currents place high thermal and
mechanical stress on conductors. Specifically
in electrical substation or transformer
grounding applications this requirement is an
overriding factor, however it is not relevant for
lightning protection application due to the
relatively low energy of the lightning
Practically all earthing applications require
mechanical strength of the conductor to resist
damage, sources of potential damage range
from driving the conductor into the ground,
scratching coating layers to fatigue from traffic,
earthwork or installation. Various standards
such as UL and IEC define clearly the
mechanical strength requirements for various
Resist corrosion over design life
Corrosion is one of the largest concerns
relating to the selection of the appropriate
earthing conductor. Corrosion can occur in
many ways from galvanic, to interaction with
corrosive soils, to oxidization. An ideal
conductor would be compatible with other
buried metals in the ground, compatible with
other metals in contact with, resist the
corrosive nature of the soil it is buried within
and resist oxidation when exposed to open air
over time. It is important to state that various
technologies perform better in certain
environments and conditions than others, and
therefore no one technology is suited for all
applications and environments.
Cost effective installation
the installation does not have influence or
knowledge of the lifetime cost benefits,
however, when the this is not the case the cost
per year installed is more likely to be taken into
account. Additionally the increase in metal
prices, specifically copper, has driven a trend
away from the amount of copper used in
applications in an effort to curb grounding
costs. The labour or equipment required to
install certain technologies may also make
them less attractive for certain applications,
therefore the ability of the conductor to be
formed (flexible), and its ease of handling
contribute greatly to a cost effective
Resisting theft
Over the past decade concern over the value
to thieves of the conductor before, during and
after installation has risen. This is driven by the
increased cost of metals, predominantly
copper, and has resulted in a clear trend in
many applications to reducing the level of
copper content used. An ideal conductor
would be one that either is difficult to steal, has
little re-sale value or at least deters or cloaks
its value to potential thieves.
Ease and
A conductor is only as good as its connection
method within the electrical system. Certain
conductors may be costly or difficult to
interconnect with the electrical system. This
may range from galvanic concerns requiring
special termination procedures, to the flexibility
of the conductor, or the cost and availability of
the connectors designed or approved to be
compatible with the conductor. Copper is a
versatile material to terminate, is easily formed
and compressed compared to steel which is
ideally required to be welded or bolted.
Exothermic welding methods, especially in
below grade applications over time have
proven to be reliable and permanent.
The cost of the conductor, for the life of the
installation is critical in practically every
application. Initial purchase cost is the
dominant factor specifically when the owner of
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
Basic Choice of Metals
There are a handful of basic metals to choose.
There is a clear trend and history in combining
these metals together to achieve the
advantages of each, which is a reflection on
the fact that no one metal is perfect for all
aspects on the application. Each of these
material properties as it relates to the
application is summarized below. Copper,
Aluminium, Steel.
High conductivity, easy to form and terminate,
resistant to most underground corrosion. The
reasons not to use copper would be the high
value of the metal, compatibility with other
metals or corrosion concerns. Copper is a
noble metal and is almost totally impervious to
corrosion in many types of soils.
Co-ordinated earthing for protection of railway
signalling and communication systems
bonded or clad coatings do not increase the
current carrying capability of the steel
Stainless Steel clad or solid stainless steel
used in the application of grounding
termination is well documented to provide an
excellent corrosion resistance below grade.
The overall below grade performance of
stainless steel in both resisting soil corrosion
and galvanic corrosion is well documented.
Stainless steel however is currently a relatively
more expensive technology, and also has a
characteristic higher resistance and contact
resistance with the soil over time than copper
based or copper bi-metallic conductors.
As stated in point 4 of the Six Point protection
plan, it is critical to provide an earthing layout
that eliminates earth loops and differentials by
creating an equipotential earth plane under
transient conditions.
Good conductivity, easy to form, subject to
corrosion in soils and is anodic compared to
most other metals. A suitable metal to be used
in above grade grounding applications or
insulated below grade. Special attention is
required for connection methods to other
conductors due to bi-metallic concerns and
oxidation that may develop resulting in poor
Steel (including Bi-metals)
Relative compared to copper or aluminium,
steel has a low conductivity, difficult to form
during application, subject to corrosion above
and below grade, requires a coatings or
treatment to resist corrosion rate. In the
grounding application, steel is typically used
with a copper bonded layer or copper cladding,
zinc plating or a stainless treatment or plating.
It is well documented that copper bonding of
steel will provide “some” of the corrosive and
conductivity of copper in the find product, at a
lower cost/value and higher strength when
compared to using solid copper alone.
Likewise plating steel with zinc provides
excellent cost effective corrosion protection
above grade. Below grade the zinc coating
provides a limited lifespan when compared to
other more noble and “thicker” coatings, a fact
that is well documented and accepted for the
Zinc coatings, unlike copper
Figure 1 shows an example of an earthing
system with a “single” point connection of
mains power and communications equipment
earth wires to the ground ring. If a surge
arrives at the facility via the mains power
supply, the surge protection equipment will
divert excessive energy to earth, and the
telecommunications and lightning protection
grounds will rise equipotentially with all other
grounding / ground points as they are closely
bonded together. There is therefore little
opportunity for potential differences between
ground points creating earth loops, or causing
sparking or side flashing.
Figure 1: Single point earthing arrangement
Figure 2 shows a “non-ideal” system with
multiple connection points to the earth-ring.
Although adequate surge and lightning
protection equipment on both the power and
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
communications interfaces is provided, the
separated electrical and communications earth
are located some distance apart (as shown by
the parameter ‘d’ in figure 2). Regardless of
the impedance of each individual earth, for a
very short time the potential of the electrical
ground will be higher than the communications
ground. As a result the excess energy has two
potential paths to follow to reach the lower
potential communications earth, thus creating
a dangerous ‘earth loop’ that will damage
sensitive electronic equipment in the
equipment room.
Co-ordinated earthing for protection of railway
signalling and communication systems
• Special materials or compounds can be
used to reduce earth impedances at
locations where the earth resistivity is high
such as in rocky, sandy or mountainous
areas with large particle soil sizes. A
relatively new international standard now
exists for such materials or compounds.
• Interconnections of conductors are a weak
point in any earthing system, more-so
below grade, the use of Exothermic a
molecular bonding processes (copper-tocopper or alloys and copper-to-steel or
alloys) for earthing and lightning protection
systems provide connections that are
• permanent;
• low impedance;
Finally, regular testing of the earthing system is
recommended across various seasons to
determine and change in performance over
time, and to trigger the need for maintenance.
A number of indicative tests are available to
diagnose grounding problems and to evaluate
the true transient performance of a grounding
system prior to a real lightning event.
Figure 2: non-ideal earthing arrangement
The following key points to consider for an
effective co-ordinated earthing system for the
mitigation of the direct and secondary effects
from lightning.
• A single point earthing system arrangement
is preferred, when this is not possible or
practical, a multiple path bonding (or mesh
system) system is a compromise.
• A low-impedance earthing system ensures
a more reliable flow of energy to ground,
minimising earth potential rise. The use of a
radial earthing technique allows energy to
diverge as each conductor takes a share of
the current. This acts to lower voltage
gradients leading away from the injection
point reducing voltage “step potentials”
affecting equipment or people.
• The selection of below grade earthing
conductors will affect the life and
performance of the earthing system.
Electrolytic copper-bonded steel conductors
provide a cost-effective means of earthing
for most standard applications.
An effective earthing system is the foundation
of the overall facility protection system, and
provides the means for the effective operation
of other elements such as surge protection and
lightning protection.
Surge protection, when used as part of a coordinated earth system, is an effective means
to protect electrical equipment from surges and
transients originating from lightning or other
means. In the simplest terms it provides a
bond to earth for service conductors, either
power, signal or communication, a diversion of
excessive energy from those conductors to
earth instead of electrical equipment, limiting
the voltage from those conductors with
reference to earth potential.
This paper looks at surge protection from to
main aspects, the desired performance of the
device itself, and then from how the device
should be effectively applied while forming part
of the co-ordinated earth system.
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
There are three critical performance aspects of
the device itself.
Two issues need to be considered when
determining the surge rating of a Surge
Protection Device (SPD) for a specific location:
What is the largest surge impulse the
site is likely to require protection
against; and
Will this rating provide sufficient
operational life under the more
frequent smaller impulses?
Competition between SPD manufactures has
seen ever-increasing surge ratings being
offered to the market, to the point where
surges of this magnitude are unlikely to ever
occur in nature. A number of sources provide
information on the statistical distribution of the
current discharge of the direct lightning strike.
Many studies have shown that peak lightning
discharges above 100kA are likely to occur
less than 5% of the time. Combined with the
fact that most discharges do not strike the
service conductor directly but are magnetically
or capacitive coupled to it, and that even under
a direct lightning discharge the energy will split
in either direction and be attenuated by the
distribution arrestors and line losses it is not
difficult to determine that a smaller fraction of
the initial lightning energy typically enters the
facility in question. It’s important to note
however that some Rail Locations are very
isolated and are often the only building in a
particular area.
The NFPA80 [6] Lightning Protection standard
has classified the “point of entry” environment
for power conductors as 40kA 8/20us or 20kA
In contrast to the above IEC62305 [2] standard
defines some differing guidelines, describing
protection zone concepts.
A “zone” is where the lighting electromagnetic
environment can be defined/controlled. The
zones are characterised by significant changes
of electromagnetic conditions at their
boundaries. These will typically be building
Co-ordinated earthing for protection of railway
signalling and communication systems
boundaries, or the point where protection is
As it can be shown, protection equipment for
power supply systems are classified as follows
according to its task.
• Class I SPD - Lightning Arrestors
• Class II SPD - Overvoltage Arrestor
Lightning current arrestors must be capable of
conducting lightning currents or major
components of them without being destroyed.
Overvoltage arrestors are only used for limiting
over-voltages at relatively smaller surge
currents. The different “protection zones”
assume the division of the initial lightning
current from zone 0 or external exposed
conductors to subsequent zones within the
facility. For zone 0, it is required for the user to
select the lightning protection class, from I –
IV: (i.e. these refer to max energy within a
direct lightning strike)
Protection Level I: 200kA (10/350us)
Protection Level II: 150kA (10/350us)
Protection Level III - IV: 100kA (10/350us)
The above levels can be selected based on
statistical level of protection required. A
lightning current of 200kA (10/350us) can be
expected for the Protection Level I.
This lightning current is divided as follows in
the most exposed sites:
50% (100kA, 10/350us) discharges via the
earth system.
50% (100kA, 10/350us) flows into the supply
systems connected to it. (i.e. power supply
system, signal or communications system,
metal pipes etc)
In the worst case the power supply system is
present only with two conductors (L;PEN), then
this is loaded with 50% of the lightning current,
or 50kA (10/350us) per conductor.
In summary, if the IEC standard were selected,
the highest surge current expected at Zone 0,
with Protection Level I, for single phase two
conductor is 50kA (10/350us) or 25kA
(10/350us) for three phase 4-wire systems. In
contrast, the lowest required surge current
expected at Zone 0, with Protection Level IIIIV, for single phase two conductor is 25kA
(10/350us) or 12.5kA (10/350us) for three
phase 4-wire systems.
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
It is a matter of selecting the product with the
required surge rating for the exposure
application, and the standards mentioned are a
good guide in making this selection.
such as disconnected neutrals in unbalanced
three phase WYE systems, or by missinstallation of SPDs. UL1449 now specifies the
following over voltage requirements:
Firstly when an over voltage of 110% of
nominal voltage is applied, the device must
remain functional and safe. Secondly when an
abnormal over voltage of 125% is applied, the
device is allowed to permanently stop
functioning, but must not become unsafe.
Finally when the full phase voltage according
to Table 2 is applied, the device is allowed to
permanently stop functioning, but must not
become a fire or safety hazard.
The role of the SPD is to limit the transient
voltage due to lightning or the more common
switching transient from reaching the
equipment, thereby preventing possible
transient voltage damage to the equipment. No
SPD device is perfect, and some resulting letthrough voltage above the nominal service
conductor voltage will still reach the
equipment. This is acceptable provided that
the let-through voltage is within the range that
the ‘protected’ equipment can withstand. Note
over time a lower let-through product will
increase the equipment reliability, as the
equipment would be less stressed, and
multiple stresses over a period of time may
cause ageing and eventual failure.
Electronic equipment is sensitive not only to
the magnitude of the let-through, but also to
the rate of the pre-clamped voltage rise, i.e.
the slope of the leading edge. This is known as
the dV/dt, change in voltage over time.
This issue generally relates to the construction
of the product, i.e. strength, accessibility of live
parts and equally important to the maximum
voltage the SPD can withstand without
becoming a fire or safety hazard. The latter is a
very important safety issue. Traditionally SPDs
could not differentiate between slower overvoltages and the faster transient voltages. For
continuous operating voltage of 20% above the
normal voltage was selected; as to make this
much higher would also increase the letthrough voltage (under transient impulses) to a
point where the equipment may not have been
sufficiently protected. There are however many
known cases where the nominal voltage has
exceeded this 20%, causing the SPD to clamp
on each half cycle and build up sufficient heat
to become a hazard.
developed the UL1449 [5] standard, to address
this growing problem. Over voltages can be
caused by poor power regulation, wiring faults
Although this Full Phase Voltage test is an
extreme over-voltage, it is recommended that
even with sites with known good power
regulation that UL1449 compliant products, or
those with equivalent MCOV ratings, are
selected, as wiring faults and accidents can
occur. test or the full phase voltage test.
The International (or European version) safety
and testing standard for surge protection
devices is IEC61643. This standard is similar
to the UL1449 standard in regards to product
requirements under over voltage conditions.
However both the UL1449 [5] and IEC61643
[3] standard as extremely important in
specifying the safety requirements for surge
protection devices.
To a user of SPDs, devices marked with a UL
and CE mark reassure compliant to both these
safety and performance standards. It means
the markings of the products are standardised
and able to be compared by users and that
products meet basic safety in operation
requirements. To state that compliance with
the above two standards, in some countries
mandatory standards, is of little importance is
under-estimating the need for product safety
under the most testing conditions.
Many different types of SPDs and technology
are available on the market. To enable the
selection of effective protection at the best
value for money, one needs to make a
selection based on the most important
technical performance specifications. In
addition to the above-mentioned critical
factors, the following selection parameters
should be considered:
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
Voltage (MCOV) withstand
Indication & Life
Physical & Environmental issues
Protection of land-based communications,
signal and data lines into the facility are an
Transients up to 20 kA 8/20µs injected onto
telecommunications and signal lines can
damage and destroy sensitive terminal
equipment and lead to facility down time.
equipment necessitates the same concepts as
used in Point 5, however due to the differing
exposure levels, much lower operating
currents and voltages and increased sensitivity
of equipment, the parameters of each surge
protector need to be carefully selected.
Knowing where to install surge protection can
be difficult. To ensure cost-effective protection
is provided for data, signalling and control
circuits, two issues need to be considered:
• Where should the SPDs be installed?
• What type of SPD is appropriate for each
circuit type and location?
Communications devices are at risk from
interconnecting signal lines. The use of surge
protection barriers, installed at either end of the
lines, provides cost effective mitigation. The
highest risk is posed by communication or
signal lines that enter or exit the building. In
such circumstances, protection devices should
be installed at the point of entry or at the
equipment termination itself. Internal wiring
which extends more than 10 – 15m should also
be protected. Twisting or shielding of cables
provides a level of protection, however this
should not be regarded as sufficient for the
sensitive interfaces that characterize todays
communication devices.
Five parameters must be considered to ensure
that surge protection devices for use on data,
signalling or control circuits are effective and
do not adversely affect operation of the circuit.
sustainable by the equipment, yet
should not interfere with the normal
signalling voltages. As a guide, the
SPD clamping voltage should be
selected to be approximately 20%
higher than peak working voltage of
the circuit.
2. The line current rating of the SPD
should be sufficient to handle the
maximum expected signalling current.
3. The SPD bandwidth should be
sufficient to allow correct operation of
attenuation. This ensures that the
attenuation of the SPD at the nominal
operating frequency of the system
does not exceed the stated limit. For
most SPDs, frequency attenuation
data or a maximum recommended
baud rate is generally specified.
4. The connection termination, mounting
method, number of lines to be
protected and other physical aspects
must be considered.
5. The SPD surge rating should be
appropriate for the intended location.
For circuits internal to the location,
surge ratings of 1-5kA 8/20us are
generally sufficient. For the protection
of circuits that connect to exposed
lines entering or exiting the location,
10-20kA 8/20us is recommended.
It is vital to consider, as per Point 5 of the Six
Point Plan, the surge rating and let-through
voltages are considered when selecting
appropriate protection for communications
and/or data lines. The optimum level of
protection is provided using multistage (or
series) connected products.
Single-stage, “gas arrester only” circuits
provide cost-effective protection for less
sensitive electro-mechanical or discrete
component-type terminals and supplement
circuits with “built-in” protection. Multistage
stage protectors employing gas arrester
primary and decoupled semi-conductor
secondary protection stages can provide lower
clamp (let-through) voltages than the singlestage protectors.
1. SPDs are designed to clamp the
excess transient voltage to safe levels
AusRAIL 2015
24 – 26 November, Melbourne
Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
The correct application of surge protection is
equally as important at the features of the
surge protection device itself. In addition to the
importance of the actual below grade earthing
system quality and layout is the following three
The lead-length is defined as the conductor
length from the active conductor to which the
surge protection device is connected to earth
point itself. The actual “earth point” also needs
to be defined in the application also, however
Figure 3 defines two basis connection means
of an SPD, one being defined as a “T”
connection (non-preferred) and the other as a
series connection (preferred).
As the surge current is diverted from one
conductor to the other through the SPD, the
resultant voltage is the sum of the SPD letthrough at that surge current, and the voltage
drop along the connecting leads. This voltage
drop is determined by the equation:
Equation 1:
V = R.i x L.dV/dt
where: R
- resistance of the leads
- inductance of the leads
- rate voltage rise of the surge
- peak surge current.
Due to the inherent sharp rise time of surges,
the inductance of the connection leads can
lead to voltage drops of approximately 1V/mm
of standard 4-6mm2 wire, for typical magnitude
surges. Clearly a surge protection arrangement
that allows for a series line/load through
connection will allow for optimal performance
of the SPD as it acts to drastically reduce the
let-through voltage protected equipment will be
exposed to under surge conditions.
Figure 3 – SPD Connection methods
From an electrical perspective, the term clean
and dirty wiring refers to the zone or position of
the wiring with reference to surge protection or
filtering devices. Dirty wiring from the aspect of
the design, refers to wiring that is upstream of
the surge protection and therefore considered
to be exposed to surges or transients. From
earlier sections of this paper, it could also be
described as wiring within zone 0 or zone 1,
and accepted within the design to potentially
carry high levels of surge energy. Clean wiring
is considered to be downstream of the surge
It is critically important in the design of the
protection system to provide a physical or
electrical means of separation of the dirty and
clean wiring otherwise it is possible for
unwanted energy from the dirty wiring to be
transferred or induced onto the clean wiring,
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Matthew Caie
effectively bypassing or negating
effectiveness of the surge protection.
Co-ordinated earthing for protection of railway
signalling and communication systems
Separation can be achieved by physical means
such and not installed them both in the same
cable tray or conduit, or shielding them
magnetically from each other when physical
separation is not practical.
These principles are not new, shown in figure 4
is a typical wiring layout within a trackside
signalling bungalow in North America which
shows traditional surge protected terminal
blocks inherently designed to separate the field
“dirty” signal wires from the “clean” equipment
connected wiring through providing a
termination system that allows for a series
connection with a low-impedance earth plane
perpendicular to the wiring to provide a
transition from dirty to clean.
The IEC62305 [2] standard defines these two
basic earth system layouts as described in
Figure 5, defining the layout from the
equipment down to the bonding network, being
either an external earthing system or structural
bonding system that forms part of the earthing
system itself. The star configuration, or single
point earthing system is ideal in creating an
environment that eliminates earth potential.
Star Configuration
All metal components (e.g. cabinets,
enclosures, racks) of the internal
systems shall be isolated from the
earthing system
Shall be integrated into the earthing
system only by a single bonding bar
acting as the earth reference point
Can be used where internal systems
are located in relatively small zones
and all lines enter the zone at one
point only (at the ERP)
Achieves Equipotential by keeping
surge currents out of the equipment
earthing lines
Mesh Bonding Network
All metal components (e.g. cabinets,
enclosures, racks) of the internal
systems are not to be isolated from the
earthing system, but shall be
integrated into it by multiple bonding
Is preferred for internal systems
extended over relatively wide zones or
over a whole structure, where many
lines run between the individual pieces
of equipment, and where the lines
enter the structure at several points
Achieves Equipotential by having
many parallel low-impedance paths,
Figure 4 – Typical field wire termination system
within a trackside bungalow.
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Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
Figure 6: Multi zonal earth system layout.
There are two key aspects to consider when
selecting and applying surge protection for
power, signalling and communication systems.
That is the important performance features of
the surge protection device itself, and the
installation of the surge protection device
including the correct connection means and
the co-ordination with the earthing system
design and layout.
The key performance features to select for the
application of the SPD are:
Figure 5: Star and Meshing Bonding
For larger sites, or for more distributed facilities
or equipment it is possible to apply the star
configuration method more effectively by
defining separate zones. The concept is to
define zones as illustrated by the dotted line
box in figure 6 where all cabling crossing the
zone boundary requires a local SPD
referenced to one single earth bar within that
zone. An example of a multi zonal star earth
system layout is shown in figure 6. This design
considering the earthing upgrade on existing
locations as it is often practical to apply.
Surge protection for the environment
Let-through voltage compatible with
the tolerance of the equipment being
Safe operation, defined by standards
compliance, testing and marking.
The key installation parameters of the SPD
Wiring lead length and means to
reduce it within the application.
Separation of clean and dirty wiring to
arrangement to earth through either a
star point earthing arrangement within
a single zone or a mesh bonding
It is acknowledged that in certain or special
applications there are other secondary
parameters to consider that those stated
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Matthew Caie
Co-ordinated earthing for protection of railway
signalling and communication systems
Direct lightning strikes and over-voltage
transients create major equipment failures and
cause downtime. Analysis of damage has
shown that no single protection device can
provide lightning immunity. Comprehensive
protection is provided only by employing an
integrated Six Point Plan approach to lightning
protection of facilities and assets.
[1] Insurance Information Institute, NY, (NY
Press Release 11 August 1989
[2] IEC62305-1, Protection of structures
against lightning, Part 1: General principles
[3] IEC62561-7, Requirements for Earth
Enhancing Compounds
[4] IEC61643-1 Surge Protective Devices
distribution systems. Part 1: Performance
requirements and testing methods.
[5] UL1449 Edition 4, Standard for Surge
Protection Devices, 2016
[6] NFPA780, Installation Guide for Lightning
Protection Systems, 2014
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