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How to apply protection
Transient overvoltages are conducted into the sensitive
circuitry of electronic equipment on power and data
communication, signal and telephone lines. Protection is
recommended for:
–– All cables which enter or leave the building (except
fibre optic)
–– The power supply local to important equipment
–– Electronic equipment outside the main building(s)
Protecting incoming and outgoing electrical services
Lightning strikes between clouds or to ground (and objects
upon it) can cause transient overvoltages to be coupled on to
electrical cables, and hence into the sensitive electronic
equipment connected to them.
To protect the electronic equipment inside a building, all
cables that enter or leave the building must be protected.
Cables leaving the building can also provide a route back into
the building for transients.
For each building protect incoming/outgoing:
–– Mains power supplies (including UPS supplies)
–– Data communication and local area network cables
–– Signal, control, instrumentation and alarm lines
–– CCTV, satellite, TV and antenna cables
–– Telephone and telemetry lines
Protect the power supply locally to important equipment
In addition to installing protection on the mains power supply
as it enters/leaves the building, protection should also be
installed locally to important equipment. Protection at the
main LV (low voltage) incomer(s) is necessary to prevent large
transients from entering the building’s power distribution
system, where they could have far reaching effects.
However, where the cable run to equipment exceeds
10 metres (to BS 7671 Clause 534.2.3.1.1), transient
overvoltages may appear on the mains after the protector at
the main LV incomer. These transients can result from:
–– The electrical switching of large inductive loads within
the building
–– A lightning strike to the building - as lightning currents flow
through down conductors transient overvoltages can be
induced on to nearby power cables
–– The natural inductance and capacitance of long cable runs,
‘amplifying’ the voltage ‘let-through’ the protector at the
main LV incomer
Additionally, local protection guards against the possibility of
a supply which enters/leaves the building being overlooked
and left unprotected.
Protect data lines locally
Generally, the biggest risk to data, signal, telecom and
network wiring is associated with cables that enter and leave
the building.
These should always be protected. However, data cables
within a building can additionally have transients induced on
to them when loops between data and power cables ‘pick up’
voltages from the magnetic field caused by a lightning strike.
As part of the overall SPM, IEC/BS EN 62305 advocates the
use of metal in the structure, and a Faraday cage lightning
protection system to help exclude magnetic fields.
Cable management practices eliminate loops by routeing data
and power cables along the same general path. In these
cases, the need for local data line protection is minimal.
However, where these steps are not possible, data line
protection, local to the equipment requiring protection, should
be considered.
Protect electronic equipment outside the building
On site or field based electronic equipment with mains power,
data communication, video, signal or telephone line inputs will
need to be protected against transient
overvoltages. It may be helpful to think of each equipment
cabinet or cubicle as a separate building with incoming/
outgoing cables to be protected.
Complementary techniques
As well as the use of transient overvoltage protectors,
IEC/BS EN 62305 outlines additional protection techniques
(e.g. shielding measures), which can be used to help reduce
the transient threat as part of the overall SPM.
These are described further in the Furse Guide to
BS EN 62305 Protection Against Lightning. Where these
can be used, principally on new build or refurbishment
projects, they need to be supported by the use of SPDs.