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Furse, Wilford Road, Nottingham, NG2 1EB • Tel: +44 (0)115 964 3700 • Email:
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www.furse.comTSC-0912 - 09.10.12
Compatibility
The protector must not interfere with the system’s
normal operation:
mains power supply SPDs should not disrupt the
normal power supply such as creating follow
current that could blow supply fuses, or cause high
leakage currents to earth
SPDs for data communication, signal and
telephone lines should not impair or restrict the
systems’ data or signal transmission
Survival
It is vital that the protector is capable of surviving the
worst case transients expected at its installation
point/LPZ boundary.
More importantly, since lightning is a multiple event,
the protector must be able to withstand repeated
transients.
The highest surge currents occur at the service
entrance (boundary LPZ
0
A
to LPZ 1). For buildings with
a structural LPS, the lightning current SPD could be
subject to as high as 25 kA 10/350 µs surge currents per
mode on a 3-phase TN-S/TN-C-S mains system (up to
2.5 kA 10/350 µs per mode on a signal or telecom line)
for a worst-case lightning strike of 200,000 A.
However, this 200 kA level of lightning current itself is
extremely rare (approx. 1% probability of occurring)
and the peak current the SPD would be subject to
further assumes that a structure is only fed with one
metallic service.
Almost all structures have several metallic services
connected to them such as gas, water, mains, data &
telecoms.
Each service shares a portion of the lightning current
when the protected building receives a strike, greatly
reducing the overall current seen by any single service,
and as such any SPD fitted to the electric service lines.
Transient overvoltages caused by the secondary effects
of lightning are considerably more common (lightning
flash near a connected service up to 1 km away from
the structure) and therefore are unlikely to have
currents exceeding 10 kA 8/20 µs.
Let-through voltage
The larger the transient overvoltage, the greater the
risk of flashover, equipment interference, physical
damage and hence system downtime.
Therefore, the transient overvoltage let through the
protector (also known as the voltage protection level
U
p
of the SPD) should be as low as possible and
certainly lower than the level at which flashover,
interference or component degradation may occur.
Transient overvoltages can exist between any pair of
conductors:
phase to neutral, phase to earth and neutral to
earth on mains power supplies
line to line and line(s) to earth on data
communication, signal and telephone lines
Thus, a good protector (enhanced SPDs to
BS EN 62305) must have a low let-through voltage
between every pair of conductors.
Enhanced performance SPDs - SPD*
BS EN 62305-2 details the application of improved
performance SPDs to further lower the risk from
damage.
The lower the sparkover voltage, the lower the chance
of flashover causing insulation breakdown, electric
shock and fire.
SPDs that offer lower let-through voltages further
reduce the risks of injury to living beings, physical
damage as well as failure and malfunction of internal
systems.
All Furse ESP protectors offer such superior protection
and are termed as enhanced performance SPDs (SPD*)
in line with BS EN 62305.
Enhanced SPDs can also satisfy more than one test
class/category by handling both high-energy partial
lightning currents of 10/350 µs waveshape whilst
offering very low let-through voltages.
Such enhanced SPDs may be suitable for changing a
lightning protection zone from LPZ
0
A
right through to
LPZ 3 at a single boundary or installation point.
As such they provide both technical and economic
advantages over standard SPDs.
End of life
When an SPD comes to the end of its working life it
should not leave equipment unprotected.
Thus in-line protectors should take the line out of
commission, preventing subsequent transients from
damaging equipment.
SPDs for data communication, signal and telephone
lines and protectors for low current mains power
supplies are usually in-line devices.
Where SPDs are installed at mains power distribution
boards it is usually unacceptable for these to suddenly
fail, cutting the power supply.
Consequently, to prevent equipment being left
unprotected, the SPD should have a clear pre
end-of-life warning, which allows plenty of time
for it to be replaced.
How to get effective protection