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Total Solution to Earthing & Lightning Protection |
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Table 2:
General indication of system impairments, of which manufacturers of transient overvoltage protectors should provide details
Protectors for mains supplies
Protectors for data lines
Low frequency
Radio frequency
Parallel protectors In-line protectors
protectors
Network protectors protectors
Nominal operating voltage
•
•
•
•
•
Maximum operating voltage
•
•
•
•
•
Leakage current
•
•
•
•
•
Nominal current rating
–
•
•
•
•
Max continuous current rating
–
•
•
•
•
In-line impedance
–
•
•
•
•
Shunt capacitance
–
–
–
•
•
Bandwidth
–
–
•
•
•
Voltage standing wave ratio
–
–
–
•
•
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 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 and 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 IEC/BS EN 62305)
must have a low let-through voltage between every pair
of conductors.