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Guide to
| Surge Protection Devices
8
Surge protection devices are classified according to their
standard into different types
• Type 1 - SPD which can discharge partial lightning current
with a typical waveform 10/350 μs. Usually employs spark
gap technology.
• Type 2 - SPD which can prevent the spread of over-
voltages in the electrical installations and protects equipment
connected to it. It usually employs metal oxide varistor (MOV)
technology and is characterized by an 8/20 μs current wave.
• Type 3 – These SPDs have a low discharge capacity. They
must therefore only be installed as a supplement to Type 2
SPD and in the vicinity of sensitive loads. Type 3 SPD’s are
characterised by a combination of voltage waves (1.2/50 μs)
and current waves (8/20 μs).
I
imp
– Impulse current of 10/350 μs waveform associated with
Type 1 spd’s
I
n
– Surge current of 8/20 μs waveform associated with Type
2 spd’s
U
p
- The residual voltage that is measured across the terminal
of the SPD when I
n
is applied.
U
c
- The maximum voltage which may be continuously
applied to the SPD without it conducting.
Terminology & selection criteria