Page 229 - IPD Product Catalog V17

Lightning & Surge Protection

Primary and secondary protection
AS 1768 requires primary protection SPDs to be installed at the LPZ0 and LPZ1 boundaries to protect downstream equipment.

Secondary protection SPDs are located downstream of primary protection SPDs at the LPZ1 and LPZ2 boundaries.
In a main switchboard with an MEN system (TN-C-S earthing system), AS 1768 requires primary protection SPDs to be installed between each phase
conductor and neutral. Often, SPDs in a main switchboard are Type 1 or Type 1+2. 1 1

RCBO RCBO O
O
R
B
C

B
C
R

For secondary protection RCBO RCBO O

R
C
B
O

C
B
R
SPDs, AS 1768 requires SPDs M Main switch h

i
sw
a
tc
in
r
C
o
to be installed between RCBO or CB B

C
B
O
R

C
B
R
each phase and neutral RCBO O
and between neutral
and earth.

If the SPD is located in a
main switchboard but more
than 10 metres from the
MEN link, then the SPDs
shall be installed between
each phase and neutral as
well as neutral to earth.
Typical SPD location for MEN system Typical SPD location for non-MEN system
Voltage protection level
The Voltage Protection level U , is an important consideration in the selection of an SPD, and describes the let through voltage of an SPD for a specified
p
test waveform. Electrical devices are characterised by overvoltage categories, or impulse withstand voltage U , as defined in IEC 60643-4-443 and
w
referred to in AS 1768, which nominates the maximum impulse voltage the equipment must be capable to withstand.
Required minimum impulse withstand voltage (kV)
Nominal
voltage of the Category IV Category III Category II Category I
installation (V) (Equipment with very high (Equipment with high impulse (Equipment with normal (Equipment with reduced
impulse voltage) voltage) impulse voltage) impulse voltage)
230/400
240/415 6 4 2.5 2.5
Selection of the appropriate surge protection devices should consider the impulse withstand rating of the equipment to be protected. In the IEC
standards the voltage protection level U is measured at the I (nominal discharge current) of the SPD.
p n

The voltage protection level U should always below the impulse withstand voltage U of the equipment to be protected. IEC 62305-4 advises selecting
p w
an SPD with a lower value U compared to the equipment U results in a lower stress to the equipment that may result in not just lower probability of
p w
damage, but also a longer operating life.
SPD Technology
SPDs are designed to mitigate transient over-voltages to provide protection for down stream equipment. They can be voltage switching, voltage
limiting, or a combination of these techniques.
Voltage switching – An SPD that has a high impedance when no surge is present but will have a sudden change in impedance to a very low value in
response to a voltage surge. Common examples of voltage switching SPDs are spark Gaps, triggered spark gaps, gas discharge tubes (GDT)
and thyristors.
Voltage limiting – An SPD that has a high impedance when no surge is present but will reduce impedance continuously with increased surge current and
voltage. Common examples of voltage limiting SPDs are metal oxide varistors (MOV) and avalanche breakdown diodes.
Installation requirements
AS 1768 requires conductor lengths connecting the SPD to both line and earth must be kept short. AS 3000 Wiring Rules advises total lead length (line to
SPD and SPD to earth) to be ideally not more than 300 to 600mm. The minimum cross-sectional area of the cable allowable is 6mm . 2
IEC 62305-4 assumes at the line entrance to a structure the volt drop is 1 kV per metre, so it is easy to see how the lead length to/from the SPD can affect
the overall voltage protection level provided to the equipment.
1300 556 601 | ipd.com.au | V17-R05 229