The Surge Protection Device (SPD) is a component of the
electrical installation protection system.This
device is connected in parallel on the power supply circuit of the loads that
it has to protect . It can also be used at all levels of the power supply
network.This is
the most commonly used and most efficient type of overvoltage protection.SPD connected in parallel has a high impedance. Once the
transient overvoltage appears in the system, the impedance of the device
decreases so surge current is driven through the SPD, bypassing the sensitive
equipment.
Principle
SPD is
designed to limit transient overvoltages of atmospheric origin and divert
current waves to earth, so as to limit the amplitude of this overvoltage to a
value that is not hazardous for the electrical installation and electric
switchgear and controlgear.
SPD eliminates overvoltages
- In common mode, between phase and neutral or earth;
- In differential mode, between phase and neutral.
- Conducts the energy to earth, in common mode;
- Distributes the energy to the other live conductors, in differential mode.
The three types of SPD
Type 1 SPD : The Type 1
SPD is recommended in the specific case of service-sector and industrial
buildings, protected by a lightning protection system or a meshed cage.
It
protects electrical installations against direct lightning strokes. It can
discharge the back-current from lightning spreading from the earth conductor to
the network conductors.Type 1 SPD
is characterized by a 10/350 µs current wave.
Type 2 SPD : The Type 2
SPD is the main protection system for all low voltage electrical installations.
Installed in each electrical switchboard, it prevents the spread of
overvoltages in the electrical installations and protects the loads.Type 2 SPD
is characterized by an 8/20 µs current wave.
Type 3 SPD : These SPDs
have a low discharge capacity. They must therefore mandatorily be installed as
a supplement to Type 2 SPD and in the vicinity of sensitive loads.Type 3 SPD
is characterized by a combination of voltage waves (1.2/50 μs) and current
waves (8/20 μs).
Characteristics of SPD
International
standard IEC 61643-11 Edition 1.0 (03/2011) defines the characteristics and
tests for SPD connected to low voltage distribution systems.
Common characteristics
- Uc: Maximum continuous operating voltage : This is the A.C. or D.C. voltage above which the SPD becomes active. This value is chosen according to the rated voltage and the system earthing arrangement.
- Up: Voltage protection level (at In) : This is the maximum voltage across the terminals of the SPD when it is active. This voltage is reached when the current flowing in the SPD is equal to In. The voltage protection level chosen must be below the overvoltage withstand capability of the loads. In the event of lightning strokes, the voltage across the terminals of the SPD generally remains less than Up.
- In: Nominal discharge current : This is the peak value of a current of 8/20 µs waveform that the SPD is capable of discharging minimum 19 times
Main applications
- Low Voltage SPD : Very different devices, from both a technological and usage viewpoint, are designated by this term. Low voltage SPDs are modular to be easily installed inside LV switchboards.There are also SPDs adaptable to power sockets, but these devices have a low discharge capacity.
- SPD for communication networks : These devices protect telephon networks, switched networks and automatic control networks (bus) against overvoltages coming from outside (lightning) and those internal to the power supply network (polluting equipment, switchgear operation, etc.).
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