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Surge Protection Devices are used for electric power supply networks, telephone networks, and communication and automatic control buses.
2.4 The Surge Protection Device (SPD)
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 (see Fig. J17). 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.
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 electrical switchgear and control gear.
SPD eliminates overvoltages:
- in common mode, between phase and neutral or earth;
- in differential mode, between phase and neutral. In the event of an overvoltage exceeding the operating threshold, the SPD
- 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).
SPD normative definition
2.4.1 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 (see Fig. J19).
– 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 (see section 3.2). 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 15 times.
Type 1 SPD
This is the peak value of a current of 10/350 μs waveform that the SPD is capable of discharging 5 times.
Applicable only to the spark gap technology.
This is the current (50 Hz) that the SPD is capable of interrupting by itself after flashover. This current must always be greater than the prospective short-circuit current at the point of installation.
Type 2 SPD
This is the peak value of a current of 8/20 μs waveform that the SPD is capable of discharging once.
Type 3 SPD
2.4.2 Main applications
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.
These devices protect telephone 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.).
3 Design of the electrical installation protection system
To protect an electrical installation in a building, simple rules apply for the choice of
3.1 Design rules
The logic diagram in the Figure J20 below illustrates this design rule.
The other characteristics for selection of an SPD are predefined for an electrical installation.
- number of poles in SPD;
- voltage protection level Up;
- operating voltage Uc.
This sub-section J3 describes in greater detail the criteria for selection of the protection system according to the characteristics of the installation, the equipment to be protected and the environment.
3.2 Elements of the protection system
An SPD must always be installed at the origin of the electrical installation.
3.2.1 Location and type of SPD
The type of SPD to be installed at the origin of the installation depends on whether or not a lightning protection system is present. If the building is fitted with a lightning protection system (as per IEC 62305), a Type 1 SPD should be installed.
For SPD installed at the incoming end of the installation, the IEC 60364 installation standards lay down minimum values for the following 2 characteristics:
- Nominal discharge current In = 5 kA (8/20) μs;
- Voltage protection level Up (at In) < 2.5 kV.
The number of additional SPDs to be installed is determined by:
- the size of the site and the difficulty of installing bonding conductors. On large sites, it is essential to install an SPD at the incoming end of each subdistribution enclosure.
- the distance separating sensitive loads to be protected from the incoming-end protection device. When the loads are located more than 30 meters away from the incoming-end protection device, it is necessary to provide for additional fine protection as close as possible to sensitive loads. The phenomena of wave reflection are increasing from 10 meters (see chapter 6.5)
- the risk of exposure. In the case of a very exposed site, the incoming-end SPD cannot ensure both a high flow of lightning current and a sufficiently low voltage protection level. In particular, a Type 1 SPD is generally accompanied by a Type 2 SPD.
The table in Figure J21 below shows the quantity and type of SPD to be set up on the basis of the two factors defined above.
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