Common characteristics of SPDs according to the installation characteristics
From Electrical Installation Guide
Operating voltage Uc
Depending on the system earthing arrangement, the maximum continuous operating voltage Uc of SPD must be equal to or greater than the values shown in the table in Figure J23.
|SPDs connected between (as applicable)||System configuration of distribution network|
|TN system||TT system||IT system|
|Line conductor and neutral conductor||1.1 U / √3||1.1 U / √3||1.1 U / √3|
|Line conductor and PE conductor||1.1 U / √3||1.1 U / √3||1.1 U|
|Line conductor and PEN conductor||1.1 U / √3||N/A||N/A|
|Neutral conductor and PE conductor||U / √3[a]||U / √3[a]||1.1 U / √3|
[a] these values are related to worst-case fault conditions, therefore the tolerance of 10 % is not taken into account.
N/A: not applicable
U: line-to-line voltage of the low-voltage system
Fig. J23: Stipulated minimum value of Uc for SPDs depending on the system earthing arrangement (based on Table 534.2 of the IEC 60364-5-53 standard)
The most common values of Uc chosen according to the system earthing arrangement.
TT, TN: 260, 320, 340, 350 V
IT: 440, 460 V
Voltage protection level Up (at In)
The IEC 60364-4-44 standard helps with the choice of the protection level Up for the SPD in function of the loads to be protected. The table of Figure J24 indicates the impulse withstand capability of each kind of equipment.
|Nominal voltage of the installation[a] (V)||Voltage line to neutral derived from nominal voltages a.c. or d.c. up to and including (V)||Required rated impulse withstand voltage of equipment[c] (kV)|
|Overvoltage category IV (equipment with very high rated impulse voltage)||Overvoltage category III (equipment with high rated impulse voltage)||Overvoltage category II (equipment with normal rated impulse voltage)||Overvoltage category I (equipment with reduced rated impulse voltage)|
|For example, energy meter, telecontrol systems||For example, distribution boards, switches socket-outlets||For example, distribution domestic appliances, tools||For example, sensitive electronic equipment|
|1500 d.c.||1500 d.c.||8||6|
[a] According to IEC 60038:2009.
[b] In Canada and USA, for voltages to earth higher than 300 V, the rated impulse voltage corresponding to the next highest voltage in this column applies.
[c] This rated impulse voltage is applied between live conductors and PE.
[d] For IT systems operations at 220-240 V, the 230/400 row shall be used, due to the voltage to earth at the earth fault on one line.
Fig. J24: Required rated impulse voltage of equipment Uw (table 443.2 of IEC 60364-4-44)
Fig. J25: Overvoltage category of equipment
The "installed" Up performance should be compared with the impulse withstand capability of the loads.
SPD has a voltage protection level Up that is intrinsic, i.e. defined and tested independently of its installation. In practice, for the choice of Up performance of a SPD, a safety margin must be taken to allow for the overvoltages inherent in the installation of the SPD (see Figure J26 and Connection of Surge Protection Device).
The "installed" voltage protection level Up generally adopted to protect sensitive equipment in 230/400 V electrical installations is 2.5 kV (overvoltage category II, see Fig. J27).
If the stipulated voltage protection level cannot be achieved by the incoming-end SPD or if sensitive equipment items are remote (see Elements of the protection system#Location and type of SPD Location and type of SPD , additional coordinated SPD must be installed to achieve the required protection level.
Number of poles
- Depending on the system earthing arrangement, it is necessary to provide for a SPD architecture ensuring protection in common mode (CM) and differential mode (DM).
|Phase-to-neutral (DM)||Recommended[a]||-||Recommended||Not useful|
|Phase-to-earth (PE or PEN) (CM)||Yes||Yes||Yes||Yes|
|Neutral-to-earth (PE) (CM)||Yes||-||Yes||Yes[b]|
[a] The protection between phase and neutral can either be incorporated in the SPD placed at the origin of the installation, or be remoted close to the equipment to be protected
[b] If neutral distributed
Fig. J27: Protection need according to the system earthing arrangement
- Common-mode overvoltage
- A basic form of protection is to install a SPD in common mode between phases and the PE (or PEN) conductor, whatever the type of system earthing arrangement used.
- Differential-mode overvoltage
- In the TT and TN-S systems, earthing of the neutral results in an asymmetry due to earth impedances which leads to the appearance of differential-mode voltages, even though the overvoltage induced by a lightning stroke is common-mode.
2P, 3P and 4P SPDs
(see Fig. J28)
- These are adapted to the IT, TN-C, TN-C-S systems.
- They provide protection merely against common-mode overvoltages.
1P + N, 3P + N SPDs
(see Fig. J29)
- These are adapted to the TT and TN-S systems.
- They provide protection against common-mode and differential-mode overvoltages