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Common characteristics of SPDs according to the installation characteristics

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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 System configuration of distribution network
TT TN-C TN-S IT with distributed neutral IT without distributed neutral
Line conductor and neutral conductor 1.1 Uo NA 1.1 Uo 1.1 Uo NA
Each line conductor and PE conductor 1.1 Uo NA 1.1 Uo \definecolor{bggrey}{RGB}{234,234,234}\pagecolor{bggrey}\sqrt 3 Uo Vo
Neutral conductor and PE conductor Uo NA Uo Uo NA
Each line conductor and PEN conductor NA 1.1 Uo NA NA NA
NA: not applicable

NOTE 1: Uo is the line-to-neutral voltage, Vo is the line-to-line voltage of the low voltage system.
NOTE 2: This table is based on IEC 61643-1 amendment 1.

Fig. J23Stipulated minimum value of Uc for SPDs depending on the system earthing arrangement (based on Table 53C 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 443-4 section of IEC 60364 standard, “Selection of equipment in the installation”, 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 Required impulse withstand voltage for kV[c]
Three-phase systems[b] Single-phase systems with middle point Equipment at the origin of the installation
(impulse withstand category IV)
Equipment of distribution and final circuits
(impulse withstand category III)
(impulse withstand category II)
Specially protected equipment
(impulse withstand category I)
- 120 - 240 4 2.5 1.5 0.8
- 6 4 2.5 1.5
400/690 - 8 6 4 2.5
1,000 - Values subject to system engineers

[a]  As per IEC 60038.
[b]  In Canada and the United States, for voltages exceeding 300 V relative to earth, the impulse withstand voltage corresponding to the immediately higher voltage in the column is applicable.
[c]  This impulse withstand voltage is applicable between live conductors and the PE conductor

Fig. J24 Equipment impulse withstand category for an installation in conformity with IEC 60364 (Table 44B).

  • Equipment of overvoltage category I is only suitable for use in the fixed installation of buildings where protective means are applied outside the equipment – to limit transient overvoltages to the specified level.
Examples of such equipment are those containing electronic circuits like computers, appliances with electronic programmes, etc.
  • Equipment of overvoltage category II is suitable for connection to the fixed electrical installation, providing a normal degree of availability normally required for current-using equipment.
Examples of such equipment are household appliances and similar loads.
  • Equipment of overvoltage category III is for use in the fixed installation downstream of, and including the main distribution board, providing a high degree of availability.
Examples of such equipment are distribution boards, circuit-breakers, wiring systems including cables, bus-bars, junction boxes, switches, socket-outlets) in the fixed installation, and equipment for industrial use and some other equipment, e.g. stationary motors with permanent connection to the fixed installation.
  • Equipment of overvoltage category IV is suitable for use at, or in the proximity of, the origin of the installation, for example upstream of the main distribution board.
Examples of such equipment are electricity meters, primary overcurrent protection devices and ripple control units.

Fig. J25Overvoltage 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).

Fig. J26Installed Up

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[a]

[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. J27Protection 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.

Fig. J281P, 2P, 3P, 4P SPDs

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

Fig. J291P + N, 3P + N SPDs