Common characteristics of SPDs according to the installation characteristics

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.

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)

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	U / √3	1.1 U / √3

• N/A: not applicable
• U: line-to-line voltage of the low-voltage system

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.

Fig. J24 – Required rated impulse voltage of equipment Uw (table 443.2 of IEC 60364-4-44)

Nominal voltage of the installation (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 (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
120/208	150	4	2.5	1.5	0.8
230/400	300	6	4	2.5	1.5
277/480
400/690	600	8	6	4	2.5
1000	1000	12	8	6	4
1500 d.c.	1500 d.c.			8	6

Fig. J25 – Overvoltage category of equipment

	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.

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. J26 – Installed 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).

Note:

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).

Fig. J27 – Protection need according to the system earthing arrangement

	TT	TN-C	TN-S	IT
Phase-to-neutral (DM)	Recommended	-	Recommended	Not useful
Phase-to-earth (PE or PEN) (CM)	Yes	Yes	Yes	Yes
Neutral-to-earth (PE) (CM)	Yes	-	Yes	Yes

Note:

• 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. J28 – 1P, 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. J29 – 1P + N, 3P + N SPDs