Section 443 of BS 7671:2018+A2:2022 contains requirements for protection against transient overvoltages either of atmospheric origin or due to switching. Where the application of overvoltage protection measures are required, regulation 443.1 specifies that any surge protective devices (SPDs) should be selected and erected in accordance with Section 534 of that standard. This article looks at the types of SPD recognised by BS 7671 and their location within an installation.
SPDs can be used to provide protection against transient overvoltages either of atmospheric origin (443.4.1) or due to switching (443.4.2). Within DD CLC/TS 61643-12:2009 Surge protective devices connected to low-voltage power distribution systems — Selection and application principles, Clause 4 (systems and equipment to be protected) advises that the following two factors need to be considered when evaluating an installation in respect of the use of SPDs:
• the characteristics of the low-voltage power distribution system on which it will be used, including expected types and levels of overvoltage and current:
• system earthing arrangement (TN-S, TN-C-S, TT, IT etc.)
• nominal voltage
• cause of the overvoltage (lightning, switching or temporary overvoltage); and
• the characteristics of the equipment requiring protection:
• the rated impulse withstand of the equipment, tested in accordance with EN 60664-1; and
• the impulse immunity of the equipment, tested in accordance with EN 61000-4-5.
Based on the aforementioned, suitable Types of SPD may be selected for use at appropriate points within the installation.
SPD Types
Classification of SPD Type relates to the tests the device must be able to meet. The test requirements for SPDs for power systems shall comply with BS EN 61643-11:2012+A11:2018 Low-voltage surge protective devices. Surge protective devices connected to low-voltage power distribution systems. Requirements and test methods.
There are three classes of tests. The introduction to BS EN 61643-11:2012+A11:2018 advises that the Class I test is intended to simulate partial conducted lightning current impulses, while the Class II and Class III tests involve impulses of shorter duration.
Class I tests are carried out with a 10/350 μs current impulse, as shown in Fig 1 (Clause E.5 of BS EN 62305 1:2011). This represents a standard lightning impulse.
For Class II, tests are carried out with the normal discharge current In 8/20 μs voltage impulse (see Fig 2).
For Class III, tests are carried out with a 1.2/50 μs voltage 8/20 μs current combination wave generator (see Fig 3).
An SPD may be classified according to more than one test class. Where this is the case, the tests required for all declared test classes shall be applied to the device.
Type 1
Type 1 SPDs, meeting test Class I, are designed to divert high surge currents associated with direct lightning strikes to earth safely and limit the transient overvoltage to prevent damage to the installation wiring and connected equipment. They also protect against risks to human life.
Where protection against high surge currents associated with direct lightning strikes is required, for example when a building has a structural Lightning Protection System (LPS) or an overhead line at risk of direct strike, Type 1 power SPDs should be installed as close as possible at each origin (534.4.1.3) or point of entry of an electrical service into the electrical installation (534.4.1.6) (see Fig 4).
Type 2
Type 2 SPDs, meeting test Class II, when located at the origin of the electrical installation address the risk of overvoltage arising from an indirect lightning strike, limiting the transient overvoltage to levels which are safe for connected equipment.
Such an arrangement is suitable for installations in locations where direct lightning strikes are unlikely, such as in built-up urban areas.
Where an installation does not have a LPS installed and does not require protection against the effects of direct lightning, a Type 2 SPD shall be installed as close as possible to the origin(s) of the electrical installation (534.4.1.4).
In industrial installations, Type 2 SPDs may be installed on sub-distribution boards or closer to the equipment to be protected, downstream from Type 1 and/or Type 2 SPDs installed at the origin of the installation (534.4.1.5) (see Fig 5).
Type 3
Sensitive equipment within the installation may benefit from protection provided by a Type 3 SPD in addition to that provided by Type 1 and/or 2 SPDs.
Section 534 recognises SPDs as Type 3 where the voltage protection level (denoted by Up on the SPDs labelling) is lower than the susceptibility threshold of sensitive equipment.
It should be noted that Type 3 SPDs should only be installed in installations where Types 1 and/or 2 SPDs are present upstream of their intended position.
They may be installed within sub-distribution boards (typically Type 2+3 SPDs), near or within items of equipment deemed susceptible to damage from overvoltage (534.4.1.5), or in fixed socket-outlets or mobile socket-outlet trailing leads.
Type 3 SPDs may also protect equipment from switching transients originating from within the premises (see Fig 6).
Combined Type SPDs (e.g. Type 1+2, Type 1+2+3)
An SPD may be classified according to more than one test class (for example, test class I (T1) and test class II (T2)). In this case, the tests required for all declared test classes shall be performed.
Type 1+2 SPDs are installed close to the intake position, such as at the first distribution board, in buildings which are at risk of direct lightning strike having a lightning protection system and/or are fed by an overhead line.
Rated impulse voltages and equipment overvoltage categories
In respect of overvoltages entering the installation at its origin, the amplitude of that overvoltage will be most severe close to the origin, with the amplitude falling as the electrical installation extends into the premises.
Electrical equipment within an installation shall have a rated impulse voltage not less than the value given for its overvoltage category in Table 443.2 of BS 7671. Table 1 of this article, based on Table 443.2, describes the required impulse voltages of equipment for a nominal voltage of 230/400 V AC.
Fig 534.2 of BS 7671, reproduced in this article as Fig 7, summarises where the various SPD Types should be located within an installation.
Summary
This article has considered Section 443 of BS 7671 and the requirements for the protection against transient overvoltages occurring in an installation due to the effects from lightning or due to switching. Where overvoltage protection measures are required, SPDs should be selected and erected in accordance with Section 534.
We also discussed the different types of SPDs recognised by BS 7671 and their suitability within an installation.
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