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Gary Parker, senior technical support engineer, ECA, takes a closer look at surge protection and the changes made to the 18th Edition.

Surge protection devices (SPDs) are used in electrical installation systems to protect against transient over-voltages (or voltage spikes) that can cause damage to electrical equipment by overloading components. These electrical surges can result from sources such as lightning or the switching on or off of large electrical loads elsewhere in the network. SPDs are designed to divert the excess current to earth, protecting electrical appliances and systems. 

The 18th Edition of the IET Wiring Regulations, which as readers will know came fully into effect on 1 January 2019, has brought important changes to SPD requirements. With our increasingly complex dependence on electronic devices, automation and IT, in both domestic and commercial environments, the possible consequences of over-voltage damage to vulnerable equipment and components – ranging from the loss of critical data on computer systems to the disruption of industrial plant, machinery or vital infrastructure – can be significant, costly and hazardous. 

Another factor that has driven change has been the growth in the deployment of renewable energy sources such as solar photovoltaic (PV) panels and wind generators. These installations rely on many electrical components and monitoring systems, but because of their locations – above properties, in fields or on hilltops – they can be vulnerable to surges caused by lightning.    

What causes surges?

Lightning strikes produce the largest transient over-voltage events in electrical installations, creating surges of thousands of volts lasting less than a millisecond. These can result from direct strikes, but also strikes up to a kilometre away from the installation. 

Man-made surges arising from switching event transients on the network, though much smaller in scale, are more frequent and can stress electronic components, causing unplanned outages, damaging or degrading equipment – and ultimately result in equipment failure.

When is surge protection needed?

Chapter 44 of the 18th Edition of the IET Wiring Regulations deals with ‘Protection against voltage disturbances and electromagnetic disturbances’. Section 443 has been redrafted, removing the earlier 17th Edition’s ‘AQ criteria’ risk assessment process that considered external environmental conditions that influence lightning. 

Instead, the 18th Edition establishes that SPDs need to be installed by electrical contractors to protect against transient over-voltages in situations where there may be serious consequences. Effectively, the process has been simplified and the emphasis in approach has shifted from considering whether SPDs should be installed, to installing SPDs unless it’s established that they’re not required. 

The 18th Edition states:

‘Protection against transient over voltages shall be provided where the consequence caused by over-voltage could:

(a)        result in serious injury to, or loss of, human life, or
(b)        result in interruption of public services and/or damage to cultural heritage, or
(c)        result in interruption of commercial or industrial activity, or
(d)        affect a large number of co-located individuals.’

If a risk assessment is not carried out, the regulation states that SPDs shall be fitted in all cases.

What types of surge protection are required?

SPDs are classified into three different types (Types 1, 2 and 3) according to their standard and the types of transient over-voltage they are required to handle: 

Type 1 SPDs 

Also known as equipotential bonding SPDs, type 1 SPDs are designed to be deployed at the origin, where services enter a building or facility, and protect against major surges from direct lightning strikes. They are not, however, designed to protect sensitive electronic equipment or electrical systems from switching transients. They should therefore be used and coordinated with Type 2 devices. 

Type 2 and 3 SPDs 

These are designed to protect electrical and electronic equipment from the secondary effects of indirect lightning and against switching transients, by reducing the transient over-voltage to safe levels. Type 2 SPDs can help prevent the spread of over-voltages in electrical installations and are usually installed in distribution boards. 

Type 3 SPDs, which have a low discharge capacity, can be used to supplement Type 2 SPDs to provide protection for sensitive and critical loads. These SPDs should be installed close to the equipment to be protected. 

The use of a combination of SPD types, effectively acting as a cascaded protection system, is recommended to provide coordinated protection from transient over-voltages. Many types of SPDs are available, including combined SPDs in a single unit (such as Type 1+2+3) for simplification of installation and maintenance, and reduced cost. 

The correct installation of SPDs is also vital to ensure maximum protection, not only for electrical safety, but also because poor installation techniques can reduce their effectiveness. For example, connecting conductors should be kept as short as possible, as this will minimise any additive voltages on the connecting cables. 

Full guidance on the specification for connections is available in Chapter 44 section 443 of the 18th Edition.



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