Transient Overvoltage Protection

INTRODUCTION

Transient overvoltages are short duration surges in voltage between two or more conductors, generally resulting from:

• Atmospheric origin (lightning activity through resistive or inductive coupling) and/or

• Electrical switching of inductive loads

Computers, other electronic equipment and systems are extremely vulnerable to lightning induced transient overvoltages via power, data, signal and telecommunication lines, often resulting in serious equipment damage leading to unexpected failures, data loss and expensive downtime, or risk of fire/electric shock due to flashover if insulation breaks down.

Global surge protection standards and surge protection designs have been updated to reflect the major trends of growing reliance on electronic systems and renewable energy.

Society is relying ever-more on sophisticated electronic devices and systems to support our everyday lives. At the same time, electronic systems are shrinking in size, making electronic components more vulnerable to damage from electrical surges. As a result, there’s more need for surge protection.

Another driver for surge protection is the integration of renewable energy. Solar and wind facilities rely on electronics in inverters, converters and condition monitoring – and these need to be protected from the electrical surges that arise from lightning.

One of the leading companies in the field of earthing, lightning and electronic systems protection solutions is ABB Furse on whose website can be found a wealth of technical literature. This includes:

• A guide to IEC/BS EN 62305 Protection against lightning

• IET Wiring regulations BS 7671 18th Edition Amendment 2  - Transient overvoltage protection

PROTECTION STRATEGY

Protection against transient overvoltages is achieved through the installation of a coordinated set of surge protection devices (SPDs) at appropriate points in the electrical system, in line with the guidance contained in BS 7671:2018 - 18th Edition IET Wiring Regulations as amended.

A key update in the 18th Edition relates to the protection of electrical and electronic systems against transient overvoltages, either as a result of atmospheric origin (lightning) or electrical switching events.

Essentially the 18th Edition require all new electrical system designs and installations, as well as alterations and additions to existing installations, to be assessed against key consequences caused by transient overvoltages and, where necessary, protected using appropriate surge protection measures (in the form of Surge Protection Devices SPDs).

Under BS 7671 Amendment 2 (2022), protection against transient overvoltages has to be provided where the consequence caused by the overvoltage could result in:

• serious injury to, or loss of, human life

• failure of safety service (defined in the BS as an electrical system for electrical equipment provided to protect or warn persons in the event of a hazard, or essential to their evacuation from a location)

• significant financial or data loss.

For all other cases, protection against transient overvoltages shall be provided unless the owner of the installation declares it is not required due to any loss or damage being tolerable and they accept the risk of damage to equipment and any consequential loss.

It should be noted that as far as this part of BS 7671 is concerned, the risk assessment method contained in BS EN 62305-2: Protection against lightning – Risk management must be used for high risk installations such as nuclear or chemical sites where the consequences of transient overvoltages could lead to explosions, harmful chemical or radioactive emissions thus affecting the environment

The result of new wiring regulations is that electrical professionals will need to specify SPDs to protect electronics in mains, signal and telecoms applications in commercial, public and industrial projects. It is important to use suitable SPDs that meet the latest product standards, particularly in terms of safety. This will ensure that they will offer the right protection against repeated “worst case” transients.

To assist electrical contractors and end users/specifiers, ABB Furse has published the following Table (which is understood to have been drafted by the IET).

Table 1 - Examples of overvoltage protection requirements to BS 7671

 (It is worth remembering that British Standards and British Standard Codes of Practice do not have the force of law but they do represent best practice and may be referred to in a court of law in the event of an accident or building/plant failure.)

For complete protection against lightning damage ABB Furse would normally advocate a “total solution” package, comprising both structural lightning and electronic systems protection. Whilst in many situations this will be appropriate, buildings are frequently encountered containing valuable and, more particularly, highly vulnerable IT and other electronic systems and equipment for which there may be no requirement for structural lightning protection under the BS EN. Owing to the fact that the risk of damage and disruption from lightning induced transient overvoltages is far greater than that of direct lightning strike, such circumstances shall not preclude the installation of transient protection.

TYPES OF SPDs

Three Types of SPD are available (Types 1, 2 and 3) and these are cumulatively known as a co-ordinated SPD set. There are also Combined Type SPDs (e.g. Type 1+2+3) that handle high surge currents with safe overvoltage protection levels for sensitive equipment. These have technical, practical and economic benefits over multiple single Type SPDs.

Type 1 SPDs should be installed at the service entrance to protect against loss of life or serious injury in the case of a direct lighting strike. They protect against

dangerous sparking where the voltage of a surge exceeds the impulse voltage rating of equipment or cable insulation. Another term for Type 1 SPDs is equipotential bonding SPDs and it is important to know that they do not provide any effective protection for sensitive electrical or electronic systems.

As a result, additional Type 2 and Type 3 SPDs are required inside structures that contain electrical and electronic systems. They are overvoltage SPDs that further reduce the size of transient overvoltages to safe levels.

Type 2 SPDs provide overvoltage protection for fixed equipment, whereas Type 3 SPDs are used to protect sensitive or critical loads, particularly against switching transients.

SPDs should comply with EN 61643: Low voltage surge protective devices - Surge protective devices connected to low-voltage power systems, telecommunications and signalling networks

- Performance requirements and testing methods. 

The surge protection extension leads that commonly retail from Argos, B&Q etc for as little as £10, whilst possibly offering some degree of protection, do not provide the complete solution owing to serious doubts as to EN 61643 conformity and, more to the point, that such devices aim to provide local protection only, as compared with an overall scheme of protection.

PROTECTION DESIGN AND INSTALLATION

Part of the services offered by ABB Furse includes a free site survey from which the appropriate scheme of protection covering mains power supplies, data communication and telephone lines and other services is specified, and a quotation for the supply of the necessary equipment submitted. Armed with the specification, equipment would be ordered and installed by an electrical contractor appointed by the Policyholder. If required, Furse can supply a list of contractors specialising in this field.

Another company in the field is PD Devices Ltd www.pddevices.co.uk. who also provide a similar service.

All SPD’s manufactured or supplied by these companies meet with the testing requirements of the EN 61643 series.

PROTECTION COSTS

Supply and installation costs will vary considerably depending on the size and complexity of the scheme of protection. However, theories that transient protection schemes are extremely expensive and beyond the budget of all but the largest businesses are unfounded. Small installations, typically comprising of the protection to incoming and outgoing power and telecoms lines and selected equipment downstream are unlikely to involve a total outlay much in excess of £3000. These can therefore be extremely cost effective within the SME sector and above, as protection against expensive equipment repairs/renewals and down time.

INSPECTION AND MAINTENANCE OF THE LIGHTNING PROTECTION SYSTEM (LPS)

BS EN 62305 recommends a robust approach to the maintenance of lightning protection systems (including SPDs), the key elements of which include:

• Inspection of the LPS should be conducted by a lightning protection specialist.

• The inspector should be provided with the LPS design report, together with the previous maintenance and inspection reports where applicable.

• The LPS should be visually inspected at least annually, including a check that there have been no additions or alterations to the structure for which additional protection would be required.

• In respect of critical installations such as structures containing sensitive internal IT/telecoms systems or where a high number of people may be present, annual inspection and electrical testing of the LPS is advocated.

• Lightning protection systems on sites involving explosive materials should be inspected every 6 months and tested annually.

• SPD’s without a visual status indicator need to be tested in accordance with the guidelines provided by the manufacturer.

   

SURVEY EXPECTATIONS

Consultants should remain alert to the transient overvoltage risk and he need to ascertain via enquiry and observation the extent of protection provided, and the inspection and maintenance arrangements conducted.  

RISK IMPROVEMENTS

The revised edition of BS 7671: IET Wiring Regulations places a greater risk-based emphasis on the importance of transient overvoltage protection which cannot be ignored. Consequently, the requirements of the revised BS as detailed in the earlier Table are to apply retrospectively on the basis of established best practice and sound risk management strategy.

Where major exposures are present, such as encountered in hospitals, care homes, schools, hotels, high-rise residential buildings, data centres and significant retail, commercial and industrial facilities, risk improvements are to be categorised as “Requirements” (experience in the field indicates that transient protection in such facilities is rapidly becoming the norm).

For other cases, including smaller commercial and industrial facilities, risk improvements (including categorisation), should be based on an informed judgement of which the potential IT/business exposure, amongst other factors, is a key consideration. In this respect it is important that a selective risk-based approach is adopted, rather than defaulting to raising a risk improvement for transient protection for every case surveyed.

ADDITIONAL REFERENCES

The subject of transient protection is also dealt with in RISCAuthority RC3 Recommendations for the loss prevention in electronic equipment Part 6: Protection against electrical disturbances.