The inclusion of surge protection devices or SPDs in the latest amendment to the 17th edition of the BS7671 wiring regulations shows how far the complexity of modern installation has come

Changes to the regulations came about as a result of the demands of the client in the expectations they have to the quality and safety of the work and reliability of the installation.
To be clear from the start, SPDs or surge protection devices are not about protecting the installation from lightning strikes, only a fitted lightning protection system to BS62305 will prevent a direct strike from damaging the property.

The point of the SPD is to ‘plug the holes’ in the typical domestic installation, in the form of conductive conductors bringing a surge current in from outside via the supply or transmitting a surge within an installation.

Installation protection using SPDs

The SPD is therefore best located at the origin of the installation and will divert the surge current to earth before this surge has a way to propagate about the wiring of the installation. This also removes the requirement for the use of the trailing sockets with surge protection that a great many of us have. But in reality these still allow a surge to travel all around the wiring of the house and causing damage as it goes and many of these cheaper socket devices present a fire risk if the protective surge components are not protected with a thermal cut-out that prevents thermal runaway or fire.

Whilst the new standard and section 534 in particular does not state ‘thou shall install SPDs’ the consequence of not installing them is the question we have to ask ourselves. To this end the section 443 asks this question in the form of a simple risk assessment – if the exclusion of the SPD represents an issue to human life, public service or industrial activity, the fitting of the SPD is recommended.

We can also ask ourselves, can we afford to be without our electrical appliances or computer in the home? As the standard now calls for protective measures to be taken would an insurance company pay out if there was a strike?

The new sections added to BS7671 Amendment 1

The new section 443 relates to the number of thunderstorm days being in excess of 25 per year when, in reality, no part of the UK exceeds this figure, but we should not lose sight of the fact most surges imposed on the network or within the house wiring are man-made.

The sources of man-made surges are the switching of inductive loads like motors, HVAC, lifts and the like. Fuse breaking, contactors switching and the re-establishing of power after an outage are other causes. The digging up of cables by roadworks contractors is another valid concern for users of sensitive equipment. All these issues can be addressed with the use of surge devices.

The SPD will not deal with power cuts or sags or swells in voltage over many cycles, just transient over voltages over a short time base.
Installation as defined in the standard is simple enough, the device is connected with cables to and from the unit no longer than a metre in total, and a short path to earth will ensure the effective discharge of the surge to earth.
For the most part a simple two-pole device for single phase TT or TNS will be enough. In TNCS systems a single pole unit may be sufficient as only the live conductor comes to the property that can bring a threat from outside influences affecting that supply.

Specific design features of surge protective devices

The device must have some form of indication on it to advise of the end of life of the unit, and better still a remote indicator, as it is no good if the device flags up it has failed by way of the flag alarm on the unit and no-one has access to see the flag has changed from green to red. A remote indicator using the volt free change over contact option will ensure the SPD is not over looked.

In addition there are three basic types of surge devices:
• Type 1 is used on buildings that have an overhead supply or a lightning protection system fitted on the roof. Another application would be when the site is a hospital or military and sites that warrant protection as the consequences of failure do not bear thinking about.
• Then there are the type 2 units. These would be suitable for all other installations at the incomer or after a type 1 at sub distribution levels. The modern home with an underground cable supply would be a typical use of a type 2 unit.
• The final type is type 3, this is used after a type 2 for localised over voltage protection in control cabinets or sensitive electronic equipment for example.

The SPD can be installed in a consumer unit on the DIN rail or beside it in an additional box or enclosure that gives some separation from the clean circuits. This may also help to meet the 1 metre total installation cable length requirements of the standard.

The device needs to meet the requirements of the standard IEC 61643-1 and EN 61643-11 and have KEMA safety approvals for peace of mind at the end of the devices life.
The type 1 and 2 SPD is a parallel connection so it fails in a way to not take out the supply, some additional fuse may be needed in the SPD ‘spur’ in larger installations.

In any event full attention to the requirements laid out in the standard and the installation guide supplied by the manufacturer of the device would insure an effective installation.