Wired into the detail

Associate Member Dehn takes a closer look at the new elements of Section 443 of the updated wiring regulations and explains why they’re not as simple as you might think



If you have your copy of the new regulations, you’ll notice that the risk assessment process for SPDs in Section 443 has changed.


Gone is the ground flash density map, CRL risk assessment calculation and choice of environmental factor, replaced by three simple idents that determine if a structure requires SPDs.


But why are there other pages in Section 443?


And what are they all about? Let me explain…


Regulation 443.1.1: Decision making


Let’s start at the beginning with Regulation 443.1.1, where there is a decision-making process to be undertaken.


Does the structure have a fitted lightning protection system (LPS) to BS EN 62305, or is it otherwise protected from the effects of a direct lightning strike?


If it is, we need to go to the lightning protection standard BS EN 62305 and also Section 534 in BS 7671 to roll out the installation of SPDs.



The protection of structures from direct strikes is not in the scope of BS 7671.


Next, we see that if no transient overvoltage protection against disturbances of atmospheric origin is installed, protection against switching overvoltages may need to be provided.


This acknowledges that within an electrical installation there may be possible sources of overvoltage and protective measures that need to be taken into account, more of which later.


If there is no fitted LPS we move to regulation 443.4.1, where we see the three consequences to requiring SPDs outlined in the article on the previous pages, i.e. injury or loss of life, failure of a safety device, or financial or data loss.


“we see that if no transient overvoltage protection against disturbances of atmospheric origin is installed, protection against switching overvoltages may need to be provided”

Those indents apply only to structures that don’t have a fitted LPS and there is no need to apply the requirements to structures like high-risk tower blocks, data centres, schools or areas with an ATEX zone. The structures to be covered will all be smaller, e.g. a dwelling converted to a surgery or small business unit.


The inclusion of safety circuits may introduce SPDs into all installations that have mains-powered smoke alarms or fire detection and fire alarm panels or emergency lights as they’re supplied by a safety circuit meeting the definition in Part 2. So the inclusion of a safety circuit makes SPDs in domestic installations very likely.


Some additional text in the regulation goes on to say: “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.”



There is no other part of BS 7671 that promotes the idea of consent with the owner for the inclusion of a device. Given how the cost of SPDs has been driven down with increased uptake of the units, it would be the smallest installations of no consequence that will have no SPDs fitted.


That would appear to be the end of the risk assessment – but Section 443 has many more regulations to be explained and followed.

Regulation 443.4.2: Transient overvoltages


We turn our attention to the next regulation, which deals with transient overvoltages caused by equipment where protection against overvoltages shall be considered in the case of equipment likely to produce switching overvoltages or disturbances exceeding the applicable rated impulse voltage of equipment.


Examples of this include where inductive or capacitive equipment, such as motors, transformers, capacitor banks, storage units or high current loads are installed.


This is a common situation for anyone who’s fitted LED lights which don’t last as long as described because other installed equipment, e.g. lifts or machinery, is inducing surge voltages beyond the level tolerated by the drivers, i.e. the rated impulse voltage.


It is therefore important to understand the technical specification of the drivers and the standard to show that the driver has the required “resilience” is BS EN 61000-4-5. This is specifically mentioned in BS 7671 in regulation 443.6.1,


This standard for the surge immunity of the equipment under test (EUT) shows the voltage impulse ratings that the EUT will withstand. Other parts of the BE EN 61000 series can also appear in the technical specification of drivers but those test conditions are less onerous and the corresponding withstand voltage is lower, hence higher failure rates being flagged up in certain installations.


One of the test conditions in other parts of the BS EN 61000 series is to test to just 2kV. The target value we need to meet in BS 7671 is not more than 2.5kV for category 2 appliances in Table 443.2.


The result of this means that the LED drivers rated at 2kV would require a local Type 3 SPD to provide the additional level of protection to ensure a long life in the installation with resilience from over voltages.


Type 3 SPDs mounted within lighting circuits would have a voltage protection level (Up) of approx. 1.5kV so the lower impulse withstand voltage of 2kV (Uw) is not exceeded.

Summing up


Section 443 is a risk assessment only for structures that have no fitted LPS. Those structures with fitted LPS have had a risk assessment undertaken in BS EN 62305 and SPDs shall be fitted as part of the measures to mitigate the damage from lightning strikes.


If any of the three indents in the list of consequences of not offering SPDs are met then SPDs shall be fitted. For all applications, the designer and installer also need to be aware of the impulse withstand voltage of the most sensitive piece of equipment and install SPDs to protect it not just from lightning surges but also other equipment in the structure.


The installation of the SPDs as a result of following the requirements in Section 443 shall be erected as per the requirements in Section 534.

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