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3 Dec 2019, 11:20

The 18th edition of BS7671 and surge protection devices

The new 18th edition of BS7671 has been out for over a year now and one area that still raises the most questions is that regarding surge protection devices.

There are two sections in the standard to be aware of, firstly section 443 which is the risk assessment. For some installation types however no risk assessment is required, they are listed below.

 Protection against transient overvoltages shall be provided where the consequence caused by overvoltage affects:

  • (a)         results in serious injury to, or loss of, human life, or
  • (b)         results in interruption of public services and/or damage to cultural heritage,
  • (c)          results in interruption of commercial or industrial activity, or
  • (d)         affects a large number of collocated individuals.

For those installations covered by the consequences above, surge protection devices or SPDs are just fitted.

The other area missing from above is smaller sites, like the domestic installation and a risk assessment is carried out and if the risk assessment is not carried out then SPDs shall be fitted anyway.

So, for those working in the FM market or inspection and test, there will be some important things to be wary of.

Firstly, the standard is not retrospective. It is not required to flag up that SPDs have been missed off older installations, as those sites had been completed under an older standard. You can flag up a C3 that the installation of SPDs for a data centre rack within an older site may be a good idea, but it is not a requirement to install.

Secondly, the new EICR has a couple of tick boxes for SPDs, are they fitted and are they working. Most SPDs are a parallel connection so after they fail the site is still functioning, but the SPD is off and will need replacement.

The other section in BS7671 is 534 and this tells us how to install SPDs once section 443 and /or the BS EN 62305 lightning standard tells us we need them.

The two key points in the new section are the LPZ or lightning protection zone concept and then the SPDA or the surge protection device assembly.

The LPZ concept breaks the installation down into zones.

LPZ0 is the first zone and is the exterior. Once any fitted LPS or lightning protection system is fitted the outside LPZ0 becomes LPZ0a and LPZ0b.

The LPZ0a is still the outside where lightning can strike but LPZ0b is the external area protected under the fitted LPS on the roof for example, or under the cone of protection of installed masts.

Then we have the internal zones, LPZ is the whole internal volume of the structure in basic terms.

Then within LPZ1 we have metal boxes that are screened so they are LPZ2, these boxes are metal consumer units or distribution boards, control panels or motor control centres, (MCC).

In the new edition of BS7671, we have the following regulation.

534.4.1.2 In accordance with the LPZ concept, where a cable crosses the zonal interface, further SPDs shall be installed to preserve the zone integrity.

So, this means each cable that comes from the outside LPZ0 into LPZ1 the internal zone needs an SPD to plug the holes in that Faraday cage.

Then if there are then further metal DBs or control panels deeper in the installation then they are protected by another SPD.

If we protect the main incomer on the LV supply, we cannot now miss off other metallic services that come in from outside or indeed go from inside to external loads. This will cover CCTV and phone lines or the COAX lines from TV or satellite dishes on the roof. It covers controls to HVAC on the roof and security systems, EV charging points and parking or amenity lights outside. This will also cover feeds off to secondary buildings or structures and the following regulation backs this up.

534.4.1.6 Consideration shall be given to the provision of SPDs to protect from other sources, such as:

- switching overvoltage’s produced by current-using equipment located within the installation

- overvoltage’s on other incoming services such as metallic telecommunication and signalling services

- overvoltage’s on other services feeding other structures such as secondary buildings, external installations/ lighting, power lines feeding external sensors.

These SPDs shall be installed and located as close as possible to the origin of such events.

The text regarding the origin of such events is the placing of the SPD at the LPZ 0 to 1 interface.

This will, of course, mean that there will be many more SPDs within an installation.

Now we shall look at the SPDA concept the surge protection device assembly. This is the circuit that includes the SPD and the OCPD if required that is in circuit. It also covers the size and length of the cables connected the SPD and OCPD.

The SPDA is more onerous for type 1 SPD being used if there is a lightning protection system fitted.

These type 1 SPDs must deal with at least 12.5kA going up to 25kA of lightning current so in the short parallel circuit the OCPD feeding the SPD must also be able to handle 12.5kA or 25kA of lightning currents or there is a risk that the lesser rated OCPD will be destroyed by the lightning currents.

For example, the best way to protect the type 1 SPD is either a fuse that will shrug off the 12.5kA to 25kA, or an MCCB with a higher rupture capacity. It is not permitted to use MCBs as they typically have 6kA or 10kA values which will de-rate the circuit. MCBs also typically get destroyed by lightning surges anyway.

Then the standard is very clear about the voltage protection level or Up level. This cannot be more than 2.5kV.

One way to maintain this low Up level is to use short cables as described in the standard, we find out that just 1m of cable with 10kA surging down it gives a 1kV volt drop. Add that 1kV to the typically 1.5kV value of the SPD and we are at the maximum let-through voltage protection level of 2.5kV.

This will require installers and panel builders to plan the location of the SPD, so these cables are as short as possible to meet the performance values required in section 534.

It is easy to badly install SPDs by using longer cables than the standards specify, doing this results in a higher let thru voltage which will damage the installation. The other thing to avoid is installing SPD  with small fuses or MCBs with low kA values.

There is another way to get around the issues of the shorter cables and the dimensions of the OCPD in one go. DEHN UK has released a new product range in RS that has built-in fuses. This means that no external OCPD is required, so that means there is no interlinking cable required and that means the performance is well within the 1m required.

These new SPDs are available as type 1 and 2 combined and a new type 2 range. They have spark gaps as the surge protection components which typically have a longer life than MOV designs, they also require smaller cables sizes, so the space envelope within a panel is smaller and costs are down.

Because of the new requirements in the 18th edition and the high bar that the installer is required to get over these new SPDs take the guesswork out and puts confidence in.

The type 1 and 2 combined unit is the DEHNventCI and the type 2 SPDs are the new DEHNguard ACI series.

The DEHNventCI is made up of single-pole modules so a TPN installation will have 4 poles for example.

The type 2 DEHNguard ACI units come as TPN or SPN and are suitable for TN and TT supplies with the remote indication to let you know if they have failed.

Also, in the new range of DEHN SPDs in RS are devices for satellite dishes and CAT5 /PoE Ethernet networks, so covering those other metallic services mentioned in BS7671. If the installation has a Tannoy speaker system or CCTV these can also be protected.

Following future technology developments, gadgets, music and Rugby.

3 Dec 2019, 11:20