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At present, the installation of SPDs has been considered as a last minute add-on and can therefore be installed badly. However, the guidance below will highlight the best practices.
Section 534 introduces the new term ‘surge protection device assembly’ or SPDA. This term is about the whole SPD circuit including the OCPD and the connection cables regarding lengths of cables and the sizing. This was not an emphasised topic in the 17th Edition but now to establish some best practice, the standard has defined some areas that the panel builder needs to be aware of.
The first area of interest is the location of the SPD. In section 5184.108.40.206, the SPD is located to protect zonal integrity or when the cable crosses the zonal interface. This is described in the lightning protection zone concept, or LPZ, in section 534. For example, when the cable enters at the origin, the zone LPZ0 is external and we locate the SPD as close as possible to the origin in the internal zone LPZ1.
Panels are no different in that the SPD shall be located at the zonal interface and that is likely to be the main isolator. A metal panel or DB is an internal zone LPZ2 in the building zone LPZ1. Also, in BS 7671 section 5220.127.116.11 there is the text: “These SPDs shall be installed and located as close as possible to the origin of such events”. In other words, the origin is not always where we expect the electric meter, but where the surges can enter. So other than the LV supply isolator, we would expect to see SPDs for other external circuits like those listed in 518.104.22.168, for example external loads, supplies to secondary building, external sensors, CCTV, lighting, EV charging and roof mounted chiller and AHUs.
These SPDs for the LV supply can be wired across the main incomer and other SPDs on the external loads located typically at the base of the cabinet where these cables enter, assuming conventional methods of construction.
It makes no sense to locate the SPD within the general internal area of the panel, as what the designer is saying is: “I am happy to allow the surge to enter the panel and as the surge travels within the cable containment or slotted trunking it can affect other cables that need not be otherwise influenced.”
Another area of interest for best practice is the sizing of the OCPD. Section 522.214.171.124 states the OCPD can be located externally or within the SPD itself. Given that with the 18th Edition we shall be seeing more and more SPDs installed, a SPD with internal fusing will be an advantage in terms of space and the cost of additional parts saved. Also, there is a section regarding the way that the internal fuse inside the SPD can achieve much lower voltage protection levels and that is what we are looking for (see 5126.96.36.199).
This lower protection level is met as the manufacturer of the SPD can declare a let through voltage for the whole OCPD/SPD combination and that is normally not more than the let through for SPD alone. So, in practice then, the SPD with the built-in fuse has a far better performance in the DB or panel it is fitted in and saves on an external OCPD and those links.
Another aspect of the fusing is addressed in section 5188.8.131.52 and this is about the external fusing. The OCPD selection with type 1 SPDs needs consideration for the rupture capacity of the OCPD and the SPD performance. So, consider that the minimum performance of a type 1 SPD is 12.5kA of the current that it diverts to ground. Any OCPD in the surge circuit cannot restrict that value. So, it will not be acceptable to have a 63amp MCB in the circuit as the capacity of typical MCBs is 6-10 kA.
If the surge exceeded the value of the rupture capacity of the MCB, then the MCB would be damaged and disconnect the SPD. This means the rest of the surge may go into the installation as the protection has been removed. Also, more importantly, a damaged MCB gives no failure indication as it is in a parallel circuit to the SPD. Power is still going to the installation, but the SPD is removed and the remote indication on the SPD flags up SPD failure not MCB failure. As for the type 2 SPD, the minimum value for the surge current is 5kA, so here 63amp MCBs with a 6kA rupture are OK.
Finally, section 534.4.10 shows us that the minimum cable size for type 1 SPDs is 16mm2 and 6mm2 for type 2. The length of those cables from the L or N to PE via the SPD with/without OCPD cannot exceed 1m due to the 1kV volt drop in 1m of cable with 10kA flowing. Add the 1kV to the declared 1.5kV of the SPD and you can see we are at the maximum let through voltage permitted in BS7671 of 2.5kV (see section 5184.108.40.206 and section 534.4.8).
It is very important that the 1m long cable rule or 2.5kV let through of the SPDA is not exceeded or what is the point? Appliances and equipment can be damaged by higher than 2.5kV let through voltages. So, following the standards regulations in conjunction with the manufacturer instructions will ensure the best SPD circuit performance.
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