SPDs on commercial installations

31 Mar 2006
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United Kingdom
I’ve been asked to act as a consultant between the contractor and the client at a charity run community centre who are looking to replace their incoming mains distribution.

The job comprises of installation of a new 250A TP&N Schneider panel board feeding submains to approximately nine existing distribution boards which are a mixture of TP&N and SP&N boards, and immediate replacement of one TP&N board, with at least two other boards planned to be replaced in the not too distant future.

It has been decided that the installation of SPDs is a good idea as there are some expensive pieces of sensitive kit within the installation such as the fire alarm panel, BMS panel, gas boilers, lighting dimmer racks, computers etc.

I don’t really have any experience of designing SPDs into an installation like this, and I am wondering whether standard practice would be to just install a single SPD at the panel board covering the entire installation, or if it is more desirable to have a local SPD installed in each sub board.

Any information you can offer would be greatly appreciated.
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I don’t really have any experience of designing SPDs into an installation like this, and I am wondering whether standard practice would be to just install a single SPD at the panel board covering the entire installation, or if it is more desirable to have a local SPD installed in each sub board.
I must be about the farthest as I could be from any sort of authority on such things but, if you are to have such devices, then you may have to consider (if you haven't already) what type of surges/transients you want to try to 'protect against' - those present on the incoming supply (mainly 'atmospheric', I would think) or those generated within the installation (due to 'switching' etc.) .... or both.

If the former, then I would think that if the SPD gives protection to anything, it must similarly protect everything within the installation. If the latter (the likelihood of which probably depends in part on the nature of installed equipment), then I'm not so sure - I suppose it might be more effective if an SPD is located close to the source of the 'overvoltage' within the installation.

Kind Regards, John
I think a lot of the bigger companies; the likes of Dehn, furse, etc will do a bit of a design if you go to them (In the same way lighting companies do), of course they have a motive to sell their kit, etc

Its a bit of a difficult area and I've sat through manufacturers training and still not come away thinking that I fully understand it all.

A single SPD at the origin does not always provide full protection, apparently if you have longer feeds going off then you can get resonance or reflections in the cables and not fully protect further down the line.

Three basic types (but you need to select on energy levels as well)

Type 1 is needed where you might have to deal with a lightning strike on the building or installation
Type 2 is for standard mains bourne surges, not incorporating a direct strike.
Type 3 is intended to protetct individual senstive fixed equipment, such as local to the fire panel.

Most of the three phase units you'll come across are type 1&2 combined (but you can buy just type 2 if you want to) , but you still need to look into energy levels.
When you buy a domestic consumer consumer unit with SPD, its generally a type 2

If you are protetcing a building against a lightning strike you need to protect at the point a surge can enter (the lpz), and you need to have an equipotential bonding* mindset when doing this, not only do you need to consider the incomming service cable, but if you have feeds to lighting columns which are outside the zone of protection. Also these days its becomming common to take a supply out to feeder pillars with a board in for EV charging, its also being common to pull a cat5 in as some makes of EVSE seem flaky on the mobile phone networks, now if you deem submain in need of SPD, then the data will need it too, you can get suitable devices, however if involved from the start on such situation, probably better run fibre rather than copper data

*type 1 are sometimes refered to equipotential bonding SPDs.

You'd start with an type 1/2 combined SPD for the new panel, ensure it can be checked without isolating anything, and it can be isolated for replacement when it comes necessary without isolating whole installation, supply via a 63A MCCB is normally sufficent. Keep conductors to it less than 500mm and avoid tight bends

As to final boards needing replacement, if they are a resonable distance away from the main panel then a type 2 SPD is probably sensible, and in the case of SPN boards, if using the domestic type boards, its normally a no-brainer as it doesn't cost much anyway. If you have a feed leaving the LPZ then you'd have ensure type 1, rather than just type 2, but if its just suppling stuff in/on building then type 2 is good. I wouldn't worry about SPDs on boards reaonably local to the main panel SPD (so if you have a TPN board bushed to the side of main panel, back to back, next room, you wouldn't need to duplicate protection there) but if you had a long final cirucit to the fire panel, then It'd probably be a good idea to stick a type 3 device at the far end.

Its been a bit of a ramble, and I've gone down some tangential assides, but thats my understanding of it all anyway!
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Thanks Adam that’s been really useful, and funny you should mention EV charging as the upgrade has been designed with this as a thing which may happen further in the future.

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