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SPD(s) for an outdoor TT install

psychicwarrior
Good morning


I am trying to get my head around surge protection - the *basic type and positioning* really- and taking an example situation please where surge protection may be added:


A moorland, overhead supplied, residential property; TN-C-S.


Outdoor regular wall meter cupboard in which is the DNO head, meter and some existing consumer switchgear (been as it is for over a decade with RCDs, MCBs from when the supply was TT) in an enclosure fed via a supply splitter block [henly] , which is there to supply two consumer units each at either end of the property.


Also fed from the splitter block, a supply to another nearby enclosure in which resides switch gear for TT outdoor services - lamp columns and electric gate 30ma RCBO final circuits and a  RCD type s distrib circuit to a 'garden work pod' which has a little sub-board.


(rough block diag attached to help description)


1) What would be the positioning and type of any surge protection  (where there is no LPS on the building rightly or wrongly)   ?


my questioning thoughts on this:

- near the origin (as in the source of energy) a  Type 1 as it is overhead fed, but not sure if CT1 or CT2 connection 

- in the consumer units a type 2  CT1 connected

- for the TT outdoor final circuits a type 2  , does this have to be upfront of RCBO being TT and  CT1 or CT2

- for the TT pod  distrib circuit, a type 2 and none  in the 'pod' sub board, or  none and type 2 in the pod subboard


- does the outdoor TT install even require SPD; can a 'surge' propogate back uptream into other parts

- future considerations, if alterations made e.g.


2) Lastly, is SPD simply about protecting connected equipment, or is it also about protecting  conductor insulation from voltages that might damage it ?


It does seem that adding type 1 SPD at the 'origin' (in meter box - other one is possible)  into this situation is nigh impossible without major works, so unless its possible to simply add it to each consumer unit/subboard as such... it seems academic, but still   it would be interesting to know where it should go ideally.


Regards

Habs


(perhaps one's ambition outweighs one's abilities)

7b9cf34fb3da18dccd3044d711131e43-huge-ietwrforumspd1.png


(last para edited for clarity ... well that was the intention)
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  • 0
    As far as I recall BS 7671 only demands type 1 SPDs where the building has lightning protection - but there does seem to be generally accepted industry advise that type 1 should also be used for overhead supplies. So I think it's a case of you pays your money and takes your choice on that one.


    I think you'll need SPDs in the TT part too. SPDs operate by shorting conductors together (e.g. L and PE) - so tend to raise the voltage on PE just as much as they reduce it on the L - indeed on a TN-C-S supply you might well find that the surge is pretty similar on both the L and PEN conductors in the first place so the SPD might have little to do. However take the live conductors into a TT area and you're almost back to square 1 again as the TT earth won't have have had the benefit of being 'pulled up' by the SPD in the TN installation. For instance, say we had a 10kV surge on the supply L - the TN SPD shorts L to PE so bringing L down to not much above 5kV and pulls the TN PE up to almost 5kV - the TN installation only sees the difference and so is happy, but the TT installation, with it's PE still at 0V sees 5kV between L and PE. (Or even worse case a 10kV surge is imposed on both the supply L and PE - TN SPDs have nothing to do, but the TT installation then sees the full 10kV).


    Generally there shouldn't be type 1 or 2 SPDs downstream of 30mA RCDs - both because the RCD is likely to nuisance trip if a surge occurs (if you're lucky) and (if you're unlucky) the RCD itself is damaged by the surge as it lacks upstream SPD protection. S-type (time delayed) RCDs can be upstream of type 2 SPDs - but often it's better to have CT2 wired SPDs before any RCDs - although a lot of care is needed to ensure that there's no possibility of a L-PE fault before the first tier of RCDs in a TT system.


       - Andy.
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  • 0
    As far as I recall BS 7671 only demands type 1 SPDs where the building has lightning protection - but there does seem to be generally accepted industry advise that type 1 should also be used for overhead supplies. So I think it's a case of you pays your money and takes your choice on that one.


    I think you'll need SPDs in the TT part too. SPDs operate by shorting conductors together (e.g. L and PE) - so tend to raise the voltage on PE just as much as they reduce it on the L - indeed on a TN-C-S supply you might well find that the surge is pretty similar on both the L and PEN conductors in the first place so the SPD might have little to do. However take the live conductors into a TT area and you're almost back to square 1 again as the TT earth won't have have had the benefit of being 'pulled up' by the SPD in the TN installation. For instance, say we had a 10kV surge on the supply L - the TN SPD shorts L to PE so bringing L down to not much above 5kV and pulls the TN PE up to almost 5kV - the TN installation only sees the difference and so is happy, but the TT installation, with it's PE still at 0V sees 5kV between L and PE. (Or even worse case a 10kV surge is imposed on both the supply L and PE - TN SPDs have nothing to do, but the TT installation then sees the full 10kV).


    Generally there shouldn't be type 1 or 2 SPDs downstream of 30mA RCDs - both because the RCD is likely to nuisance trip if a surge occurs (if you're lucky) and (if you're unlucky) the RCD itself is damaged by the surge as it lacks upstream SPD protection. S-type (time delayed) RCDs can be upstream of type 2 SPDs - but often it's better to have CT2 wired SPDs before any RCDs - although a lot of care is needed to ensure that there's no possibility of a L-PE fault before the first tier of RCDs in a TT system.


       - Andy.
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