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SPD arrangement

lyledunn
24e119d041831991ba57ac52261a5c57-huge-e2f7a6a0-5147-4eb4-b505-80e94f626124.jpg
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    The above follows manufacturers instructions. Am I right in assuming that the neutral module is not used as protection is afforded by the N/E bond on a TN-C-S system?
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    If there's an overvoltage between a L and N, then the effective total cable length is the brown from DB to SPD, plus the E all the way from the SPD back to the N/E bond point - which is likely to be a lot more than 0.5m. So it will be very poor at surpressing such transients as far as I can see.
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    I wouldn't worry too much, since the damned things don't work in any case.
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    lyledunn:

    The above follows manufacturers instructions. Am I right in assuming that the neutral module is not used as protection is afforded by the N/E bond on a TN-C-S system? 


    There are only 2 types of connection in BS 7671 for three-phase systems with Neutral - CT1 (Fig 534.3 in BS 7671) and CT2 (Fig 534.4 in BS 7671).


    What is shown in your picture appears to be neither of those, if the system has a Neutral ... but yes you are correct, in this case if the N and PE are bonded close to the SPD the N-PE for CT1 may not be necessary.


    In TN-C-S systems, because the Neutral separates from PE downstream of the service head, especially in premises with LPS, there may well be impulses between N and PE by the time you get to the distribution board / switchboard. It may therefore be OK to use this at the main switchboard / distribution board, but Type 2 or Type 3 SPDs downstream would need all 4 SPDs.


    I still think connection of the N-PE SPD would be the best approach (since it's been provided), as shown in Fig 543.3 and Fig 16A5 in BS 7671 - but the OSG says refer to manufacturer's instructions !

     


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    Exactly Graham. However, both CT1 and CT2 are presented as an “example” of such connection arrangements. That implies that there is more than one way to comply. 

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    Sorry Graham, I only saw the first sentence of your post for some strange reason then suddenly downloaded the rest!
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    lyledunn:

    Sorry Graham, I only saw the first sentence of your post for some strange reason then suddenly downloaded the rest!


    So did I!


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    I think it would look better if there was a bit of blue wire in the end slot. How close is the TNC-s bond - more than a meter or so round trip and I'd say it is not safe to assume that they are at the same voltage for the sort of short duration very high current transient these devices are supposed to address.

    Mike
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    I guess it would be around 15m of 16mm2, Mike. I will have the instructions emailed to me for a better look. I note, however, some manufacturers referring to TNC systems which might explain confusion with installer.
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    lyledunn:

    Exactly Graham. However, both CT1 and CT2 are presented as an “example” of such connection arrangements. That implies that there is more than one way to comply. 

     


    Apologies for that, I typed a response which really didn't say what I wanted to say to be honest, and edited it afterwards. The original response began the same way.


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