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Best location for a SPD

AncientMariner
Our existing supply arrangement is underground TN-S into the DNO's external access meterbox where the Cut-Out, Meter and DNO supplied 2-pole Isolator are located. From the Isolator 25mm tails through the wall to a Proteus metal cased 100 Amp Switch Fuse (Approx 19 x 31 x 6 cm) located where the original meter and CU were located. From the Switch Fuse, a 25mm sub-main of about 10 metres to Hager CU (plastic) in the attached garage.

The above all from when the house was rewired circa 2002.

We seem to have more than our fair share of lamp failures, GU10 Halogens and the GU10 LEDs that I replaced them with - Crompton and B&Q. Also 6-foot LED fluorescent replacements. Add to those, Broadband Routers and VoIP adapters. We are about 150 metres from the sub-station (which supplied underground).

As I see it, the only two locations for a SPD would be either at the Switch Fuse or at the Consumer Unit. I'm assuming that at the Switch Fuse would be preferable. Also, no other earth connection than the existing DNO supplied one from the TN-S supply?


Thanks.  Clive
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    There is an extra consideration with SPDs in that if the downstream path is 10m or more, you can get some some of resonance thinggy (which I do not claim to understand) which can double the spike. So an SPD at the switch fuse reduces any transient to 1.5kV at that point , but at the CU the spike is now 3kV. So best to put the SPD near the CU.
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    +1 for at the CU - as wally says better to have the SPDs not too far from what's to be protected, plus better placed there to provide protection from surges due switching etc. on one circuit affecting other circuits.


    If main bonding is taken from the intake/switch-fuse rather than the CU however, there is an argument for having SPDs there - as the L-c.p.c. protection by more local SPDs could be undermined somewhat by class 1 items being in contact with extraneous-conductive-parts. I'm sure the manufacturers would recommend SPDs at every stage of distribution.

     

       - Andy.
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    really you want it where the conductors you want the SPD to crowbar together for the duration of any overvoltage come physically closest. What you do not want is any additional inductance at about 0,7- 1 nH per mm for meter tails that is in the shunt path.
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    Hah, I am enlightend, I was thinking that on say a supply that has a switchfuse within 3m of the cut out then feeding a CU say up to 10m away from it would be option one to clamp the voltage near the switchfuse for protection of incomming external transients or better still for option two both at the switchfuse and also at the CU  but more cost. You`ve made me rethink now.
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    "I'm sure the manufacturers would recommend SPDs at every stage of distribution. "


    Agreed AJJ
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    ebee:

    "I'm sure the manufacturers would recommend SPDs at every stage of distribution. "


    Agreed AJJ


    Slightly different scenario: intake into the garage/workshop, which is nearest to the street. Lighting and various socket circuits. It is most unlikely that any sensitive equipment will be plugged in there, so why have SPD at this point?


    Switch fuse protecting distribution circuit to the house. Isn't the best place at the CU? If you are changing a CU (or installing from scratch) getting one with SPD already fitted seems to be a bit of a no-brainier.


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    I was working on the theory of clamping the voltage as it comes in before getting further, seems I was wrong

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