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SPDs - retrofitting practicalities?

AJJewsbury

Having had yet another LED driver/ballast go phut at home - and this time rather obviously during a period when we had several thunderstorms - I have started to wonder if retrofitting some surge protection to my mid-17th Ed installation might be a good idea.

I am a bit wary of SPDs in one respect though - having learned the hard way that having them in the wrong position (or rather not in all the right positions) can do more harm that good. Quite a few years ago a PC with analogue modem, fed by one of those Belkin extension leads with in-built surge protection for both power and phone line, resulted in the phone system (PBX) getting fried (twice) before I figured out that the surges must have been coming in on the overhead phone line and being drawn through the PBX to the SPD in the extension lead. Disconnecting the modem phone line from the SPD in the extension lead and adding an SPD to the main incoming phone line before the PBX did seem to be a long term solution.

I could imagine something similar happening with the TV aerial if I had SPDs only on the LV side - with surge currents being drawn through the TV and associated boxes.

So Fig 534.1 of BS 7671 seems to make sense - I'd need to protect everything that could introduce a surge into the installation, not just the LV.  BS 7671 seems to cover the LV side well enough - but what about everything else?

I my case, I've got LV arriving through an underground (PILC) cable into the cellar with a couple of sub-mains one to the main house CU and another to a detached garage (TT'd) . Phone (&ADSL) overhead from a pole in the street where it goes underground, a chimney mounted TV aerial and a PV array on the roof. So what to put where? I'm presuming type 2 SPDs at the main intake and both CUs for starters.

Ideally everything would enter close together to minimise voltages between SPDs - while the phone line and TV aerial cable enter close to each other on the 1st floor - they're some distance from the house CU and a long way from the intake position in the cellar. The aerial feeds a multi-outlet distribution system so already has the cable screens earthed (via a 4mm2 G/Y to the CU). The PV d.c. cables come through the roof at almost the furthest possible distance from the CU. 

So lots of questions -

  • What sort of SPDs would I need for the d.c. side of the PV? Would ordinary type 2s do or would I need something special? I can imagine that d.c. side of PV poses a few possible problems - both as d.c. arcs don't extinguish themselves - and the limited current available wouldn't open any fuse intended to disconnect the SPD should it fail short-circuit.
  • What sort of SPD for the TV aerial cable? - I've not seen much on the market and would it have any impact on the digital TV signal?
  • Would an SPD on the phone line have any impact on the ADSL performance?
  • How to cope with the distances between the different points of entry? They're orders of magnitude higher than the usual 0.5m limit for SPD connections. An SPD on the aerial cable might limit the voltage between core and screen at the point of entry, but with the large surge currents involved, that might still leave a huge difference in voltage between screen at that point and PE (and N & L) at the TV set. Ditto for the PV system and the rest of the installation.
  • What sort of size conductors would be needed for the TV, Phone and PV SPD earth connections? As the points of entry would seem to be the ‘origin of the installation’ for those systems BS 7671 could be read as suggesting 6mm2 minimum for type 2s - but that's rather too large to be convenient (I could just about manage 4mm2 as the c.p.c.s are already that) but still feels far too small to limit the voltage between different SPDs at the distances involved.

 

Or is it all just too hard to do properly, and should I just give up and resign myself to occasionally replacing the odd bit of electronics?

   - Andy.

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  • 0

    Thanks guys.

    This to may introduce a problem for ‘a’ that was not there before.

    Indeed - the more I think about it the worse it seems. SPDs seem easy enough when there's only one connection to a piece of equipment, but once you get two or more, especially heading off in different directions, it seems to get much much uglier - with far more risk of (a portion of) the surge current being drawn right through the equipment you're trying to protect.

    Fig 534.1 (the concentric circles of lightning protection zones with external services heading off in all sorts of different directions) looks good on paper but seems to rely on the bonding forming each circle having near negligible impedance … but given that most of us don't live or work in something akin to a landlocked submarine, that doesn't seem particularly realistic.

    As you say, having everything enter at a common point makes things a lot simpler … but isn't particularly practical, especially with PV & aerials on the roof and main services coming out of the ground. Maybe for a new build or major renovation it would be possible to arrange things that way (PV d.c. cables coming down the outside wall to an inverter next to the intake, likewise phone and TV aerial), but isn't going to wash for me now.

    The only simple policy I can see that might work would be to have SPDs on all connections immediately next to every piece of equipment you're trying to protect (with a common PE connection) . So for the PV I'd need SPDs on the d.c. side right before the inverter, then another set on the a.c. side - both connected to the same c.p.c., then another two sets in the CU - one on the bus-bar and another after the PV circuit's RCBO (if I didn't want it to be subject to the surge current). Likewise the TV aerial system would need SPD both fore and aft of the distribution amplifier and then again on both the aerial and mains at the TV. Cost and convenience count strongly against that approach however.

    I do wonder about the value (or indeed the costs) of adding SPDs to the LV installation without considering everything else in the building.

       - Andy.

Reply
  • 0

    Thanks guys.

    This to may introduce a problem for ‘a’ that was not there before.

    Indeed - the more I think about it the worse it seems. SPDs seem easy enough when there's only one connection to a piece of equipment, but once you get two or more, especially heading off in different directions, it seems to get much much uglier - with far more risk of (a portion of) the surge current being drawn right through the equipment you're trying to protect.

    Fig 534.1 (the concentric circles of lightning protection zones with external services heading off in all sorts of different directions) looks good on paper but seems to rely on the bonding forming each circle having near negligible impedance … but given that most of us don't live or work in something akin to a landlocked submarine, that doesn't seem particularly realistic.

    As you say, having everything enter at a common point makes things a lot simpler … but isn't particularly practical, especially with PV & aerials on the roof and main services coming out of the ground. Maybe for a new build or major renovation it would be possible to arrange things that way (PV d.c. cables coming down the outside wall to an inverter next to the intake, likewise phone and TV aerial), but isn't going to wash for me now.

    The only simple policy I can see that might work would be to have SPDs on all connections immediately next to every piece of equipment you're trying to protect (with a common PE connection) . So for the PV I'd need SPDs on the d.c. side right before the inverter, then another set on the a.c. side - both connected to the same c.p.c., then another two sets in the CU - one on the bus-bar and another after the PV circuit's RCBO (if I didn't want it to be subject to the surge current). Likewise the TV aerial system would need SPD both fore and aft of the distribution amplifier and then again on both the aerial and mains at the TV. Cost and convenience count strongly against that approach however.

    I do wonder about the value (or indeed the costs) of adding SPDs to the LV installation without considering everything else in the building.

       - Andy.

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