This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Where to install a surge protection device - single phase domestic / light commercial supply

A surge protection device can be installed in two locations:

1. The consumer unit. This is the easiest to install but it takes up two spaces which may not always be available.

2. The isolator switch. This is more difficult to install as it involves having to remove the big fuse but it avoids taking up two spaces in the consumer unit.

Does anybody have any more comments or advice on the best location?

Proteus sells an attractive looking isolator switch with a 2 module SPD although it is a bit pricey compared with SPD modules for consumer units. Has anybody got experience of it?

Who actually owns an already installed isolator switch? Is it National Grid, the utility company, or the property owner?

  • The DNO's responsibility stops at the cut-out these days, so the the isolator is usually owned by the supplier (or their metering agent) - although not always (mine is owned by me as I fitted it for my own convenience as my supplier seemed to be one of those that preferred not to fit their own by default). Nothing stopping you putting a "REC2-stype SPD" immediately downstream of the supplier's isolator though (subject to the usual rules of what can go in a meter box of course).

    As for positioning, I'd suggest as close to the "MET" as possible (i.e. where all the main bonding branches off) - there's little point in keeping the leads to the SPD as short as practical (e.g. <0.5m) if you're then going to by-pass the c.p.c. to a point that's perhaps several metres away. If the intake and CU are some distance apart you might even be best installing SPDs in both positions.

        - Andy.

  • Wylex (and doubtless others) sell an SPD/isolator in a 4 module casing (REC4).

    Normally, an isolator is fitted in the consumer's tails so it belongs to the consumer, but it would be at the supply end so in a cabinet if there is one.

    I raised the question of positioning of an SPD at a NAPIT event in the spring. IIRC, somebody had mentioned that protection is provided only for a finite length down the load side. It follows that SPD protection is best placed in (or immediately adjacent to) final distribution boards rather than at the origin.

    If I have misunderstood the situation, please let me know.

  • Note that Wylex now do a single-module SPD (which doesn't need protecting by an MCB as long as there's a 100A fuse upstream) which fits directly on the busbar (with terminals for N and E leads). Obviously only good for Wylex CUs though.

    As regards length, at more than 10m from the SPD the spike voltage can be twice what it is at the SPD (e.g. a 4kV spike reduced to 1.5kV at the SPD becomes 3kV further downstream) although I don't pretend to understand why.

  • I have noticed this Wylex single module SPD that slots onto the busbar.. Crabtree also sells one that looks like it's made on the same production line.

    Is it any good? Are there any disadvantages compared with a two module wide SPD?

    It appears that it will fit most consumer units. Not just Wylex.

  • Where best to fit an SPD all depends on where the surge is generated, and where the victim is, and the type of damage mechanism the victim is likely to experience.
    The simplest  is when the impulse is external to the building, and is common mode relative to ground - that is the voltages on  L and N fly up together- rather than voltages moving   in differnent directions - that antiphase  case  is differential mode..

    In such a case an SPD near where power enters the building is enough. But, surges may come from other places - wiring to outbuildings ,workshops with welding and machines, and may be a mixture of common and differential modes.

    In a perfect world each needs strangling as near the origin as possible - but this is not always practical. Situations where the victim equipment straddles two zones - perhaps mains wiring and a phone line or cable tv, then it may only be possible to tie things together  the victim to avoid the electrical equivalent of the splits...

    there is no one size fits all solution that always works.


  • This ought to be a simple question!

    If there is a main DB at the origin with distribution circuits to different parts of a house and its outbuildings, there seems to be nothing at risk before the final DBs.

    If, for example, it is desired to replace the final DBs in order to provide additional protection in the form of RCDs, doesn't it make sense to put the SPDs in the final DBs, particularly now that DBs with SPDs already installed are widely available?

  • Does it indeed make sense to put SPDs at both the main board and all dist boards both local and intermediate? perhaps it does but the cost versus risk needs be analysed too. In fact do you all use an SPD trailing socket to plug in computer stuff to the nearest socket outlet as well?

  • This ought to be a simple question!


    I make an intelligent guess that only 1% of all houses in Britain have a SPD in the incoming supply.

    There are still plenty of houses with prehistoric consumer units fitted with rewirable fuses. Even more with plastic / bakelite consumer units where the original fuses have been replaced by MCBs.

  • Where best to fit an SPD all depends on where the surge is generated, and where the victim is, and the type of damage mechanism the victim is likely to experience.

    Would many homeowners possess the knowledge of this in order to determine the optimum location of the SPD?

    If they had a workshop with several machine tools, then an SPD for the workshop supply physically located in the workshop could be a good idea in addition to one in the main consumer unit.

    For a conventional house an assumption might be made that the surge is external rather than internal.

  • the cost versus risk needs be analysed too

    If your final DBs are (over)due for replacement, the extra cost of ready-installed SPDs is hardly going to break the bank. Given that we have not had a damaging surge here over the past 27 years, the risk must be pretty small.