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.

Mike

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?

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?

ebee said:

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.

wise words Chris. just employing murphy`s law

But is the risk increasing?

I would say yes the risk is increasing because we are using more and more appliances that are susceptible.

Not many years ago most stuff was just simple current drawing items, telephone lines were the only things we had much local surge protection for. 

I am highly dubious about the merits of SPDs rated for 'domestic use'. Indeed, I haven't bothered with one for my own installation. Can't say I have ever experienced anything untoward as a consequence.

That is pretty much my attitude, but it could be different if I lived on the top of a hill with an aerial supply.

However, if you are going to install a new CU (either for yourself or in trade) is there any good reason for not including an SPD?

I live at the top of a hill but my supply is underground.

To answer your question - Yes I would, and have fitted them as a swap-out of an existing 3036 job, but I would never recommend a separate retro-fit to an existing installation.

In-CU or Out-CU I don't believe they are effective in either case, but Fusebox do em cheap enough to be a no brainer, and the best of all is that if they 'turn red' nobody will bother replacing them, nor will the customer notice any change in the characteristics of their supply!

whjohnson said:

best of all is that if they 'turn red' nobody will bother replacing them, nor will the customer notice any change in the characteristics of their supply!

That's very uncharitable! People will check them when they press their test buttons twice a year.

Oh of course they will - silly me!

I think that unless you have mission-critical kit, such as a server farm for a merchant bank, and can afford 'proper' SPD protection, then you are tiddling into the wind when it comes to domestic, especially when the things are constructed with dime store jelly bean priced components marked up 100s of percent. Still, it ticks another regs form box, and that seems to be all they are interested in these days.

I ask you - how many instances have there been whereby a house fire has been blamed on the electrics and the insurance company has refused to pay out on account of the cu being an old BS3036 rewirable with no RCD, no SPD and no AFDD present?

Personally, I've never heard of such an instance, and I don't believe there will ever be one. If the insurers went down that route, then no one other than mortgage holders would ever bother with house insurance again.

whjohnson said:

no one other than mortgage holders would ever bother with house insurance

Quite a few folk do not bother with insurance  as it is article in actuarial post suggests ~ 1/4 UK homes no content insurance and more like half for tenants though it is not clear from the articles I suspect the un-insured are those at the poorer end of the market, so les likely to have the all new fuseboard anyway.
Mike