Of far greater concern should be induced currents on external antenna feeders, which if your antennas are to be any good, will be at a significantly greater height than the incoming underground mains (!), and likely to bring in far more in the way of  atmospheric static and thunderstorms. Depending how much you intend to transmit, gas discharge devices across the coax or from any open line feeders to ground , at the main access panel or earth marshaling point, or the base of the antenna if it is a vertical tuned against ground, are worth considering. They are pennies to those of us who can solder  The Three legged ones are very good for balanced open wire line, the single cartridge ones are better for coaxial cables.

Be aware though that you need to have GDTs with a breakdown voltage a lot less than the peak RF that will appear on the lines, or they will suddenly light up mid transmission and short the PA, which is not good for the kit or the continuity of the QSO.. If you are not considering long bursts of unattended operation, then a sacrificial  relay that shorts all the antennas to ground when the shack mains is off is  a simple belt and braces expedient.

I'd also suggest that an RF filter rather than the BS7671  type surge suppressor will be more use for a radio shack- it will keep any stray RF  of yours out of the mains cable (reducing the risk of domestic conflicts), and put a bit of a damper on any mains born interference coming to you, as well as smearing out and flattening any short duration spikes into a lower amplitude longer duration 'ring' without lifting your local earth potential.

(the standard surge suppressor simply dumps the KV surge into the local earth which in your case, where there is a mix of inside and outside, is not so good.)

Mike.

I thought the requirement for an SPD didn't apply to individual domestic properties? In the 40 years I've lived in this road there gave been no lightning strikes (I can't even remember the last time we had a power cut). Given the intended use of the garden building it wouldn't be occupied during a thunderstorm so the risk to life is low.

My antenna is an HF end fed half wave fitted with a lightning arrestor that is connected to an earth rod. I propose to move the arrestor to the building and connect it to the mains earth rod.

Mike

it is a requirement to consider it.  You may consider it, to be spurious on this case ;-)

The sort of consideration you are supposed to do is

This includes a simplified assessment for when SPDs are required, stating that:

“Protection against transient over-voltages shall be provided where the consequence caused by over-voltage:

1. Results in serious injury to, or loss of, human life or
2. Results in interruption of public services and/or damage to cultural heritage or
3. Results in interruption of commercial or industrial activity, or
4. Affects a large number of co-located individuals.”

Maybe a shack full of gear that is unplugged or DP isolated when not in use does not really need this.

Mike

A rather long but interesting doc - but bear in mind written largely by the makers of the things.

The current rules for fitting SPDs are:

Must, if loss of life etc as listed in (1)..(4) above; else

if single domestic and cost outweighs value of stuff protected, then can skip;

else if can't be bothered to do risk assessment, then Must;

else if risk assessment says yes [this isn't a subjective thing - it's the calculation based on location in the country and the characteristics of last km of supply - except plus the dodgy urban/suburban distinction] , then you Must, else can skip.

The domestic exception is becoming mostly meaningless these days, as the value of TVs, laptops, LED lighting, boiler controls etc are almost always going to far outweigh £60 for an SPD.

The assessment includes the arrangement of the HV feeding the sub-station, and a lightning strike can be quite far away and not directly hitting the supply and still cause a transient.

I'm pretty certain a RA would conclude it's not necessary but if they really are £60 then the hassle would outweigh the cost. Where is the SPD installed - house or garden building?

And please tell me I don't need an AFFD ...

Mike

Where is the SPD installed - house or garden building?

Typically in each distribution board/CU - so potentially both. They act to momentarily short the wires together, so tend to lift the voltage on your Earth connection as much as lowering it on the other wires - so you'd get a very poor result from the point of view of the TT'd shed if the only SPD was in the house and connected to the TN earthing system.

There's also a nasty effect that can result in the surge getting larger again at a distance downstream of the SPD - perhaps twice the size it was limited to by the SPD initially - so circuits of  over 10m length feeding sensitive equipment tend to have extra downstream SPDs too.

I suspect that might also be a risk of SPDs in the shed introducing a surge from the Aerial system to the L+N conductors of the supply - which will conduct back into the house - where they'll be a a very different potential to the TN Earth - risking Class I equipment in the house if there wasn't another SPD there.

   - Andy.

It is these concerns, plus the risk of induced RF current in the wiring from transmitting , that led to my suggestion of filtering to spread and lower any spikes, rather than just simple clipping - it will take up rather more space on the wall and doubtless raise an eyebrow, but given the application ,may be more appropriate.

I do not think it is relevant for the sort of power in a Ham set up where it is a few hundred watts of RF at the very most, but for larger transmitters, the RF can trigger the mains SPDs into thinking there is a surge, which can be awkward !

A filter is only series inductance to hold off sudden current  changes, by allowing voltage drop across the 'L' and small capacitors to present a low impedance to any really high frequency components and fill in peaks and troughs at the tens to hundreds of  of nano-second scale, at which level of zooming in, a 50Hz sinewave is quasi-static. It can be combined with an SPD on the RF cold side if desired.

There are many types ready made, or there is the option to roll ones own for the application.

Mike.

Hmm this is becoming quite a project! Plenty of scope for the "electricians" on youtube to use their favourite expression "it's a grey area ..." (c:

OK so SPDs at both CUs then.

I operate SSB and limit my power to 100 Watts because I have to demonstrate compliance with the ICNIRP regulations and there is a public footpath at the bottom of the garden.

Mike

I am G8FNR. I suspect that the surge suppression is unlikely to work very well if you happen to get a discharge close to your aerials. I would tend to go with Mike here, as you are very unlikely to fully escape any damage, and a surge suppressor (Type 1) in the feed cable from the house might also be necessary to preveny damage there. The aerial case is a bit difficult because it is not really that considered for surge suppressors, these are intended to stop small surges on the mains supply incoming as much as anything, not big local cases. However in all the years I have had high aerials I have never had any damage anywhere, despite lightning happening around about. You need to be very careful too to stop much RF getting into the VDR suppressors, it may well cause severe RFI due to their inherent non-linearity, and much consumer type equipment in the shack can be a severe cause of HF noise. You may care to view youtube.com/watch?v=v9OXRtISCjM for some more information on aerials.

Hi David. I'm G8GYW, back on the air after 40 years away. Thanks for the link.

Mike