Wiring a garden building

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

I made that video about 40 years ago! Still worth watching. 73.

I made that video about 40 years ago! Still worth watching. 73.

Fascinating video David, thanks for sharing.

As for my wiring conundrum, I'm not looking forward to the discussions with the electrician / building control surveyor as I doubt they would have come across this before. Perhaps an SPD in the house and mains filter in the shack is a starting point.

Mike