Chris Pearson: 
 

gkenyon: 
And of course, I don't know whether the stable has a concrete floor or wooden floor … if concrete, re-bar recommended to be main-bonded regardless of supply earthing arrangement (not just PME) see 705.415.2.1.

Does Section 705 always apply to stables? The picture that I got from the OP was that the stables and garage were in a domestic setting.

I guess that's up to the designer. If the premises (or part of them) are not classified as agricultural or horticultural premises, then I agree 705 will not apply (and of course the converse).

However, would you as a designer recommend Section 705 for the stables in order to protect horses (and yourself from being sued)? I think I would lean towards that to be honest.

gkenyon: 
However, would you as a designer recommend Section 705 for the stables in order to protect horses (and yourself from being sued)? I think I would lean towards that to be honest.

The floor of the particular stables that I am thinking of is made of sets (with appropriate channelling for urine to flow into the drains). There are no extraneous conductive parts and I don't see why a stable would require them. The only exposed conductive parts are the lighting switch drops in steel conduit. I cannot see how a PME supply would be a threat to anybody: two-legged or four.

I am sure you can Chris, the stable floor (Earth) and the neutral may be at significantly different potentials when the horse decides to bite/lick at the conduit/switch, which they do sometimes! Why seems to be something to do with horse psychology, but they are very susceptible to shocks. TT should reduce the time where there is a live/Earth fault to a very low chance indeed, whereas PME may not.

But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

Just as an aside - a modern TT installation may have a CT2 wired SPD before the first RCD - so what happens if the N-PE SPD element fails to short? (as I gather is possible, which is why no element is connected L-PE in the CT2 scheme). The TT system then ‘degrades’ to TN, no protective device opens (as the N-PE voltage difference is low) and the “fault” remains until probably the next EICR (whenever that might be). What's the magnitude of L-PE fault currents now?

    - Andy. 

AJJewsbury: 
 

But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

Just as an aside - a modern TT installation may have a CT2 wired SPD before the first RCD - so what happens if the N-PE SPD element fails to short? (as I gather is possible, which is why no element is connected L-PE in the CT2 scheme). The TT system then ‘degrades’ to TN, no protective device opens (as the N-PE voltage difference is low) and the “fault” remains until probably the next EICR (whenever that might be). What's the magnitude of L-PE fault currents now?

    - Andy. 

Same situation if there's any form of N-PE fault upstream of the first RCD … or indeed N-PE short in a CT1 wired SPD upstream of the first RCD … but of course CT2 is preferable for TT because this would require two SPD faults to cause an L-PE short which may be more problematic, at least initially before any other faults?

I guess, though, you are making the point because of 534.4.7 and TT supplies via overhead supply?

 

AJJewsbury: 
 

But if the stable has no extraneous-conductive-parts and is of wooden construction, perhaps with a blue plastic water pipe only, that puts a different complexion on things.

Just as an aside - a modern TT installation may have a CT2 wired SPD before the first RCD - so what happens if the N-PE SPD element fails to short? (as I gather is possible, which is why no element is connected L-PE in the CT2 scheme). The TT system then ‘degrades’ to TN, no protective device opens (as the N-PE voltage difference is low) and the “fault” remains until probably the next EICR (whenever that might be). What's the magnitude of L-PE fault currents now?

    - Andy. 

What is 3a on page 509 then?

Also fault protection as defined in Chap 41 shall remain effective in the protected installation even in the event of SPD failure. 536.4.6.

Also table 534.5 applies. SPD only downstream of R.C.D.

Edit. Add. Having just spoken to a very nice young lady at Surge Protection Devices Ltd. she assured me that their S.P.D.s can not short circuit permanently. They have a ten year guarantee and can work for up to 100,000 hours.

Z.

Can these things really go permanently short circuit?

Zoomup: 
 

B

Also table 534.5 applies. SPD only downstream of R.C.D.

 

Z

534.4.7

Can these things really go permanently short circuit?

Yes, or at least with the sort of fault currents you will see in a TT system they are not always enough to blow clear. I suspect in TNS, TNC-s there is enough PSSC to vaporise something smartly, so they will eventually fail to an open state .

The same things have been found inside well designed electronics for years, and they do fail more or less dead short in the face of any event that takes the spot dissipation inside higher than it can handle.

Mike.

mapj1: 
 

Can these things really go permanently short circuit?

Yes, or at least with the sort of fault currents you will see in a TT system they are not always enough to blow clear. I suspect in TNS, TNC-s there is enough PSSC to vaporise something smartly, so they will eventually fail to an open state .

The same things have been found inside well designed electronics for years, and they do fail more or less dead short in the face of any event that takes the spot dissipation inside higher than it can handle.

Mike.

Even gas discharge tubes as found in the Wylex neutral S.P.Ds?

Z.