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PeterW

Connecting up an ASHP

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@JSHarris

I didn't  imply anything,

I just said it wasn't proper.

And being qualified doesn't make you competent or knowledgeable, you should know that.

 

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Re neutral faults on a concentric incomer.

 

It is actually a well known fault condition. If the outer sheath of the concentric cable gets punctured, water can get in and corrode the outer (unsheathed) PEN conductors. Eventually it will give out and you have no neutral.

 

Inside the EZ it is surprisingly not much danger, but an exported earth to say a shed can rise to a lethal potential wrt local earth Carvans are the big risk where the metal bodywork is usually connected to earth and if you fed one of those with an exported PME earth you could have a lethal situation as you step in or out of the 'van, that's why the regsdemand they are fed from a TT earth.

 

Up here, the DNO's token gesture to make TNC-S a PME is a little pig tail of "earth" wire buried in the connection pit alongside the potted joint box.
 

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9 minutes ago, Steptoe said:

@JSHarris

I didn't  imply anything,

I just said it wasn't proper.

And being qualified doesn't make you competent or knowledgeable, you should know that.

 

 

You implied that our installation was bodged.  Specifically you gave my name and said that I had, quote:

Quote

" sort of (for want of a better word), bodged a bit of a half headed attempt at it"

That's both bloody rude and untrue.

 

The DNO instructed that the TT rod be removed when the main box was switched from TT to TN-C-S, not me. 

 

I very specifically added a local earth rod at the workshop, so that CU is TT, because the earth was being exported via fairly long length of SWA, and although all the impedances were within limits, I wanted to ensure that the local earth (the slab) in the workshop was equipotential with the earth at the workshop CU.  That's a damned good reason for doing it, in my view.

 

Finally, whether you or I like it or not, only those with the right bit of paper are now deemed competent.  It matters not how qualified or experiences anyone os, if you don't have the flavour-of-the-month bit of paper then you are deemed to not competent.  That's not just in this area, either.  My sideline for years has been as giving expert evidence in court, in civil cases (almost always insurance-related).  Those cases hinge entirely on competence, as shown on a bit of paper, rather than on common sense, qualifications, experience etc.  It's not right, in my view, but it's the way the world seems to now work.

 

 

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14 minutes ago, ProDave said:

Re neutral faults on a concentric incomer.

 

It is actually a well known fault condition. If the outer sheath of the concentric cable gets punctured, water can get in and corrode the outer (unsheathed) PEN conductors. Eventually it will give out and you have no neutral.

 

Inside the EZ it is surprisingly not much danger, but an exported earth to say a shed can rise to a lethal potential wrt local earth Carvans are the big risk where the metal bodywork is usually connected to earth and if you fed one of those with an exported PME earth you could have a lethal situation as you step in or out of the 'van, that's why the regsdemand they are fed from a TT earth.

 

Up here, the DNO's token gesture to make TNC-S a PME is a little pig tail of "earth" wire buried in the connection pit alongside the potted joint box.
 

 

And one of the reasons I TT'd my workshop, as it was a separate EZ and I wanted it the earth at the CU to be equipotential to the local earth under my feet.

Edited by JSHarris

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I think this topic has strayed a long way from ASHP's. Can we bring the earthing discussion to a close please, or start a new thread to discuss that aspect.

 

And play nicely please.
 

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9 minutes ago, JSHarris said:

 

And one of the reasons I TT'd my workshop, as it was a separate EZ and I wanted it the earth at the CU to be equipotential to the local earth under my feet.

You said your workshop was TN-C-S,

that's why I said adding a rod to it was a half hearted bodge at a localised PME,

So, all this pedantic talk of earthing types and terminology actually has a point.

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Maybe, but the general tone detracts from the underlying points.  If you upset people, then they tend to stop listening, and if they don't listen, then they won't learn and this kind of defeats the point of the debate. :)

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I just wish I understood what you lot are talking about, give me woodwork any day ?.

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17 minutes ago, TerryE said:

Maybe, but the general tone detracts from the underlying points.  If you upset people, then they tend to stop listening, and if they don't listen, then they won't learn and this kind of defeats the point of the debate. :)

But I think it further reinforces the fact that a little knowledge can be a dangerous thing, 

What was being described was a TN-C-S system with localised PME, which is entirely different from TT, 

One is completely acceptable, the former requires a lot more understanding than just banging a rod in and measuring the resistance, 

The other misunderstanding was it was said to be impedance being measured, which would be a TN system, so again, not TT as is now being said.

These things aren't just pedantic terms, they are lifesaving systems, and it is of vital importance to get them correct.

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2 hours ago, Steptoe said:

You said your workshop was TN-C-S,

that's why I said adding a rod to it was a half hearted bodge at a localised PME,

So, all this pedantic talk of earthing types and terminology actually has a point.

 

In that case that's an error, as it's TT.  Dave and I discussed this, either here or on Ebuild, back when I was doing it, and IIRC we were both of the same mind that exporting an earth down a moderate length of SWA was OK as far as the regs were concerned, but that I still felt happier having equipotential earthing at the workshop end, with the rod making the local earth in the CU the same potential as the slab.

 

Exporting the earth over that distance was fine in term in terms of impedance, it was solely a personal "comfort factor" that made me put an earth rod in.  In case anyone is interested, I fitted the earth rod in an unusual way, and it may be an idea someone wants to copy.  I found that the standard, copper-coated steel, screw together sections, type earth rod is a nice sliding fit in (I think) 12mm OD plastic pipe.  There's a short length of this set into the workshop floor slab that goes right down through the slab, insulation and type 1.  Into this I drove a long rod (two screwed together, so well over 2m, I think).  It's at least a metre and half into the underlying soil, maybe more, as the termination plastic box is only about a foot of the floor, and screwed to the inside wall.  There's a bit of rigid black conduit running from the green box up to the CU, with a 16mm2 earth lead in it.  I don't have the paperwork to hand here, but IIRC it was a damned good earth, probably because the sub-soil under the slab is most probably always a bit moister than the surrounding open ground.  The rod is also down below the level of a soakaway, that's a few metres away further upslope, and that may well help keep it damp.

 

It's a neat way of keeping an earth rod well protected, I've seen some outside ones that have been hit by stuff, had multiple coats of paint over them when close to outside walls and generally in places that are subject to damage.  The only downside is that it is not easy to pull it out, as it's so close to the wall and right in a corner.  I have pulled it out, because originally it was only a single length, but when the temporary one was removed from the main incomer, when the house wiring was completed, I pulled it out, screwed the two together, to double the length.  It made not one jot of difference to the effectiveness, mind!

Edited by JSHarris

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1 hour ago, Steptoe said:

But I think it further reinforces the fact that a little knowledge can be a dangerous thing, 

What was being described was a TN-C-S system with localised PME, which is entirely different from TT, 

One is completely acceptable, the former requires a lot more understanding than just banging a rod in and measuring the resistance, 

The other misunderstanding was it was said to be impedance being measured, which would be a TN system, so again, not TT as is now being said.

These things aren't just pedantic terms, they are lifesaving systems, and it is of vital importance to get them correct.

 

 

Some confusion has crept in here, and that's in part to me making an error (highlighted earlier) and in part by the change in the earthing system demanded by the DNO, part way through the build.  The latter was, in my view, barking mad and illogical.

 

 We applied for, and had installed, a permanent domestic supply.  I'd already fitted the meter cabinet, put in an isolator fused DP switch, an earth block, Henley block and a feed to an external 4 way IP68 CU.  That CU had a 100A DP switch and three DP RCBOs, one of which was (and still is) connected to a 16A Commando socket.  All this was tested and signed off by the first electrician, and provided a connection point for the house supply (via an 80A fused, 100A DP isolator), and power for the sewage treatment plant, the borehole water supply pump and the 16A Commando.  At no time did we apply for, or pay for, a site supply.  I expected that the DNO would just fit the head, ready for the suppliers to fit the meter the next day, but they refused point blank to connect it, because of the lack of a TT earth, saying that their policy was that a "temporary site supply" MUST be TT'd.  I tried explaining that this was the permanent house supply, not a temporary site supply, and that they were never coming back to this job again once they'd fitted the head, but they were adamant.  I ended up driving a 20 mile round trip to get an earth rod, box and bit of conduit, whacking it in and calling my electrician to come in and test and wire it, before the DNO would make the supply live, ready for the suppliers to come in the next morning to fit the meter.

 

The next day the suppliers turned up to fit the meter, and asked why the tails I'd left included a TT earth, when there was a perfectly good TN-C-S earth available under the cover on the head.  I told the the tale and they just shook their head in amazement.

 

When we'd finished wiring the house, my electrician asked the same question, why is the meter box TT'd with a dodgy looking rod banged into the soil next to it?  I explained, and even pointed out that the earth impedance on the TN-C-S earth had been measured by the DNO and written in felt pen inside the box!  At this point my electrician rang the DNO, who told him he could go ahead and pull the plastic cap out and fit the earth to their connection, removing the "temporary" TT earth.

 

Now, I can fully understand why, when fitting a temporary site supply (for which there are separate charges) it would make a lot of sense to have a TT earth.  It's the same logic that caused me to fit one in the workshop, it gets the site earth and electrical connection earth at the same potential.  However, I cannot see why the same installation, with no changes at all except a DP fused isolator switch being thrown for the first time, should be OK on TN-C-S, when with that same switch off it wasn't, and could only be run TT'd.

 

If our account was for a temporary site supply, rather than a domestic supply, then I could understand it; I'm sure they have a safety policy that says all temporary supplies have to be TT'd (makes sense, as a temporary supply could include a generator, I guess).  I cannot for the life of me understand why they applied this policy to a standard domestic supply, though.  All answers welcome, because it's been bugging me for ages.....................

Edited by JSHarris

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We had exactly the same experience regarding the site supply having to be TT earth. So we had the strange situation of NIE fitting a new transformer, running the cable across a field, carefully inserting earth points for a TN-C-S connection, connecting up the the incoming cable to a meter in the meter box in the wall of the house - and then telling us we had to knock in a rod in because we couldn't use their carefully crafted earth connections until we had everything signed off. 

Absolutely insane 

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3 minutes ago, Alphonsox said:

We had exactly the same experience regarding the site supply having to be TT earth. So we had the strange situation of NIE fitting a new transformer, running the cable across a field, carefully inserting earth points for a TN-C-S connection, connecting up the the incoming cable to a meter in the meter box in the wall of the house - and then telling us we had to knock in a rod in because we couldn't use their carefully crafted earth connections until we had everything signed off.

Absolutely insane

It's not insane, it's the rules.

 

The TNC-S earth is there ready for you to use when the supply is feeding the house, no need to get the DNO back again. it's usually there accessible on the side of the cut out, though I am noticing more often than not they now fit an earth block next to the cut out. In the mean time while it's feeding a building site, just for the moment ignore their earth and connect to your own rod.  In the case of a static caravan, that must remain on a TT earth.

 


 

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7 hours ago, JSHarris said:

 

 

Yes, the upfront master RCD  leakage is rated above the smaller RCBOs, so I get the right trip sequences, and I accept that the higher leakage current is slightly less safe as a consequence.  

 

 

 

 

 

Just noticed this,

And it may be something that most of you don't realise,

The actual tripping current is a bit of a red herring here,

It could actually be rated the same, or lower than the other devices, 

Its the tripping time characteristics that are important here, 

Simply using a 'normal' 100mA RCD will not provide any discrepancy with a 30mA RCD or RCBO, 

You require one with a Selective or Time Delay inbuilt (S) or (TD)

 

Edited by Steptoe

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7 minutes ago, Steptoe said:

 

Just noticed this,

And it may be something that most of you don't realise,

The actual tripping current is a bit of a red herring here,

It could actually be rated the same, or lower than the other devices, 

Its the tripping time characteristics that are important here, 

Simply using a 'normal' 100mA RCD will not provide any discrepancy with a 30mA RCD or RCBO, 

You require one with a Selective or Time Delay inbuilt (S) or (TD)

 

 

+1. 

 

https://www.tlc-direct.co.uk/Products/WYWRMT100slash2.html

 

Double the price if you want MK.

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