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Earth to Earth - when not to connect earths together.


MikeSharp01

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1 hour ago, JSHarris said:

 

And it's easier and more practical to do, too, especially with a nice high wattage bulb so you get a decent earth current.  The snag is that you don't have an exact number to enter into some form to show that the impedance is low enough, not that knowing the exact impedance makes it any safer, it just keeps the paperwork police happy.............

 

 

Might be a bit tricky to get a calibration cert for the ad hoc "meter"...

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3 hours ago, JSHarris said:

 

Probably - the 15V battery for the AVO definitely should be, as they aren't that easy to buy any more.

 

You could always make one:

 

https://martin-jones.com/2013/07/31/avometer-8-blr121-15v-battery-replacement/amp/&ved=0ahUKEwjt1pb854vXAhUEBBoKHXqzBuAQFghKMAE&usg=AOvVaw34FE_3xShduiPBJRRpcOcx&ampcf=1

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Is THIS the perfect test device for my new earth rod driven in today? It's the trip device in the old house. Although it has a weakness, it basically protects nothing much, the trip is based on leakage to the earth wire and the test button does not work of you remove the earth and behave very carefully. I connected it to the new earth rod and it works perfectly so the new rod must be working at a basic level. No certificate though! Won't UKPN have a test device I can blag a test from / with? 

Edited by MikeSharp01
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8 hours ago, MikeSharp01 said:

Ah - the brother in law. Ok will await his test. 

 

If you're stuck I've an earth loop impedance tester you can borrow. 

An LCB2500/2:

 

www.industrialcalibration.co.uk/downloads/LCB2000%20User%20Guide.pdf

 

Does RCDs too along with ramp testing. An oldie but a goodie!

 

IR tester too, dedicated capacitance meter and so on.

 

 

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Ok its all done, installed and connected. Just thought I would update with a few pics and the test results.

 

The panel as finally complete, rewired to remove all vestiges of single insulation as suggested by @ProDave with split Henly blocks and using the piranha nuts as suggested by @JSHarris and others. All the earths are connected together here both the PME supplied earth and the rod earth I installed. The gland, lower left is screwed into the top of the piranha nut and allows me to shrink wrap (still to be shrunk ) the three cores of the SWA feeding the Garden room and maintain the double insulation, inside the gland, while allowing them to split out.

 

Installed - the earth rod for the utilities centre (This will eventually be inside the building once it gets its frame and cladding):

 

The rather fetching main meter cabinet with the new meter (reading 000000) installed and connected to our main isolator:

 

Interesting to report the readings:

 

Ze for the earth rod was 4.1 Ohm, while Zs was 0.3 Ohms, prospective Fault current 811A and Sort Circuit current of 1324A

 

Down at the Garden room CU (without the local earth rod connected, still to run the cable) the figures were:

 

Ze 0.25 Ohms (Not sure I understood this immediately but think I do now - why is it so much lower than the 4.5 Ohms of the earth rod down the 45m of cable at the utilities centre) Zs 0.37 Prospective fault 693A and short circuit of 928 I am told these reasonable to good!

 

20171025_154941.jpg

20171027_171041.jpg

20171027_171151.jpg

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10 hours ago, MikeSharp01 said:

Interesting to report the readings:

 

Ze for the earth rod was 4.1 Ohm, while Zs was 0.3 Ohms, prospective Fault current 811A and Sort Circuit current of 1324A

 

Down at the Garden room CU (without the local earth rod connected, still to run the cable) the figures were:

 

Ze 0.25 Ohms (Not sure I understood this immediately but think I do now - why is it so much lower than the 4.5 Ohms of the earth rod down the 45m of cable at the utilities centre) Zs 0.37 Prospective fault 693A and short circuit of 928 I am told these reasonable to good!

 

Ze is the earth loop impedance at the origin , Zs is the earth loop impedance of a particular circuit.

 

So you have Ze 0.3 ohm from the PME earth and 4.1 ohm from the rod. These are both connected in parallel. So your actual Ze at the origin will be a bit under 0.3

 

Ze at the sun room will be Ze at the origin plus the impedance of the cable, so 0.25 sounds suspiciously low.

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3 hours ago, ProDave said:

Ze at the sun room will be Ze at the origin plus the impedance of the cable, so 0.25 sounds suspiciously low.

Yes, as I said I was not sure I understood this at the outset - when he did the test in the Garden room I mentioned that I did not understand why it was so low, he said that is can vary and repeated the test but the max reading was 0.27 the electric board people gave Ze (They called it Earth Loop Impedance) at the PME junction box as 0.1 Ohm so maybe the 0.3 at the distribution panel (350mm of 16mm2 earth tail away from the PME earth) was out  because 4.1 and 0.1 can get to 0.25 I suspect even down 45m of 16mm2 cable.

Edited by MikeSharp01
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  • 2 years later...
On 24/10/2017 at 09:19, Jeremy Harris said:

I was peripherally involved in some of the consultations for the LV Directive and harmonisation, and what was interesting was how focussed the UK was on earthing, and how almost disinterested some other member states were about it.

 

From talking to some who were involved in the decision to adopt PME, which sort of morphed into TN-C-S (although they are not always the same) I gather the major concern at the time (it was before I'd even qualified, let alone was teaching) was that earth rods were often found to be damaged, the cables or clamps broken and that a lot of the time the real earth protection was being provided via earth bonded iron pipes, rather than the earth rod. 

 

There seems to have been a lot of resistance to the idea of PME, which I think stemmed from the shift of responsibility from providing the PE from the householder to the supply corporation (as it was at the time).   My view is that they would have been better off tackling the cause of earth rod/mat failures, as they have in the current regs for a TT connection, and specified protection for both the cable clamp to the rod and the PE cable coming into the house.

 

The other relatively recent change that questions the PME/TN-C-S model is that the incomer rarely comes into the house now, anyway.  External meter boxes have been the norm for decades and some of us here have gone one step further and fitted the box away from the house altogether.  That then creates the "caravan problem" for the house, in effect, although with far less conductive material around in a house the risk is  great deal lower.

 

I quite like the idea of having a local intermediate earth, for the same reason as I wasn't too keen on exporting the earth out to my workshop, so wired that as a local TT installation, with a very well protected earth rod.  It's reassuring to know that you have a fair degree of local protection should the local distribution network develop a CNE fault.  Having said that, so many appliances are double insulated now, and as RCD or RCBO protection is mandatory, the risk for users has reduced a great deal.

When the regs change to Harmonisation and went to RCD's as protection to replace the faraday cage within a faraday cage (bathrooms) it solved the problem of checking if local earthing was present but the problem now is that no one presses the test button and as a result RCD's stop working!

 

BTW, it isn't compulsory for building sites to use 110V. It is allowed in the regs (or was) to use 230V in combination with an RCD but almost all sites don't allow it for policy reasons.

 

The short reason that PME isn't allowed on sites is that the regs say so and the long reason is that unlike in a house there isn't a faraday cage present to protect.

Edited by Markblox
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