Just joined, would like some advice on ELI measurement

[Philipn]

Hello everyone
I'm not an electrician, but I was in the electrical business most of my working life before I retired. My background is electronics with the last few years working in industrial control systems - control engineering, etc - so I hope you will make allowances for me not being fully conversant with domestic electrical installations.

My question relates to checking ELI. My property, being rural, has TT earthing and I'd like to know how measure Ze as accurately as I can using my older loop tester - Robin KMP 4116DL. All this tester has is a mains lead and an earth probe. If I plug this into a socket in my house and carry out a test I get a reading of 120 ohms - providing it's on the 2k ohms range, lower settings will trip the RCD.

It seems to me that this method is not giving me a true Ze because of the parallel earth circuits that may be in place - although our water supply pipes are plastic and we don't have gas, so that may not be the case. For a true reading surely I need to disconnect the earth connection at the input and measure the impedance around the loop through ground to the incoming phase. Is this correct and, if so, how can I do that with my loop tester?

 

[telectrix]

that's about as close as you will u get, unless you cobble up some leads . if you water bonding was giving you a parallel path it would probably be a lot less that the 120 ohms.

 

[johnduffell]

Sounds like you don't have any extraneous conductive parts (technically the earth rod isn't one even though it fits the definition otherwise), so there won't be any other paths.
It's slightly safer not to have any, so you should be fine.
You will actually be measuring part of the circuit as well, but that will add negligible impedance on a tt system.

 

[Philipn]

Thank you both for your helpful replies.

 

[Wilko]

Hi - can I ask, what got you interested to make the measurement? Have you been having any troubles with the RCD tripping ?

 

[Philipn]

No, no problems at all. It was just that I didn't like the look of the existing earth rod - which looked pretty ancient, and was in a covered area - or the 4mm cable attached to it that ran back to the CU so I drove in 2 metres of new earth rod and replaced the earth lead with a 6mm one - I think it might need to be 10mm (?) but I had almost a full reel of 6mm and no 10mm..

 

[Wilko]

Did you measure the old one first - you've likely improved the result .

 

[Philipn]

Unfortunately I didn't.

 

[Wilko]

Another issue can be the rod is going into builder's rubble rather than nice earthy earth, if that makes any sense. The rubble is good for water drainage, but not for current flow. Although at 2m you should be through that. Could try hosing it for a bit and see?

 

[Philipn]

I just tried that, and it brought the Ze reading down a bit - 107 ohms. However, I thought the previous reading was acceptable?

 

[Wilko]

Yes it's an acceptable result and should work your RCD no problem.

 

[Philipn]

Excellent, many thanks for your help/advice.