Loop Testing

[deejaypsy]

Hi there, I wonder if someone could help with a problem I am having...
I inspected a flat on the 2nd floor of a 2 storey block.
I have inspected flats in this complex before.
All of the other flats have received a great earth loop reading of around 0.16 ohms.
Now this flat I inspected last week came back with a loop of 12 ohms. Obviously this needed to be corrected.
Normally i would line tap the earth to the neutral in the meter box.
I checked the kiosk feeding the 4 flats in this particular block, and the earth is already bonded to the neutral busbar.
This leads me to think that the earth is broken somewhere between the kiosk and the flat.
I wanted to verify this, so I connected an earth wire from the incoming junction box (around 100m from the kiosk, at the foot of the flats) directly to the earth bar in the mainboard.
I again tested the loop and it came back as a similar reading...around 11 ohms.
I also meggered the earth wire from the junction box to the earth bar and found perfect continuity.
I have not managed to gain access to the other 3 flats to test their loop readings, but I am assuming that the earth is rusted/corroded through somewhere between the kiosk and the block of flats (it's a 10mm bare earth with a 16mm 4 core armoured cable).
Would this sound like a likely assumption of what I am finding?
Your help much appreciated as the sale of the flat is registering before the weekend and I cannot issue the CoC before this is resolved...
Many thanks
Simon

[AndyD]

......I also meggered the earth wire from the junction box to the earth bar and found perfect continuity.

Why did you use a megger tester? Continuity is better measured with a standard multimeter.

You'll probably fine there's a poor termination somewhere but you'd need to get access to the other flats really.

[deejaypsy]

Why did you use a megger tester? Continuity is better measured with a standard multimeter.

Sorry, I use my megger on its bell test sometimes. I meant just checked continuity.
I couldn't gain access to the other flats and I have checked all the junction boxes and find nothing untoward.
I have started knocking earth spikes in to get an earth but this property is near the beach.
I'm 25ft down and the reading has improved from 12.5 ohm to around 5 ohm now.
Almost there!

[deejaypsy]

Well, finally got there...
17 earth spikes later!!!
6 in 1 hole, 6 in another, and 5 in the 3rd! What a mission. I would definitely have opted for an earth mat, but the complex manager insisted I use earth spikes instead as I would need to lift pavers in order to put an earth mat down!
Ah well, as long as the clients happy and someone's paying at the end of the day...

[AndyD]

I like that, 6 spikes deep is good. I also usually go 6-8x2.3m spikes deep if it's a sandy area and depending on the supply size would try for a sub1ohm impedance. Good depth usually gets a good Ra reading especially if you can get below the water table. Did you knock them in the old fashioned way with a sledge hammer or did you use a jack-hammer with an SDSMax rod driving attachment? Last time I used a sledge hammer was a couple of years ago on a remote site with no power, I thought I was having a heart attack after the first 3 rods and ended up getting a generator.

[deejaypsy]

Hey Andy,
I was told the water table was only 2m down, which was great, cos as soon as I went into that, bam, perfect earth until the copper rusts away!
But no luck for me I'm afraid...
I didn't know you can use a SDS hammer drill for this! Wow! Next time I will look into this attachment, as I already have the drill!
Very cool

[AndyD]

You should invest, believe me they're worth it. I've got a couple that are DeWalt but Bosch and several others also supply them. You can also make your own like this one from an old drill bit or chisel bit and a socket if you've got a welder.

I also have a slide hammer I made specially for earth rods, It's still manual obviously but it's a lot easier than a sledge hammer and it works really well



Getting below the water table is always good and rust on earth rods has very little, if any, negative effect on their Ra impedance reading from experience. Also over time the ground settles and compacts against the rod, if you go back and test a rod a year or two after you installed it I guarantee the readings will be far better than when it was newly installed.