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Topic Title: TT system help
Topic Summary: understanding what what should be there?
Created On: 21 April 2014 12:35 PM
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 21 April 2014 12:35 PM
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ryespark

Posts: 203
Joined: 05 March 2008

Hi Guys,

went to look at a job the other day, it is a property located in a island in the middle of the river thames. It was obviously supplied by a overhead TT supply. I have never come across a TT supply before in the area that I work. There is a split load wylex board, with the lighting on a the non RCD protected side of the board. The customer wants some alternations to the lighting in the property.

I carried out a quick ze test on the installation, the earth loop impedance is very low for a TT system at 0.15 ohms. probably understandable being on a island.

Mentioned to the customer that the lighting should be RCD protected, he was not to impressed as that the fuse board changed a few years ago, but did realise he needed certification.

My question is it possible to simply install a RCBO on the lighting circuit that i will be working on?

Or will a upfront 100ma RCD be needed to protect the whole installation as well?

what code would you give this on a ECR if you come across this?

Many thanks
Ryan

-------------------------
Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 05:14 PM
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John Peckham

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Ryan

How did you measure your Ze? That is a very low Ze(Ra) for most TT systems even if it was on an island in the middle of the Thames.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 21 April 2014 05:57 PM
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slittle

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I agree with John, That's impossibly low for a TT supply. Are you sure it's TT or is it an overhead fed supply that is in fact TNCS at the cutout ??

Either way to comply with 17th edition RCD protection would be required for the circuits fed from the mainswitch (lighting) in a domestic situation and although I wouldn't do it, an RCBO on the affected circuits should provide compliance


Stu
 21 April 2014 06:00 PM
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ryespark

Posts: 203
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Hi John,

Yes thats what i thought as well, even got my calibration box out to check my tester, tested on my multifunction tester with main earth bonding disconnected and main switch on. The water table was very high from the recent flooding but still seemed low, but do not have any experience of TT systems.

thanks

-------------------------
Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 06:05 PM
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ryespark

Posts: 203
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No its defiantly TT Stu, can checked the cutout, as can see the earth rod under the timber framed house.

cheers

-------------------------
Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 06:07 PM
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John Peckham

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Ryan

What is the earthing conductor connected to? Rod, plates pipework etc? If it is a rod any idea of the length? Did you do a high current test or a low current no trip test?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 21 April 2014 06:08 PM
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slittle

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Stupid question but you are doing your loop test L-E and not L-N aren't you ??

Easy mistake to make if you've got a three lead tester


Stu
 21 April 2014 06:11 PM
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ryespark

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Hi John,

Its a rod, no idea of the length its located under the timber framed house either its been banged in a sharp angle or its probably no longer than 1 Meter long, tested it on high current on my meter.

thanks

-------------------------
Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 06:20 PM
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slittle

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Are there neighbouring properties on the island ??

it's possible that even with the main bonds disconnected there are "copper paths" to other properties which are TNCS. I see it a lot with farm cottages that have metallic water mains, one is TT the neighbouring one TNCS. The results are similar to yours

Have you disconnected the main earth conductor from the earth bar in the DB and loop tested that with nothing else connected to it ?

Stu
 21 April 2014 06:22 PM
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John Peckham

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The Ra cannot be that low on a single 1.2m rod. Something is wrong or the rod is connected to other rods(unlikely).

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John Peckham

http://www.astutetechnicalservices.co.uk/
 21 April 2014 06:35 PM
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ryespark

Posts: 203
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Yes I did disconnect the main earth from the terminal aswell, how low have you seen ra readings in a domestic situation john? Will have to pop back there to take a closer look maybe with another meter to!

Thanks for your help guys

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Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 06:37 PM
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redtoblackblewtopieces

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Why did you test with the main switch on ? Was the earthing conductor disconnected for test ie this was the only path to earth during test, also what material are the water and gas (?) supply pipes.
Kevin

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 21 April 2014 06:57 PM
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ryespark

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Main switch was on but all circuits isolated, water and gas pipes are not plastic, could it not just be to the ground being so wet? The water completely entered the house which was about 19 inches deep in the house?

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Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 07:20 PM
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hifly

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think about it, 0.15 is better than you will get for most r1+r2's do you really think you could get this via mud/water ?

as asked above are you sure you did not have the meter set to L-N?

how did you know you were on the wire to the rod and not a bond that could be connected to next door with their PME giving 0.15?

I would be looking for 20, 30 60 Ohms or more, more than 100 times your reading.

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Vince

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 21 April 2014 07:25 PM
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Fm

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All the rods we use are threaded to allow a longer rod, why would it be unlikely?
 21 April 2014 07:25 PM
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Fm

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All the rods we use are threaded to allow a longer rod, why would it be unlikely?
 21 April 2014 07:27 PM
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ryespark

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Hi Vince,

I could visually see the length main earthing cable from the MET right to the top of the earth rod. Am going to pop back to re test with another meter!.
Sorry but have no experience of TT systems so was just looking for some advice, with re check everything.

And report back

Many thanks

-------------------------
Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 07:29 PM
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ryespark

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Fm have never installed a earth rod before, so didn't realise they threaded together, so could be longer in that case, but sounds like it still would justify my reading of 0.15 ohms i have?

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Ryan Andrews

http://www.electrician-hampton.co.uk/
 21 April 2014 07:39 PM
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hifly

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if you still get the same readings do a test between the rod and the water bond, there is always the chance they hit the water pipe with the rod.

by the way if it was TT it should already have an upfront RCD or rcd's covering every circuit in some way or another.

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Vince

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 21 April 2014 08:19 PM
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mikejumper

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Originally posted by: hifly
I would be looking for 20, 30 60 Ohms or more, more than 100 times your reading.

The proximity of water does seem to make a significant difference.

The lowest Ze on a TT system I've ever had was on a damp October day at a detached house which had streams on two sides of the garden.
The reading was 4.9 Ohms.

I agree with others though; 0.15 just seems too low.
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