TT Testing

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sparky999

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Evening guys

Testing a rewire we have done and the earthing for this job is a TT system.

I have done all the tests on circuits in the house theses are all fine.

My question is how do i find ZE is it just like normal testing of this by removing Earth and going across the phase conductors. I performed this test on the installation and got 6.69 ohms not sure if this is the reading for ZE or not.

At are firm we havent got an earth electrode tester so how do i find the reading of Ra to put on the test sheet.

Sorry if its simply but never done a test on this installation as the work we do is that there is an earth provided so i thought id ask all u intelligent gents hehe:D

 

sparky999

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Yeah i looked through that today at work and then i did the test with a three lead loop impedance multi function tester and got 6.69 ohms is this acceptable. i did a zs and this was givin 7 ohms on a 32a b type socket circuit. Some advice needed.

 

paulh

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how does testing across phase conductors give ze? ze is the resistance of the phase conductor and the earth return not phase conductors. To obtain ze is no different to any other installation with an efli tester. the reading you have is the sum of R1 and Rn.

 

sparky999

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yeah i know, sorry i never said put the croc clip on the earth which i removed then on to phase conductors.

 

Admin1

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What tester you got Mate? Are you doing a two or three probe test?

 

sparky999

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Kewtech 16th edition multi function tester. You need a three probe test for Ze on this tester.

 

Admin1

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switch power off, remove earth from the Met, Use a Croc Clip (Is easier) and use your two probes on the Phase and Neutral, SUPPLIERS Side. do test, record result, re-connect your earth and re-energise.

:)

 

sparky999

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i just said all tht above. I know how to test Ze. Its just on a TT system im a little bambuzzled. ive done ZE on it and gettin 6.69 which surely aint right.

 

seajayess

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Not sure what test papers you use we use NICEIC installation certificates, as posted before remove main earth conductor from MET and do a Ze test as any other installation, record measured result as Ra and it is recorded as test method 2, hope this helps CJS

 

sparky999

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cheers CJS yeh NICEIC sheets we use so ill put my reading of 6.69 in Ra so what goes in the Ze the same reading for Ra.

 
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i just said all tht above. I know how to test Ze. Its just on a TT system im a little bambuzzled. ive done ZE on it and gettin 6.69 which surely aint right.
WHY?

How far away is your supply transformer?

Have you got Guidance Note 3?

Pg 48.Test Method 2 (alternate method for RCD protected TT installations)

A loop impedance tester is connected between the line conductor at the source of the TT installation and the earth electrode, and a test is performed. The impedance reading taken is treated as the electrode resistance

BS7671 requires Ra In <=50V.
That tis what the book says... (the big orangey one, not the Red One! :) :D )

 

sparky999

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WHY?How far away is your supply transformer?
I dont know?

yes i have one thats y i said i looked through it 2day.

i believe it to be yellow not orangy:D

 

sparky999

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haha so special is 6.69 acceptable then?

Sorry if im sounding thick its the 1st time i have tested a tt system.

 

seajayess

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That reading for a TT is very low , unless you are very close to the transformer, I would make sure you are testing the Main Earth Conductor and not a bonding conductor, I have had this myself, switch of the main switch disconnect the Main Earth conductor and using a long lead confirm it is the Main Earthing Conductor, if you are not close to the transformer I would say you have a parallel path somewhere, but if not there is nothing wrong with the reading you are getting, it is below 200 ohms just seems low CJS

 

sparky999

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Yeah defo is main earth conductor followed it around in the floor boards and outside, o right thats fine then cheers.

 

Chris_81

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so what goes in the Ze the same reading for Ra.
I've just been loking at the form wondering that, although my Ra is astronomic compared to yours so won't be recording it just yet!!

 

sparky999

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So what happens with my Zs readings if i were to use Zs = Ze + (r1+r2) i would get 7.12 for one of my ring circuits which is over the values of a b type.

 

seajayess

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This is why you have to use an RCD for additional protection on TT systems CJS

 

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