R1+R2 - two way lighting - 2.7 ohms, can i get your opinions please?

[srl-8]

Hi All,

In preparation for a pending assessment, I have installed a new 2 way CU in my garage with all new wiring (my own property), and TT the earth. (House supply is TNC-S, PME).

I am testing the lighting circuit. (6a mcb / 2 switches / 2 way/ 1.5/1.0 cable . Have tested in all switch positions. Polarity is correct.

I am getting Ra/ZE value of 145 ohms.
I am getting measured ZS of 147 ohms at the last light

These results suggested I should get a R1 +R2 of 2 ohms.

I am getting a result of 2.7 ohms.


Being new to testing, does this R1 + R2 result seem acceptable to you ?

I would value the comments from people on here.

Regards

 

[TJ Anderson]

Hi All,

In preparation for a pending assessment, I have installed a new 2 way CU in my garage with all new wiring (my own property), and TT the earth. (House supply is TNC-S, PME).

I am testing the lighting circuit. (6a mcb / 2 switches / 2 way/ 1.5/1.0 cable . Have tested in all switch positions. Polarity is correct.

I am getting Ra/ZE value of 145 ohms.
I am getting measured ZS of 147 ohms at the last light

These results suggested I should get a R1 +R2 of 2 ohms.

I am getting a result of 2.7 ohms.


Being new to testing, does this R1 + R2 result seem acceptable to you ?

I would value the comments from people on here.

Regards

You have calculated R1 & R2 by Zs-Zdb (Ze would be at PME cutout). What was your dead test R1 + R2 for lighting circuit? Also do you have an RCD Main Switch in Garage DB as they can add quite a bit to your reading? Did you test both Zdb and Zs on three lead (No Trip) test?

 

[Plonker 3]

Zs-(R1+R2) will always be higher than a Zdb,Ra or Ze due to parralel paths. Think you need to use the tables in GN3 and work what your R1+R2 should be and see if they are satisfactory.

 

[telectrix]

i can't see why you can accept that TT Ra value when you have a perfectly good TNC-S earthing system. is there any valid reason why you've not used it?

 

[srl-8]

Measured R1 +R2 was 2.7 (highest reading I got, from all switch positions.
ZE at garage board (earth electrode/TT) 143 ohms
Measured ZS = 147 ohms

Yes RCD, 30ma main switch.

Does this result look correct ?

 

[TJ Anderson]

i can't see why you can accept that TT Ra value when you have a perfectly good TNC-S earthing system. is there any valid reason why you've not used it?

Oh no, that old chestnut!

 

[SJD]

I am getting Ra/ZE value of 145 ohms.
I am getting measured ZS of 147 ohms at the last light
These results suggested I should get a R1 +R2 of 2 ohms.
I am getting a result of 2.7 ohms.
Being new to testing, does this R1 + R2 result seem acceptable to you ?

If I understand correctly, you first measured R1+R2 (before any live tests) and got a value of 2.7 Ohm?
If so, and then subsequently measured Zs on this circuit as 147 Ohm, with a Ze of 145 Ohm, I'd say at these values, the accuracy is limited by the accuracy of the tester (you are not going to measure exactly 147.7 Ohm, the MFT is just not that accurate).

 

[telectrix]

the results looks about right, but see my last post. a 145 ohm Zdb is not, in my eyes, acceptable. what have you used for a rod, a coat hanger?

 

[TJ Anderson]

Measured R1 +R2 was 2.7 (highest reading I got, from all switch positions.
ZE at garage board (earth electrode/TT) 143 ohms
Measured ZS = 147 ohms

Yes RCD, 30ma main switch.

Does this result look correct ?

No, for a single garage lighting circuit 2.7 Ohms R1 + R2 is way to high. <0.50 Ohms would be more like it. Something is amiss there.

 

[telectrix]

switch contact resistance? some switches are diabolical.

 

[srl-8]

teltrix,

There is no parallel path to my knowledge. The garage is supplied via a 2 core swa. Because the House is TNC-S, I was advised not to export the earth in case of a netrual/earth fault.

So have TT the garage. Does that make sense.

(UPDATE: The garage was originally earthed to a dis-used copper pipe, disappearing in the floor. )

 

[trev]

teltrix,

There is no parallel path to my knowledge. The garage is supplied via a 2 core swa. Because the House is TNC-S, I was advised not to export the earth in case of a netrual/earth fault.

So have TT the garage. Does that make sense.

(UPDATE: The garage was originally earthed to a dis-used copper pipe, disappearing in the floor. )

Not really, no

 

[Leesparkykent]

export the earth?

 

[srl-8]

No, for a single garage lighting circuit 2.7 Ohms R1 + R2 is way to high. <0.50 Ohms would be more like it. Something is amiss there.

TJ, Even though this is a 2 way switch, so will measure the 3 core cable aswell.... ?

 

[trev]

TJ, Even though this is a 2 way switch, so will measure the 3 core cable aswell.... ?

How long is the 3c?