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Chris_81

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Could be more possible stupid questions here but got plenty of help off here so far so thought I'd carry on!!

Managed to sort out the high Ra reading I was getting. Done all the tests on the garage, just want to check they all read ok. (This is the first install I've tested on my own that isn't where I did my course, and that was a while ago!)

Install consists of 8 double sockets and 3 fused spurs on a 32amp mcb and 7 flourescent lamps on a 6amp mcb.

Ra 128 ohms

Prospective Fault current 1.05 kA

Ring:

R1-0.4,

Rn-0.3,

R2-0.6.

R1+R2-0.3

IR p/n-191, p/e-213, n/e-132

Zs-0.44

Lighting:

R1+R2-0.8

IR p/n-570, p/e-321, n/e-144

Zs-1.05

Main ones I'm not sure about are the IR readings as they seem low to me and the Zs reading, although I've been told these can vary greatly because of it being a TT system.

Thanks in advance!!

 

Chris_81

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Yeah it's a detatched garage with running water so had to use an earth rod and convert it to TT as I couldn't export the earth from the house.

 

Chris_81

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Yep, noted on the certificate and correctly labelled. :)

 

extension15

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Just as a matter of interest, did you measure Ze, without the bonding?

:D

 

Chris_81

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No, just removed the main earth and tested to get the Ra, didn't think there was a Ze measurement as such on a TT system? Could be wrong tho, its the first TT I've had to work on...and hopefully the last!!

 

Admin1

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Ra is the Ze on a TT. Correct, Gentlemen (and Ladies)?

 

Chris_81

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You and me both! :)

I've put the Ra result where it asks for it on the form, so I'm guessing I put the same in the Ze mesurement box....

 

extension15

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No, just removed the main earth and tested to get the Ra, didn't think there was a Ze measurement as such on a TT system? Could be wrong tho, its the first TT I've had to work on...and hopefully the last!!
7.2.6 ii OSG

:)

 

extension15

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Chris 81......

I've only ever worked on 1 TT system, and that was a leaky Microwave on the ring...

:^O

 

Chris_81

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7.2.6 ii OSG :)
Blimey, took me longer to go and get the book and find the page than it did to read the one line!! :^O

Leaky microwave on the ring.... sorry you've lost me! ?:|

 

Admin1

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Leaky microwave on the ring.... sorry you've lost me! ?:|
I trust he is referring to the ONLY time that he has dealt with a TT installation?

 

extension15

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Blimey, took me longer to go and get the book and find the page than it did to read the one line!! :^O Leaky microwave on the ring.... sorry you've lost me! ?:|
Got called out to a job, RCD tripping, it was my first TT installation..

(Faulty Microwave)

:^O

 

Chris_81

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Ah I see, sorry been one of those days, think I left my brain on the pillow when I got up this morning!! Blushing

 

cheggers

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Could be more possible stupid questions here but got plenty of help off here so far so thought I'd carry on!!Managed to sort out the high Ra reading I was getting. Done all the tests on the garage, just want to check they all read ok. (This is the first install I've tested on my own that isn't where I did my course, and that was a while ago!)

Install consists of 8 double sockets and 3 fused spurs on a 32amp mcb and 7 flourescent lamps on a 6amp mcb.

Ra 128 ohms

Prospective Fault current 1.05 kA

Ring:

R1-0.4,

Rn-0.3,

R2-0.6.

R1+R2-0.3

IR p/n-191, p/e-213, n/e-132

Zs-0.44

Lighting:

R1+R2-0.8

IR p/n-570, p/e-321, n/e-144

Zs-1.05

Main ones I'm not sure about are the IR readings as they seem low to me and the Zs reading, although I've been told these can vary greatly because of it being a TT system.

Thanks in advance!!
Your r1 and rn should be the same reading or at least no greater than 0.05 difference you may have a loose connection some where

your ir reading seem low for a new install is there damp or neon lamps in the spurs

 

Chris_81

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I'm gonna check the connections tomorrow and re-do the R1 and Rn readings. As for the IR there are neons in two of the spurs, I was always told to just make sure they were switched off and they wouldn't effect it. I agree with you about the readings seeming low, did to me too, was a very wet night before i tested which may have effected it. I'll see how it is tomorrow...

Only other thing with the IR readings is they may well go higher but the reading starts low and slowly counts up, those readings were taken after 30 secs, as I was told that was aceptable, but I'm sure they'll go higher if I left it longer. Not entirely sure how longs best, when its counting up in units of around 5 the maximum reading of 1000 is gonna take ages...

 
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Could be more possible stupid questions here but got plenty of help off here so far so thought I'd carry on!!Managed to sort out the high Ra reading I was getting. Done all the tests on the garage, just want to check they all read ok. (This is the first install I've tested on my own that isn't where I did my course, and that was a while ago!)

Install consists of 8 double sockets and 3 fused spurs on a 32amp mcb and 7 flourescent lamps on a 6amp mcb.

Ra 128 ohms

Prospective Fault current 1.05 kA

Ring:

R1-0.4,

Rn-0.3,

R2-0.6.

R1+R2-0.3

IR p/n-191, p/e-213, n/e-132

Zs-0.44

Lighting:

R1+R2-0.8

IR p/n-570, p/e-321, n/e-144

Zs-1.05

Main ones I'm not sure about are the IR readings as they seem low to me and the Zs reading, although I've been told these can vary greatly because of it being a TT system.

Thanks in advance!!
Hi Chris..

an additional point I was told with your R1+R2 and ring continuity values, is do a re-check calculating back to see if the readings tie in with the actual cable lengths used.

e.g. you could have done a circuit with 10m of cable got a reading that is within the max Zs values but the r1+r2 would imply the cable should be much longer? which indicates a bad connection somewhere!

for example in your case.. The Ring:

table 9A OSG pg 166

2.5mm = 7.41miliohm/metre.

1.5 = 12.10miliohm/metre

R1-0.4, would imply approx 54m of cable? ((0.4/7.41)*1000)

Rn-0.3, would imply approx 40.5m of cable? ((0.3/7.41)*1000)

R2-0.6. would imply approx 49.5m of cable? ((0.6/12.10)*1000)

personally I would expect the tolerances to be a bit closer?

but that may be your meter probe/croc clip connections?

That is assuming the cables are around 40m - 55m in length?

If you have only used 25m then somethings out?

or if you have used 70+m somethings is out?

R1+R2 on a ring will be equal to

(r1+r2/4) (0.4+0.6)/4 = 0.25 you got 0.3! 0.05 difference?

Not far off!

I would agree the ins res values seem low for new wiring?

but may be a quirk of your meter?

Have you tried doing an ins res on a brand new reel of cable, to see how your meter performs?

If it is loads quicker to stabilise there may be a problem with the installation.

The light circuit would be

R1+R2-0.8

If 1.0mm (0.8/36.2)*1000= 22m?

If 1.5mm (0.8/30.2)*1000= 26.5m?

not sure if that was any help?? ?:| :eek: :p

 

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