IET logo
 
IET
Decrease font size
Increase font size
Topic Title: Calculating Zs using r2, not R1+R2
Topic Summary:
Created On: 30 November 2012 12:00 PM
Status: Post and Reply
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 30 November 2012 12:00 PM
User is offline View Users Profile Print this message


Avatar for CMElectrical.
CMElectrical

Posts: 272
Joined: 20 April 2010

Hi all,

As the title states....

How does one calculate it?

Thank you

-------------------------
Regards
Carl.
 30 November 2012 12:06 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11374
Joined: 13 August 2003

I don't see how you can, not properly.

(You can prove a degree of safety using R2 alone - the old 'alternative method' - ensures that the voltage between MET and point doesn't exceed 50V if the fault current isn't sufficient to achieve ADS - still exists to some extent in reg 411.3.2.6 with the c.p.c. acting as a supp bond - but not the usual approach these days).

I suppose you could use R2 to have a guess at cable length and lookup the corresponding R1, but you'd normally have an idea of cable length anyway so could just lookup (per m) R1+R2, and that doesn't really prove that R1 is free of high resistance joints.

- Andy.
 30 November 2012 12:23 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19593
Joined: 23 March 2004

You could calculate it, subject to a few assumptions as Andy highlighted - one quick way (in an all insulated installation) would be to multiply R2 by the ratio of R1:R2 to give you a design R1 and R2 from which the addition of a known or assumed Zs and correction for temperature would give you a design value under stated or conventional conditions.

From a measured R2 in an instalation with metallic containment or multiple circuits enetring common conductive enclosures you have no chance without knowing the arrangements of R1 (or possibly of Rn, again assuming you don't have reduced neutrals present)

Regards

OMS

-------------------------
Failure is always an option
 30 November 2012 12:28 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11374
Joined: 13 August 2003

and that doesn't really prove that R1 is free of high resistance joints.

and assumes that R1 is the same length as R2 - which for some circuits - e.g. lighting (due to switch drops) - it usually isn't.
- Andy.
 01 December 2012 12:42 PM
User is offline View Users Profile Print this message



MrP

Posts: 836
Joined: 24 March 2006

Carl

I think in the 16th edition yellow book and pre there was a table for max R2 corresponding to CPD it's obviously not in the latest edition
I'm not sure if the latest edition schedule of test results gives the alternative R1+R2 or R2 method being recorded
Someone with a copy of the book will put us right on that
The table was probably removed when the powers that be realized it didn't work generally using modern installation practice

MrP raining at the moment, going home in 18days
 02 December 2012 04:56 PM
User is offline View Users Profile Print this message


Avatar for CMElectrical.
CMElectrical

Posts: 272
Joined: 20 April 2010

Hi all,

Thanks a lot for the replies, it was just a thought and proves impractical in reality

Hope you had a great w/e

-------------------------
Regards
Carl.
 02 December 2012 06:02 PM
User is offline View Users Profile Print this message



L4V15

Posts: 5
Joined: 22 November 2012

As said above if live conductor is the same csa and length as the CPC you could just double the R2 to give you roughly r1+r2. If it's a 2.5 twin and earth you could divide the R2 value by 1.67 to give you an approx R1 value. Then add together again. Then add the Ze. I suppose it would only really be practical for straightforward radial circuits (not lighting). Easier to just r1+r2 though. As said you're only really supposed to calculate Zs if you have measured r1+r2.

You can still use R2 only in the latest schedule of test results. Not often used though.
 03 December 2012 06:47 AM
User is offline View Users Profile Print this message


Avatar for sparkiemike.
sparkiemike

Posts: 1537
Joined: 24 January 2008

Originally posted by: L4V15

You can still use R2 only in the latest schedule of test results. Not often used though.


Not as a method for calculating Zs, you could use the R2 column to demonstrate "continuity of protective conductors". You would need to test for polarity and you would still need to calculate or measure earth fault loop impedance (Zs).
 03 December 2012 06:55 AM
User is offline View Users Profile Print this message


Avatar for sparkiemike.
sparkiemike

Posts: 1537
Joined: 24 January 2008

Originally posted by: MrP

I think in the 16th edition yellow book and pre there was a table for max R2 corresponding to CPD it's obviously not in the latest edition


Also in the brown book, reg 413-02-12 and Table 41C
 03 December 2012 10:00 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 11374
Joined: 13 August 2003

Originally posted by: sparkiemike

Originally posted by: MrP



I think in the 16th edition yellow book and pre there was a table for max R2 corresponding to CPD it's obviously not in the latest edition





Also in the brown book, reg 413-02-12 and Table 41C


yup - that was the 'old alternative method' I mentioned.
- Andy.
 27 May 2013 12:26 PM
User is offline View Users Profile Print this message



Zs

Posts: 2856
Joined: 20 July 2006

Good afternoon,

I've dug this thread up from the archive.

Yesterday I was testing an office in the city. As Andy pointed out earlier it is unusual but I am an R2 and Zs measurer and I don't calculate very often. I'm not desperately keen on the fiddling and linking involved in R1+R2 testing on existing installations.

I had a ream of Zs results, tempting to calculate and go home but I thought better to stick to my usual method. R2 wander lead.

One area of the office (desks) alerted me to an issue when the megger refused to test. The screen popped up 'O-L volts' It was in fact 20Volts. I then went on to find that the lighting above the area would similarly not test and registered 10.2 Volts in the metal ceiling tiles (megger users will know that's just above the annoying <10 volts which the megger uses as a zero volts point).

So, they have something to look into there. Had I calculated I'd not have known about that.

The desks are not bonded BTW.

Zs
 28 May 2013 11:25 AM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19593
Joined: 23 March 2004

I had a ream of Zs results, tempting to calculate and go home but I thought better to stick to my usual method. R2 wander lead


A tip for you - do R2 first then Zs - that way if the R2 shows up as alarming, then you don't proceed with Zs - with a poor or no earth, Zs isn't sensible from a safety perspective - and with a poor or no earth, any measured Zs you get is a pointless number.

High protective conductor currents ?

Regards

OMS

-------------------------
Failure is always an option
 28 May 2013 12:08 PM
User is offline View Users Profile Print this message



Zs

Posts: 2856
Joined: 20 July 2006

Normally yes, CIP as they taught at college and I thought someone might say that. This was a case of turning up and finding an office full of people, including the CEO, who didn't get the shut down message, so last weekend I did what I could with half the floor empty, one riser cupboard and a 58 minute UPS. Sometimes strategy has to change. Low current before anyone asks. I test down the CPC.

My point though, is that unless >50Volts a Zs test does not alert to the issue and whilst 50V is a given parameter for two legs, I want to know about lesser amounts flowing in my customers' circuits and be able to advise them accordingly. So I remain an advocate of measuring and not calculating.

D - 7 hours.....
Zs
 28 May 2013 12:38 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19593
Joined: 23 March 2004

Originally posted by: Zs

Normally yes, CIP as they taught at college and I thought someone might say that. This was a case of turning up and finding an office full of people, including the CEO, who didn't get the shut down message, so last weekend I did what I could with half the floor empty, one riser cupboard and a 58 minute UPS. Sometimes strategy has to change. Low current before anyone asks. I test down the CPC.

I see - apologies if I'm misreading this, but any chance what you were Zs testing is fed via the UPS - or had loads of "leaky" loads like PC's connected.


My point though, is that unless >50Volts a Zs test does not alert to the issue and whilst 50V is a given parameter for two legs, I want to know about lesser amounts flowing in my customers' circuits and be able to advise them accordingly. So I remain an advocate of measuring and not calculating.

For sure - I wasn't suggesting anything else - just a comment on the "order" of tests and what can sometimes be an exercise in futility - and of course the benefit od a "dead" R2 test compared to a live R1 + R2 test.

Not sure I can put my hand to it now, but somewhere I've got an oscilloscope trace of a circuit feeding a bank of PC's capturing the before, during and after of a low and high current EFLI test - if I can turn it up, I send you a copy



D - 7 hours.....

and counting - On Every Street !

Zs


OMS

-------------------------
Failure is always an option
 28 May 2013 01:58 PM
User is offline View Users Profile Print this message



MrP

Posts: 836
Joined: 24 March 2006

Hi Zs
If its any help wasn't till I read OMS post that I got that been there

In a previous life I was involved with the I&T of job centre's/dole office length and breadth of the country

Glasgow dole office all test were conducted at night the lights were done first in sections with rows and rows of flus on lighting trunking the lighting was I&T in sections giving us background light to perform the task what we found was that because the lighting set up was long lines of metal lighting trunking a voltage was induced across to any adjacent dead circuit including power poles that terminated at the desk from the suspended ceiling.

MrP
 28 May 2013 02:07 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19593
Joined: 23 March 2004

In a previous life I was involved with the I&T of job centre's/dole office length and breadth of the country


LoL- JobcentrePlus, please - we don't call it the dole - now what a project that was - what with the Design Manual, power poles, the forum area lighting, DWP Wembley, L&H, RWC's, LST and Dalkia it was enough to drive a sane man crazy at times.

And those bloody gonk things stuck in top of the computer screens in the back of house offices - now what was that all about ?

regards

OMS

-------------------------
Failure is always an option
 28 May 2013 03:09 PM
User is offline View Users Profile Print this message



Zs

Posts: 2856
Joined: 20 July 2006

Could be. Could well be. Thank you. Curious that it is only in one part of the office though. I like it a lot though. Even in the lights?

Gonk? that'll be me then. This afternoon I have lost the entire weekend's work on the laptop. You can edit in protected view but you can't save. It doesn't tell you that though, it just lets you hit save and carry on. Been working on a document that I emailed to myself. Mad as Jack Mc Mad with myself.

Gonk Zs
 28 May 2013 04:08 PM
User is online View Users Profile Print this message


Avatar for OMS.
OMS

Posts: 19593
Joined: 23 March 2004

Gonk? that'll be me then


LoL - I doubt it - these people had literally dozens of the things surrounding the PC screens - and virtually every desk had them - along with a few nutters who use to collect the little labels stuck to apples and they'd range those around the PC screen as well.

You've never seen the public sector in operation until you've seen Back of House in a JobcentrePlus - frightening, truly frightening

Could be. Could well be. Thank you. Curious that it is only in one part of the office though. I like it a lot though. Even in the lights?


That's the trouble with electronic testers (of many kinds) - the high internal impedance can't pull down any spurious voltages present from leakage, inductance and the like - on a "noisy" circuit it gets a bit confused

regards

OMS

-------------------------
Failure is always an option
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.