r2 testing

[just add water sparks]

Please forgive my lack of knowledge on this subject.Normally when i have done pir tests i have used the r1+r2 method . To obtain the Zs you obviously use the formula Ze +(r1 +r2 )=Zs example Ze =0.02 r1+r2=0.46 therefore Zs=0.48 I have just been using the R2 method and in some circumstances found this far easier as can be used at high level with just a pole and saves time using scaffold.Is the method of finding the Zs the same i.e Ze + r2 = Zs.I Like the idea the r1 +r2 confirms the phase and c.p.c alsther testers tell me they prefer r2 testing is better taking into account safety of long lead and don't bother with confirmation of c.p.c and live as this is done with the earth loop tester anyway.Could someone confirm how to compare an r2 result to any other figure to see it is acceptable and just to see who prefers the best methods and why.

 

[uksel]

Please forgive my lack of knowledge on this subject.Normally when i have done pir tests i have used the r1+r2 method . To obtain the Zs you obviously use the formula Ze +(r1 +r2 )=Zs example Ze =0.02 r1+r2=0.46 therefore Zs=0.48 I have just been using the R2 method and in some circumstances found this far easier as can be used at high level with just a pole and saves time using scaffold.Is the method of finding the Zs the same i.e Ze + r2 = Zs.I Like the idea the r1 +r2 confirms the phase and c.p.c alsther testers tell me they prefer r2 testing is better taking into account safety of long lead and don't bother with confirmation of c.p.c and live as this is done with the earth loop tester anyway.Could someone confirm how to compare an r2 result to any other figure to see it is acceptable and just to see who prefers the best methods and why.

sorry mate, you got it all wrong!

Zs is not equal to Ze plus the resistance of the CPC

it needs to include R1 (line) because current must flow through this also, adds resistance to the fault circuit and also

you will need to determine Zs by calculation or measurement, for calculation you need the value of R1.

you can do it by calc provided the conductor sizes are the same, or if using tw+e multiply the measured R2 value of the CPC by 0.6, this would give you an acceptable value for R1


hope that helps

 

[just add water sparks]

thanks i dont think i initially got it WRONG! in my first paragraph i did include the r1 in the formula.IT was because i was using a different method i wanted confirmation i.e the long lead. how do i compare or know this is acceptable if i did not know the r1 Example the r2 was 0.46 if i did not know what the r1 was how do i know if this is acceptable??

 

[uksel]

no it would not be acceptable

The R1+R2 value obtained while testing COPC is only used to deternmine Zs by calc.

you will need to measure Zs directly or calculate Zs using the resistance of Ze, R2 and R1 also

i.e. using your example your value of R2 = 0.46

with the Ze = 0.02

if wired in tw+e the value of resistance of R1 would be

R2 x 0.6 = R1
0.46 x 0.6 = 0.276 (or 0.28)

so your Zs = 0.02 + (0.46+0.28)

therefore Zs = 0.76 ohms

or wired in singles (same csa for line & cpc)

Zs = 0.02 + (0.46+0.46)
therefore Zs = 0.94 ohms

 

[NoSparks]

This is mathematically sound provided the T&E is not 2.5/1.0 (which a lot is) but to my mind totally defeates the object of the exercise which is to detect any faults in the circuit. What if the measured value of R2 was 0.46 and R1 was 1.8. Wiould this be detected? Would the fault be detected? Would the fire be prevented?

Shorly it would be better to measure Zs to be certain there was no fault, then if desired calculate R1 and R2.

If only R2 can be measured, eg no time / facility / money to errect scaffolding then let this be an agreed limitation.

Geoff

I have just re-read the original question and note there is an implication that Zs testing is done seperately and in addition to R2 testing. Personally I cannot see the need for R2 testing if there is a satisfactory Zs test. The Zs test confirms the continuity of both the phase and cpc conductors and also the polarity of the supply. The only other test that should be needed is the phase - nutral loop test which is not even called for in BS7671, since this is what controls the current when there is a short circuit and which causes the MCB to switch off. (I am assuming the fault loop is protected by an RCD and it is the RCD that will switch off if an earth fault develops). The only excuse for not doing a phase - nutral loop test is that the Zs test (assuming no faults) would give at least the same and normally a larger value. -- I would be interested in comments on this theory.

 

[Guest123]

Hi.

As it's a PIR then a limitation can be agreed where you only measure Zs to verify an earth path and not R1+R2 or R2.

I would still measure the continuity of ring final conductors, a blanket IR test (again as a lim) and RCD function.

I wouldn't use the transposed formula of Zs = Ze+(R1+R2) to calculate the R1+R2 as the reading may not be totally accurate due to any parallel paths that may exist.

 

[spadxiii1918]

The "Long lead method" is usually only used where ths circuit is wired in steel enclosures where parrallel paths to the cpc may be present and the R1+R2 value would not be a true value, or whre the cpc resistance is required for use with table 41C of the wiring regs.
If possible always use the R1+R2 method as this proves contnuitity of cpc and correct polarity of the line conductor.

Long lead method should be used to prove continuity od bonding conductors and supplementary bonding conductors.

 

[Guest123]

In addition to my previous post, a lot depends on the condition of the install when you first go to view it......also if the previous reports are available to view and compare with.

I'm a bit of a stickler when it comes to periodics and like to find out as much as I can about an installation so try and keep any limitations to an absolute minimum and will carry out all tetsing.

 

[eskimo39]

So to clarify if I am say for example testing a lighting circuit in a house I can isolate and then join out R1 & R2 at the consumer end, and then take a reading on all lights until I find the highest reading which should be the furthest light. This highest reading can then be added to Ze to give Zs?


If that is the case could I not just do a live test Z loop to find the Zs?

Sorry to be vague but I am now worried my testing method is wrong.

Could someone please clarify what tests need to be done and in what order? Thanks

 

[spadxiii1918]

Your above statement is fine and perfectly acceptable.
it is advisable to do a ELI test at the furthest point, with the cables at hteir mormal operating temperature. Sometimes it is more satisifying to do a live testthat will give a direct reading os Zs to ensure that there are no loose connections or high resistance joints are affecting the circuit. A live test is simple and leaves very little to chance.

 

[Guest123]

I agree with spadxii...it's perfectly ok to measure your R1+R2 and then use this reading to add to your Ze to obtain a Zs measurement.

That said I will always measure Zs also, as has been said you are measuring the cable at it's operating temperature and in normal service conditions.

 

[eskimo39]

Thanks for that. What other tests do you need to do and in what order?

eg:

Ze - Live
Zs - Live
RCD - Live
R1 + R2 - Dead
Insulation Resistance - Dead


Regards

 

[Guest123]

Well technically they could differ between an initial verification and a periodic inspection due to any limitations for example.

This is how I like to do it on an initial verification

1. Continiuity of bonding conductors
2. Continuity of ring final conductors
3. R1+R2 on all circuits, confirms continuity of protective conductors (also should confirm polarity)
4. Insulation resistance
5. Ze/(Ra)
6. PFC
7. Zs on all circuits
8. RCD tests.


Other people may have a different view...be interesting to see.

 

[NoSparks]

The correct order of testng is given in several documents. This list if taken from the OSG.

DEAD TESTING
Continuity of protective conductors
Continuity (including (R1+R2) testing and continuity of ring final circuit conductors)
Insulation Resistance

LIVE TESTING
Polarity
Earth Fault Loop Impedance (Zs)
Functional testing

The OSG goes on to discuss Earth Electrod Resistance measurement, and GN3 places in sequence such testing as floor and wall insulation.

RCD testing is not mentioned in the OSG sequence, but GN3 includes this in Functional Testing.

Of hyper importance is that insulation testing should be the LAST part of Dead Testing to ensure (apart from the obvious) that there is no short circuit between live conductor and CPC (ie no links have been left in place from the (R1+R2) testing) (GN3 Para 2.7.7)

 

[WayneL]

The correct order of testng is given in several documents. This list if taken from the OSG.

DEAD TESTING
Continuity of protective conductors
Continuity (including (R1+R2) testing and continuity of ring final circuit conductors)
Insulation Resistance

LIVE TESTING
Polarity
Earth Fault Loop Impedance (Zs)
Functional testing

The OSG goes on to discuss Earth Electrod Resistance measurement, and GN3 places in sequence such testing as floor and wall insulation.

RCD testing is not mentioned in the OSG sequence, but GN3 includes this in Functional Testing.

Of hyper importance is that insulation testing should be the LAST part of Dead Testing to ensure (apart from the obvious) that there is no short circuit between live conductor and CPC (ie no links have been left in place from the (R1+R2) testing) (GN3 Para 2.7.7)

.
I agree with NoSparks 100% except for one thing -

I would perform Ze and PFC tests immediately after Polarity.

Although Polarity,PFC and Ze are indeed live tests, they are performed before the installation is energised, as the cover will likely still be off the board and the Earthing Conductor will be disconnected for Ze.
If these tests are sattisfactory - cover on, energise and on with the remaining tests.