Zs

[snowplough]

Hi Pushrod,
Ithink you might have nailed it, sorry i havnt given the full drawn scenario , but it does say in the workshops the single cables are run in surface mounted steel conduit and trunking, so bearing this in mind ,my question to you is,when doing a dead test R1+R2 with the standard low ohm meter, would you still get parrell paths effecting the reading as you think above giving you a lower reading?

Regards

 

[pushrod]

you would because the conduit would be earthed

 

[snowplough]

Many thanks Pushrod,

ive loads more questions for you as im only a novice but ill start with them another night as i think ive tested you enough tonight and my head is now hurting,

Many Thanks Sincerely

LOcket

 

[Geoffsd]

If I may but in...

To find out if the students can do it the question asked the R1+R2 to be calculated from the length and resistance of the cable. That's understandable as is asking why the calculated value may be higher than the measured.

However, I am a bit confused as to why the measured R1+R2 of 0.51Ω is mentioned at all. The fact that it is so low, comparatively, seems to be confusing everyone but, surely, it is irrelevant and could have been given any value or is there something we don't know - another part of the question?

edit - I may have taken too long typing this.

 

[brucelee]

Hi Pushrod,

I see what your thinking but its on an old 2391 test paper,it gives you a Sched of Test Result sheet with a lighting circuit of 45 n long and an R1 +R2 reding of 0.52 , which must be the dead test reading. The Q. asks you to calculate the maximum expected test result value for the info given in a table showing 1.5 mm cable ,with 12.1 mohms/m value at ambient temp of 20 degrees.

To me its just asking for a like for like value of R1 +R2 cold , not any 1.2 con oper temps etc, so why should the dead test reading be more then the calc value , im confused

Regards

Locket

hi mate the measured value will include parallel paths the calculated is a more accurate value and does not include parallel earth paths
ie your conduit and the main protective bonding conductors will introduce parallel paths so not a accurate measurment

 

[Sparky Ninja]

Hi Pushrod,
Ithink you might have nailed it, sorry i havnt given the full drawn scenario , but it does say in the workshops the single cables are run in surface mounted steel conduit and trunking, so bearing this in mind ,my question to you is,when doing a dead test R1+R2 with the standard low ohm meter, would you still get parrell paths effecting the reading as you think above giving you a lower reading?

Regards

This would have been answered much sooner with this info.

 

[malcolmsanford]

When we do R1+R2 test at college we use a crocodile clip attaching say the appropriate line cable connected with the clip on one end to the cpc busbar (not disconnected appriopriate cpc) but even then where would the parrell paths be even doing it this way ,i dont understand.

locket

Have you asked your tutor why?

Just think about it. by clipping direct onto the earth bar you are connecting to all the other CPC's on that board. If that Earth bar is the installations MET, then you will also have the bonding conductors to the services, may not be gas but certainly water, that will also be on there, so you have all those parallel paths.

 

[Sparky Ninja]

I agree with malcolmsanford 100%.

Your tutor is either lazy or doesn't understand the tests fully.

 

[snowplough]

Many thanks Malcolmsfield,

If doing R1+R2 test as i said i do at college and it also shows you to do in O.S.G ,G.N.3 etc , this then obviously decreases the resistance due to parrallel paths etc as you explained , which then asks me the question is this then the true R1 +R2 reading?,
and so with or without earthed steel conduit as you said , there would still be parrallel paths.

Kind Regards

Locket

 

[ringer]

No, you should disconnect the cpc from the earth bar when doing R1+R2, the parallel paths are coming from the (earthed) steel conduit.

 

[malcolmsanford]

R1 + R2 is a multi layered test, the only true value is done on design. as a designer you will design a circuit and to ensure several pieces of criteria you do a calculated R1+R2 to confirm the cable.

On an EIC say a rewire, most electricians will before they start connecting up the CU, do IR tests and also the R1+R2 test, this should give you a very good indication of condition as the R1 + R2 should be very close to a calulated one. But of course you may think you have installed 25mteres of cable, but in fact you have installed 31 metres, so the measured and the calculated one will be close but never the same. You also have to take into account quality of conductor and if doing it via a socket then even that.

On a PIR then I'm afraid your in the lap of the gods. it can depend on so many factors. You may not be able to disconnect the supply so you only do a R2 test. The earths in the CU are not sequencial and so you have to clip the bar as you did in your test, and then you get those parallel paths, but that is what you put on your schedule of Results. In many cases you will not have diagrams. let alone design charts to compare so you just have to guestimate a lot.

 

[snowplough]

Hi ringer,

Thanks for repling,

So ,if there wasnt any steel conduit etc ,just normal pvc cable connected to the earth bar and you did the R1+R2 test as with croc clip attched to earth bar and line of theselected circuit, then did the standard test at extremity of that circuit ,would there be any parrel paths from anywhere effecting the reading?

Kind Regards

Locket

 

[malcolmsanford]

Have you asked your tutor why?

Just think about it. by clipping direct onto the earth bar you are connecting to all the other CPC's on that board. If that Earth bar is the installations MET, then you will also have the bonding conductors to the services, may not be gas but certainly water, that will also be on there, so you have all those parallel paths.

Yes as your conduit seems to have CPC installed and your bonding conductors as in post #37.

 

[snowplough]

Hi Malcolmsfield,

Thanks for replying, you mentioned might not be able to disconnect the supply , not qiute sure how you would conduct the the test with live supply? R2 only you mentioned on the earthed bar.

Many Thanks

Locket

 

[malcolmsanford]

On a PIR there maybe times in a factory where you can not isolate a supply, you of course try to arrange a time, ie out of hours, early morning at night or weekends, but no it can not be switch off.

Then on your schedule of results you will see a R1 + R2 box and next to it you will see a R2 box. What you do is clip one end of a long wander lead to your earth bar, and if it's a light circuit say, you connect the other end of you lead into you MFT and the other side of the meter.after you have either nulled it or taken the leads resistance, start taking measurements on the light fittings exposed conductive parts, once you got your highest reading that will be your R2 reading. The chances are that a switch will be your R2 reading, what this test does is confirm the continuity of the protective conductor and is Test Method 2

 

[Sparky Ninja]

This threads never gonna end



You will notice that the illustrations in the OSG & GN3 have only one circuit in the consumer unit, hence no parallel paths, hence no need to illustrate it disconnected. They also appear to be thermoplastic multicore cables and not steel conduit or any other potentially parallel path providing wiring systems.....

 

[snowplough]

Thanks for replying widdler,

I was wondering that and you have just confirmed it with the illustration,

I never thought that you could get parrallel paths effecting your reading when you do the Dead test R1+R2 (method one) connecting to the earth bar, and i thought it was just when you had metal conduit etc protecting the cable .
I wonder why we are not told at college then to discoonect cpc from earth bar when doing metod one?.

Many thanks for your feed back ,i must speak to my tutor after the holidays

regards

Locket

 

[pushrod]

If you are testing in the workshop and you can see you just have T&E everywhere, then putting a little test lead on to the earth bar or clipping onto the earth bar can save you quite a bit of time disconnecting and reconnecting, especially if you are in some form of timed assessment.

 

[snowplough]

Hi Pushrod,

I appreciate that but ii would have never thought i could be getting incorrect readings and i would have tesred like that in real life, because surely you would always disconnect to be on safe side and just test cpc disconnected knowing nothing else can encourage your readings.

All this gives me more questions then answers, It might be an incorrect R1+R2 reading on a all, but if it helps to keep the Total Zs reading lower, hence keeping within disconnection times to prevent electric shock ,does it really matter?

Thanks for your constant feedback

Locket

 

[pushrod]

Sorry locket have read that a couple of time times but not really sure what you're saying.

 

[snowplough]

Sorry push should read what i write,

I started off saying that i would never have thought you can obtain parrallel path readings by using the test method two with croc clip on earth bar on a R1+R2 dead test, which would give you a lower reading .
Then i was thinking well it might be an incorrect R1+R2 measured reading by not disconnecting the cpc from earth at test, but does it really matter when overall, it can only contribute to a lower Zs reading, which means disconnection times can be met, lowering the electric shock risk.

Many Thanks

lOCKET

 

[pushrod]

Got you now.

R1+R2 readings are fundamental when designing and installing. Checking them allows you to compare to design/check quality of installation. If you are doing an R1+R2 you would want it to be accurate.

When you have an existing installation ie you are doing a minor works or a like for like replacement/repair that is where the emphasis moves more to your Zs reading.

 

[snowplough]

Thanks pushrod for your input,

Ive loads more questions ,but ill save them for another day

akaind Regards

Locket