Zs

[Sparky Ninja]

Hi Ringer,

Cheers for your feed back, the actual question says, Explain , briefly the most likely reason why the actual test result (0.51) would normally be less then the value calculated (which i we worked out to be 1.09 )

Does a dead test R1+R2 reading (measured have parrell paths which might make the reading lower?

Regards

Locket

Parallel paths..

next question?

 

[ringer]

If the ambient temp at the time of the reading is less than 20C then that may be another reason for the actual being less than the calculated value. Though with the actual being roughly half of the calculated, suspect there may be another reason. Don't have enough practical testing experience myself to be able to answer this any better.

 

[snowplough]

Hi Widdler,
Itake it you think its parrell paths when you do adead test R1+R2 reading with the low ohm meter, it was just a guess as i thought you only got parrell paths when you did the Zs measured reading with the loop imp metre?

Kind Regards

Locket

 

[ringer]

If the question was describe 'briefly' why, then yes, parallel paths would seem to be the answer. But what sort of parallel paths would you find in a lighting circuit when doing R1+R2, because you have disconnected from the MET, unless it is referring to the commoned cpc at all points within that circuit?

 

[Sparky Ninja]

It could be many things.. depends on wiring systems, circuit arrangements and if it has been installed effectively.

 

[Sparky Ninja]

It might help if you could provide the question in full detail?

 

[snowplough]

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

 

[pushrod]

ah just re-read and seen workshop lighting mentioned - could this mean it is in steel conduit and that is your parallel path? Also can't remember was it in singles which would also make you think conduit.

 

[snowplough]

I did earlier i think

 

[ringer]

It might help if you could provide the question in full detail?

^^^^What he said^^^^

 

[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.