Zs

[snowplough]

Hi pushrod,
It says in the description box workshop lighting

Regards

LOCKET

 

[ringer]

I am a bit confused - are you asked to calculate the expected value, or are you asked to explain the reasons why the measured value will be less than the calculated value?
Bear in mind that R1 ( the brown line conductor) will be 45 metres long as it goes to light fittings, then down to switches and back, before it finally reaches the light itself. The R2 (cpc) will be shorter than this because, although cpc will be run to the switches, it will be commoned at the light fitting and so will be effectively shorter. That may be one reason. Another reason may be that the actual length, rather than the calculated length, is less than 45m. Am not entirely sure that either of these is the answer they may be alluding to, but in any case, if the actual reading is less than the designed reading, this is usually (though not always, think breaking capacity) the desirable result.

 

[snowplough]

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

 

[JulianC]

Are we saying the measured R1+R2 is considerably less than the calculated value? In which case perhaps we haven't taken a reading at the extremity of the circuit?

 

[snowplough]

Hi julian c,

The measured value is 0.51ohms , the calculated value we have worked out is 1.09 ohms, i dont think its a trick question as it says normally expect to be ie measured less then calculated ,thats saying there is no other figures i should have included in my calcs

Regards

locket

 

[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^^^^