Zs

[pushrod]

or measured value has not been taken at the true end of the circuit, or the 45 m included switch runs which were not measured when the R1+R2 was meassured at the end of the circuit. - yes i'm scatching around for answers lol

 

[snowplough]

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

 

[pushrod]

Yep , i think i would have to stick with the measured value would not include the full cable ie the lengths used on other switches.

Did it say there was just a single lighting point or can you assume there are several?

 

[ringer]

locket, please be very careful when putting numbers in to calculations. You have misquoted figures more than once throughout this thread. Apart from confusing the situation, you are likely to make mistakes in designing circuits which will potentially make them unsafe. You may think that I am being picky (for instance, 45m of 1.5 is 1.09ohms, not 1.08ohms) but you do need to ensure you get this right. You also seem to be getting confused over which is the larger resistance, the actual (0.52ohms) and the calculated (1.09ohms). This doesn't answer your original question, but it is important.

 

[snowplough]

Thanks Ringer,

Sorry for being incorrect on the calcs ,your quite right, Any how now you no the correct ones have you any ideas

Regards

Locket

 

[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