Zs

[snowplough]

Can any of you guys tell me why when using the 80 per cent rule for Zs , you can take a measurement with the tester get a reading and use the rule of thumb,
Why cant you just use the rule of thumb when your working out the zs of a circuit knowing the Ze and having worked out R1 and R2 by calculation say befor e you put in the circuit , why cant you use the rule of thumb as you would for the measured Zs instead of having to use the cable operating temps etc.

many thanks

Locket

 

[pushrod]

Not quite certain what you are asking, but ... When you are measuring a Zs and you want to know if it will meet the disconnection time for the protective device then you will often use BS 7671 for the max value permissible. However these figures are given at max operating temp and so the readings will be higher than if they were taken at a more usual ambient temp like the ones you have taken. To make a more valid comparison you scale the BS 7671 values down by 0.8 . This is not, however, completely accurate and if you wanted a more accurate value you would have to use a more complex formula. The max values in BS 7671 are accurate figures though.
When designing a circuit the designer should work out R1+R2 at the expected ambient temp. When it is installed it would be checked at that ambient temp and so would be easier to compare with the designer's specification. To start doing rule of thumb conversions would immediately start to introduce inaccuracies and prevent easy checking.

If you use the max Zs values from the OSG then they are scaled to an ambient temp of 10'C

Hope that covers what you meant

 

[Sparky Ninja]

Maximum Zs values are given for design. They accommodate the particular amount of fault current required for them to achieve the relevant disconnection time, and are designed to the operating temperature. Therefore, when designing/selecting circuits, these are the values we must use. We don't apply any rule of thumb rubbish as in real time conditions when this amount of fault current is present to achieve operation of the device, the conductor is at temperature.

The maximum 'measured' Zs is altered as the conductor is not at temperature (i.e. not carrying the fault current) and so the conductor has more conductivity (increase in temp = increase in resistance). This extra conductivity will give an inaccurate reading and so the rule of thumb factor (appendix 14) is applied in the same way as other de-rating factors.

I'm not very sure as to what exactly you are suggesting with the question, but hopefully this makes sense.

 

[snowplough]

Many Thanks,

I will have a look at your answers and try get my head round them .

Many Thanks

Locket

 

[snowplough]

Hi Pushrod, Widdler.

One of the reasons i was asking was because in a mok exam paper, a question asked why is the R1 + R2 value on the sched of test results 0.51ohms , be less then the calculated R1+R2 reading ie circuit length 45metres x 12.1x 2 for a 1.5 mm lighting cable ,
= 24.2x 45= calculated R1+R2 = 0.11 ohms.

Now why would the calulated answer 0.11 be more then the measured dead test value 0.51?, is there parrell paths maybe when you do the dead test R1+R2.

Appreciate your thoughts,

Locket

 

[telectrix]

you would get a measured value highr than the calculated one if you had 1 or more poor connections.

 

[pushrod]

My initial thought is if the circuit is 45 m then calculated R1+R2 for a 1.5mm+1mm t&e is is 1.36Ω

 

[snowplough]

Hi Telectrix,
Sorry its not 0.11, but 1.8 for the calculated value, so as i said why would this be more the 0.51 dead test measured value which is in the sched of test results table? surelly it cant be the cable conductor operating factor because its just asking to calculate R1+R2.

Rgards

Locket

 

[snowplough]

Hi Pushrod,

The value for 1.5mm cable is given in table as 12.1, it says that line and earth are both 1.5mm ,so i get
12.1x2=24.2 x 45metres long = 1,089/1000= 1.08ohms

so the question asked why would 1.08ohms be more then the Sched of test result reading of 0.52 ?

Regards

Locket

 

[pushrod]

If the R1+R2 value has been calculated from Zs - Ze, and the Zs included parallel paths ?

 

[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

 

[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.