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