Temp factor .8 and 1.2

[John-]

I have a new Metrel tester and it has a .8 factor to apply to ZS. for the temperature adjustment for the cable heating up from ambient as a result of a fault.

I see .8 mentioned a lot elsewhere, is this .8 a rule of thumb derived from the equations of : rT= R20 {1 + A 20 (0-20)} =1.2 (inverse of .8) ?

and

ZTest (Max) = 1 / f *Zs = 1/1.2 * ZS (where 1/1.2 = .83r) almost .8?

.8 as a multiplier to de-rate the tabulated value, and .8 as a divider to be applied to inflate the measured value?

Max tabulated ZS = 1.3ohm - derated to be compared with a measured value would become a (1.3*.8) = 1.04 max measured value, anything measured higher than 1.04 is too high.
If i have a measured value of ZS and i want to convert so i can compare it to a tabulated value: -
measured value = 1.04ohms /.8 = 1.3ohms - i can then directly compare the 1.3 to the given ZS of the relevant overload device?

Is that correct please?
I realise that the 1.2 is a factor that is dependent upon the type of cable, 70 or 90.

Thanks
John

 

[jc_150]

Hi John,
This is my take on it. The values of Zs in BS7671 are taken at 70 Degs. C and those in On Site guide are at 10 Degs. C. Copper changes resistance by approx. 2% for every 5 degs. C . change Therefore difference between them is 60 Degs. (12 lots of 5 degs.) so from 10Degs. C to 70 Degs C is a 24% increase ( a multiplier of 1.24.) and the reciprocal of 1,24 (1/1.24) = 0.8. Therefeore multiplying the BS 7671 values by 0.8 will give an acceptable maximum Zs when measured. Otherwise use On Site Guide of GN3 for the values and one can make a direct comparison of measured values as these have already been multiplied by 0.8.
JohnC

 

[John-]

Thanks.

 

[telectrix]

as above.it's to allow for the increase in resistance of the cable. you measure at 20 dg.C, but the cable is rated to 70.C. very pessimistic, IMO as cables are rarely loaded that high as to ever reach70.C.