Maximum permitted Zs question

[MRoach]

I am a lecturer teaching electrical installations and in reading through the on-site guide to prepare a lesson I have come across a section I have not seen before.
On page 146 near the bottom, the OSG talks about applying an ambient temperature correction factor to the maximum Zs values given in tables B1-B6 at the time of measurement. It provides the example of a 32A type B MCB having its value of 1.1, however, if the ambient temperature is 1.06 then this becomes 1.1 x 1.06 and therefore 1.17 ohms. Should we then on a test sheet record the maximum permitted Zs as 1.17 or as 1.1?

Any help would be appreciated. Thanks

 

[Rockingit]

Most test sheets (certainly the automated ones) ask you whether you're using Max or 80% figures

 

[pc1966]

Good question. I think most simply work from the OSG tables that are correcting for 10C measured to 70C operating, as that is safe enough to cover most situations. In reality most installation's circuits won't be maxing out at full CCC for PVC insulation so won't get the full 20% increase (from 20C!) and few are going to be measured colder than 10C, so just ticking with the OSG table for a standard answer for what to check against is OK.

However, there will be occasions where you fail Zs if doing so but are not really at risk, for example if measuring it hot as you only just powered the DB/circuit down for dead testing, but in reality it takes tens of minutes to get anywhere close to cold. In that situation you could well argue that having measured Zs at the computed limit based on Umin and max magnetic limit is sufficient. Thatr is Zs <= (230V * 0.95 / (In * 5|10|20)) for B|C|D curve.

 

[Vortigern]

The value recorded on the test sheet is 1.37 as this is the maximum permitted value for a 32a MCB RCBO. However when testing the "rule of thumb" is to use an 80% value i.e. 1.06. When calculating the cable size the tabulated value will need adjusting or rating down (usually) to take into account the operating temperature and other factors (C factors) are applied alternatively. The requirement of the test sheet is to always record the maximum tabulated value in BS7671, not the 80% value though. Although I often see this. Note, this is not the measured value when testing and writing results. The factors you talk of are for calculating cable size through a specific set of formula/calculations. Not to be conflated with the test sheet.

 

[grumpyjohn01]

The value recorded on the test sheet is 1.37 as this is the maximum permitted value for a 32a MCB RCBO. However when testing the "rule of thumb" is to use an 80% value i.e. 1.06. When calculating the cable size the tabulated value will need adjusting or rating down (usually) to take into account the operating temperature and other factors (C factors) are applied alternatively. The requirement of the test sheet is to always record the maximum tabulated value in BS7671, not the 80% value though. Although I often see this. Note, this is not the measured value when testing and writing results. The factors you talk of are for calculating cable size through a specific set of formula/calculations. Not to be conflated with the test sheet.

A bit of a classic instance of too much information on the Test Sheets imo (plenty of others). Hard to explain to a client if its not just a case of 'this numbers bigger than that number' and something thats easy to look up for any skilled person that needed to know and probably in many cases memorised anyway..