Can Someone Clear Up Max Zs Values For Me

[Technician]

When I put that post up I was careful to avoid any

reference to the "Rule of Thumb" as explained in

appx 14. I also carefully avoided any reference to

fault conditions which are handled in another section.

The various corrections available need to be very

precisely explained and so many are applied without

a full understanding, IMHO.

 

[unphased]

Here is something for you all to consider. Zs is a measure of earth fault loop impedance around the entire fault loop. Before we had the benefit of RCD protection it was important to limit the impedance of the fault in order for the protective device to function within specified limits under fault conditions, BS7671 had to stipulate max Zs values. Where a 30mA RCD is used on a circuit does the max Zs value really matter? It would matter where no RCD protection is afforded, but an RCD would trip the circuit before the fuse most times under earth fault conditions.

As for the differences in published max Zs values, it is not that difficult to fathom. BS7671 has tabulated values and each table is specific to the BS numbers. In order to save time on site when testing a rule-of-thumb was devised whereby if your test reading was lower than 75% of max allowed (that was the original one) it was considered acceptable. The 75% has now been relaxed to 80% and it is this value taht most charts use. The chart will state what the value is (75% or 80%). If under test conditions the 80% max value is exceeded and there is no RCD protection on the circuit, the electrician is to decide whether the circuit Zs should be improved.

Another point about Zs is that it is intrinsically tied to Ze. If Ze is high then Zs max will be reached more readily. The benefits of RCD protection in these circumstances is fairly obvious.

 

[unphased]

Page 184 of the OSG gives the correction factors for

conductor resistance at maximum operating temperature.

When measuring at 20 Cel a factor of 1.2 could be applied

to ensure that (R1 + R2) reads would not contribute to a

high Zs value at the maximum operating temperature.

If you applied the ambient temperature multipliers given on

the previous page you COULD do your correction from 15

Cel.

HTH.

How many electricians carry a thermometer around to measure tempertaures of conductors under load or ambient temperatures? The point I am making is that this is too complicated for working under site conditions that are often time pressured and stressful. You avoided rule-of-thumb in your reply (as stated in a subseqhuent post) but this is exactly where the basis of 75% or 80% max values are derived. If the r-o-t vakues are reached or exceeded than correction factors can be applied if deemed necessary by the spark. It is r-o-t that drives the chart values published for this purpose. As long as the value you are using is mentioned on the test results sheet as being 100%, 80% or 75% and the recorded values are below this then that is all that matters. But if you read my response that introduces RCD influence on circuits then that is another factor. It is about understanding what all the values mean rather than worrying about the different values published. It is not too difficult as I am sure you know, tech. The OPs question suggests he is not appreciating this.

 

[kme]

Barx

To return to your original question - and as was alluded to on the previous page:

The tables in the fron tof the NIC cert packs are "80%, rounded down, of the values given in BS7671" - "when measured at ambient temperatures UP TO 20 deg. C"

example - I`m not putting the whole table in (if you want a scan of it, I`ll email it to you):

Type B 60898 / 61009s

3A / 12.26 ohms

6A / 6.13 ohms

10A / 3.68 ohms

16A / 2.3 ohms

20A / 1.84 ohms

25A / 1.47 ohms

32A / 1.15 ohms

40A / 0.92 ohms

45A / 0.82 ohms

50A / 0.73 ohms

63A / 0.58 ohms

HTH

 

[Barx]

Cheers KME. Managed to get my grubby mits on a copy of NIC values now