max Zs, Which Value

[Sparky Ninja]

It doesn't really matter as it isn't required by the IET on documents. All they have put down is that the Zs should be less than the values given in Appendix 2 O.S.G. and Appendix B GN3. So, in theory, that would be the considered 'maximum'.

 

[brucelee]

Hi mate
If its for the sample EICs in bs7671 or gn3 your referring to then it should be the measured value at the time of test that you record as this is what it says in the notes accompanying the schedule of test results

if your talking about a schedule of test results that has two columns for Zs on it, then one will say the max100% bs7671 value and the second column is the measured Zs value which should be compared to the 80% values in OSG or GN3 which are the maximum measured values for comparing test results to and your measured value should be less than the 80% values which is the rule of thumb

at least this is what i was told and shown on courses i have just done inc 2391

which cert are you referring to the IEE version or another

 

[telectrix]

Hi mate
If its for the sample EICs in bs7671 or gn3 your referring to then it should be the measured value at the time of test that you record as this is what it says in the notes accompanying the schedule of test results

if your talking about a schedule of test results that has two columns for Zs on it, then one will say the max100% bs7671 value and the second column is the measured Zs value which should be compared to the 80% values in OSG or GN3 which are the maximum measured values for comparing test results to and your measured value should be less than the 80% values which is the rule of thumb

at least this is what i was told and shown on courses i have just done inc 2391

which cert are you referring to the IEE version or another

it was the max. permitted value of Zs i was referring to. as you say, the max. figure IMO is the one given in brb. eg 1.44 for a 32A type B. Obviously, having a measured value less than the osg. corrected figure is what we should attain. the query was who puts which figure in the max permissible columm. from posts on this thread, most use the brb.

 

[brucelee]

hi mate
if it asked for the maximum permitted value then its the 100% maximum tabulated value from BS 7671

but the IEE sample certs only have 1 column for ZS and this is the value you have to record from the test
and these certs dont ask for the max permitted only the measured value

 

[telectrix]

ah. mine do. so. query answered. i will continue to use the brb max. figures. many thanks for all contributions

 

[prideofengland]

Now I might be wrong on this but I beleive that the 80% rule of thumb is to allow for the rise in temperature under fault conditions, after all this the only time that the max Zs is important to ensure automatic disconnection. Therefore the 80% value is the value the actual Zs has to be compared with.

However the Certificate asks for the 100% value or BRB value. This to me seems daft and causes confusion especially when on a PIR and the value exceeds the 80% value but not the 100% value. My answer is to use the 80% value but indicate it on the test sheet. NICEIC inspectors have always been happy with this compromise.

 

[brucelee]

hi mate
the rule of thumb takes into account increase in resistance of conductors with the increase of temperature due to load current and errs on the side of safety

 

[prideofengland]

Yes of course your right, well I have been on the vino! My rational however was that as it would take about 160A of fault current to cause a 32A MCB to disconnect in time I would expect a reasonable temperature increase.

 

[telectrix]

can't see a huge increase in temperature within the 0.4 secs taken for it to trip. anyone got a time/temperature graph handy for cables carrying fault currents?

 

[brucelee]

yes mate it would increase the conductor operating temp uder fault conditions and yes it does need 160amp to operate the 32a breaker in the required time

and the cable has to be able to carry the fault current for a reasonable time and cope with the increase in temp due to fault current

I think the better certs in my opinion have two columns one for the max permitted bs7671 value and 1 for the actual measured Zs at the time of the test
then even without using gn3 or osg by applying the rule of thumb 0.8 value to the max permitted value you can see on the schedule that it complies

 

[prideofengland]

No Im afraid not however:

'PVC. is probably the most usual form of insulation, and is very susceptible to damage by high temperatures. It is very important that p.v.c. insulation should not be allowed normally to exceed 70°C, so the current ratings of cables are designed to ensure that this will not happen. Some special types of p.v.c. may be used up to 85°C. A conductor temperature as high as 160°C is permissible under very short time fault conditions, on the assumption that when the the fault is cleared the p.v.c. insulation will dissipate the heat without itself reaching a dangerous temperature. '

http://www.tlc-direct.co.uk/Book/4.3.2.htm

It would be interesting to know for definate.

 

[brucelee]

yes mate agreed
and the Cb will clear the fault quickly if zs is sufficiently low enough
so the cable is designed to carry a larger current for a short space of time under fault and will disapate the heatquickly once the fault is cleared

the 70C is the max it can carry constantly under load conditions and increases due to higher resistance of increased load

 

[prideofengland]

Fair point Bruce Lee, by the way have you seen 'Kung Pow', possibly he finest martial arts movie ever?

 

[brucelee]

No mate not seen that
Bruce lee was childhood hero

and through the films i watched was a encouragment to take up martial arts when iwas younger
kickboxing,karate,and a bit of thai boxing

long time ago now not done any for years and also not seen may kung fu films over last 20 years or so

 

[prideofengland]

If you get a chance watch it , bit of a **** take, but done affectionately. If you like the old martial arts films and got a sense of humor its a good nights entertainment.