max Zs, Which Value

[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.

 

[lennyallover]

hi quick question, i was debating with a colleague today about measuring R1 + R2 on a pre existing RMSO. My colleague insisted that it be measured from every point served whilst I said it should be measured from the furthest point only and as long as the other points served all checked for correct connection taking a resistance measurement from all points was not needed. Anyone clarify this?

 

[brucelee]

most of the martial arts films where and are very far fetched
this is why i liked bruce lee as he was trying to bring real martial arts to the film industry but since his death it reverted back to the tongue in cheek stuff

i will watch if i ever get the chance mate for a laugh

 

[brucelee]

hi quick question, i was debating with a colleague today about measuring R1 + R2 on a pre existing RMSO. My colleague insisted that it be measured from every point served whilst I said it should be measured from the furthest point only and as long as the other points served all checked for correct connection taking a resistance measurement from all points was not needed. Anyone clarify this?

mate what is RMSO do you mean ring final circuit if so then they should be measured at every outlet if you are referring to a lighting circuit then the furthest point is the R1+R2

 

[lennyallover]

ring final circuit mate , rmso-ring main socket outlets. thanks for the reply i'll go eat humble pie in the morning ... damn he'll be insufferable all day.lol.

 

[telectrix]

by measuring R1+R2 at every point, you verify that each point is correctly connected and you should get a slight variation from point to point. the highest reading is the mid point. if you get a reading which is at odds with all the others, then you know there is a problem at or close to it. it may just be a high resistance in a socket face. the way i do it, right or wrong is to short L/E on my break-out box, meter across at CU then move short from one point to another. then repeat with N/E shorted. on a radial, of course, the highest reading is the "end of line".

 

[lennyallover]

ty telectrix, very concise..i do however know all that know all that and have done the same. My point was , if the polarity of the ring final was being checked using continuity methods and the correct sixth and quarter values measured, is R1+R2 from every point really needed

 

[telectrix]

yes. supposing a cpc was continuous, but had come out of the socket terminal. only that particular socket would show it up when doing R1+R2 test., simply because the tester could not see the cpc when plugged in that socket

 

[lennyallover]

Yup!!! thats a solid enough answer for me, nice . Another one. if a person is in a stainless steel bath ,and the bath itself has all 4 of its feet attached attached directly to the man earth block in 16mm cable. the water in the bath is highly salted and in the most conductive state possible when an unfused live cable is placed into the water (er.. this is theoretical im not planning sod all)would the person in the bath be electrocuted? would the current pass harmlessly through the water taking the path of least resistance through the steel bath and on to earth leaving the person unscathed . or some other path?

 

[malcolmsanford]

Everyone in contact with a fault will be "electrocuted" but you limit that shock to a touch voltage of 50v and limit the duration of the fault/shock by making sure the protection device acitvates in the prescribed time.

On your post concerning the ring final circuit what "sixth" value are you refering to?

 

[lennyallover]

there is no protective device....imagine the fuses have been replaced with nails big chunky steel nails...given that the resistance of the bath water is .05ohm for each cubic ltr the current has to pass through and the resistance of the person is around 150Kohm(THESE FIGURES ARE NOT ACCURATE JUST A WHAT IF SCENARIO) would the water and bath act as a faraday cage protecting/ channeling current away from th person...my own thgoughts on this is that the person would survive though could suffer flash burns across the skin surface. disregard sixth....shouldnt post after drinkin on an empty head..was meaning th (R1 + R2)/4