system characteristics TNCS

[dogwind]

Hi i am after some help please, i am looking at a fluke schedule of inspection and circuit details/ test results, and in the box called system characteristics it asks for fault level(s), its on a TNCS and i have looked in my on site guide and my regs book for the information, could someone please point me in the right direction.
thanks for any help
Regards Ady

 

[Spudnik]

Not sure as i dont use them, however, at a guess i would think measured PFC.

 

[dogwind]

Oh right thanks, i measured a loop test Ze of 0.39 so do i just devide that into 100Amps as in my main fuse or am i heading the wrong way with this, cheers

 

[Spudnik]

If the Ze was .39 then you need to contact the DNO as this is too high. It should be a maximum of .35.

Your PFC is measured. It will be recorded as the highest of the PEFC and PSCC.

However, on a TNCS system, they should be around the same.

 

[dogwind]

Yes i did see it was high, i am going back tomorrow and will measure it again, the house is empty at the moment. Thanks for your help.

 

[pushrod]

Oh right thanks, i measured a loop test Ze of 0.39 so do i just devide that into 100Amps as in my main fuse or am i heading the wrong way with this, cheers

No - you would divide your supply voltage by your Ze. In your case your fault current would theoretically be 589.7amps (230V/.39) which you would record as 0.589kA. In reality your supply voltage will not be spot on 230 so if your meter is working it out you will probs get a higher reading for the pfc. However as Jason said if you are testing it at the main incomer and not a subboard (which would not be classed as Ze anyway) then your value of .39 ohms for a tn-c-s is not acceptable. Are you sure it is a tn-c-s and not a tn-s?

 

[dogwind]

Hi thanks for your advice, it is deffo a tncs system and when i re tested i got a lower reading than 0.35 also i realised i hadnt deducted my test lead resistance the first time i tested, i did my 2391 around 7 years ago and unfortunately the company i worked for i didnt get to do much testing so i am a bit rusty, i am self employed and looking at joining NAPIT or NICEIC (bloody part P) so i am trying to get back up to speed, i was also looking at the fault current characteristics graphs in the regs book for mcb/rcbos and unless i am missing something really obvious i couldnt see a time for as an example a type b mcb 6A what time would it disconnect at say 400Amps, i see in the box on the right it says 0.1 to 5 seconds, i wanted the disconnection time to work out a cpc size calculation, the graphs for the fuses seems much clearer for their time/current disconnection times. Thanks again,
Regards Ady

 

[pushrod]

Hi thanks for your advice, it is deffo a tncs system and when i re tested i got a lower reading than 0.35 also i realised i hadnt deducted my test lead resistance the first time i tested, i did my 2391 around 7 years ago and unfortunately the company i worked for i didnt get to do much testing so i am a bit rusty, i am self employed and looking at joining NAPIT or NICEIC (bloody part P) so i am trying to get back up to speed, i was also looking at the fault current characteristics graphs in the regs book for mcb/rcbos and unless i am missing something really obvious i couldnt see a time for as an example a type b mcb 6A what time would it disconnect at say 400Amps, i see in the box on the right it says 0.1 to 5 seconds, i wanted the disconnection time to work out a cpc size calculation, the graphs for the fuses seems much clearer for their time/current disconnection times. Thanks again,
Regards Ady

400 amps is well to the right of all parts of the 6A line so the disconnection time is < 0.1 seconds or 0.1 for your calculation. As i understand it, for the 6A line it would only be with a fault current of less than 30 amps that you would be concerned that disconnection times would not be met so once the pfc is above that figure you can take 0.1s for your cpc calcs. But it is a good point - when you are putting a time in the adiabatic equation which time do you use 0.1 or 5! When i was at college we were always shown to use the most advantageous one ie the 0.1 s time

 

[dogwind]

Thanks again, i did think it would be the 0.1 but i also tried the calc with 5 secs to be on the safe side, as i wasnt sure if i was reading the graph correctly i thought i would ask on here,i still cant see on the graph any that disconnect at 5 seconds so i dont understand why they say 0.1 to 5 it just seems confusing for no reason and it would be easy to make a mistake
Your help is much appreciated
Thank you