Ze 2.46 Ohms on TNC-S

[mini-si]

Hi all, just after a little advice. Have been reading throygh some test papers in the office at work this week and came across a problem.One of our guys have tested a small train staion( i.e a cubical feeding a help point and 2x lamp post) its out in the middle of nowhere. Our guys who is off sick at the mo has put the system down as a TNC-S but it seems abit to far out in the sticks looking at the map, also the high Ze result he has recorded makes me think its in fact a TT system. The Ze was measured at 2.46 Ohms and Pfc at 0.10ka. Switching these results around using nominal voltage/Ze would roughly equal 2.3-4ka.Im thinking he has either the two results mixed up on a TT system or the Ze is really that high in which case what do you write on the test report as it would be the supply authority responsibility to reduce the measured value surely? please correct me if im wrong have been in electrics for 7 years only been testing for 5 months so still have alot to learn any help/advice would be great guys.

 

[m4tty]

Ze needs to be below 0.35 Ohms for TNCS and your reading sounds too good to be TT but im sure more knowledgable people will be along soon to give you a better answer than me. HTH

 

[keniff]

with the tt systems i think they are starting to change them to tncs if the ze is below 0.35 (measure this by doing your standard ze test L-N instead of earth), it may have been converted at somepoint? also i dont think anywork should be undertaken on the system if the ze is that high as it would fall under inadequate earthing arrangements.... could be wrong though

 

[xraytek]

The Ze was measured at 2.46 Ohms and Pfc at 0.10ka.

So he measured PEFC and PSCC yet 100A was the highest reading?

Switching these results around using nominal voltage/Ze would roughly equal 2.3-4ka

Hows that then?

 

[keniff]

Looks like he has mixed them up, with ur readings u got, PFC would calculate at 93amps (v/r=I) and ze would calculate at 2.3 ohms (v/I=r) so ur fault current is lower than the general supply....

 

[semaphore]

mimi-si

What are the final circuits and their protection and their R1+R2?
Assuming he's written it down right and the 2 lights are on a B6A BS60898 if their Zs is <6.13 ohms you will still get ADS in <0.4
Even 100A is enough to make small size fuses and mcb's go bang.
But, if you're worried, recommend an RCD in the report. If it does turn out to be TT, you'll have no choice.

 

[Pinto Baz]

I don't understand where you are getting 2.3-4 kA from? If the nominal voltage is 230 then 230/2.46 = 93.5 A. That's roughly the 0.1 kA (100 A) as stated. If it is a TN-C-S system then the PEFC and the PSCC will be virtually the same as the neutral and earth are joined at the service cut out. I agree that the 2.46 is too high for a TN-C-S as this should be 0.35 max, so this requires further investigation.

 

[eggyboy]

never assume always proove ha ha saves your insurance also cubical might b run of a mcb from another disboard eg 60898 20a zs 2.55 happy days worth another look