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Posted By Topic: old bare earth TPS

Mike
Mar 14 2013 17:30

Hi there,is it ok to extend a new cable off an existing point with a bare earthed TPS to a new power outlet.I will be upgrading the fuse to an RCBO.Thanks.
   

Donald
Mar 14 2013 19:57

Sleeve the bare earth.
   

make
Mar 14 2013 20:46

sure you meter out the cable looking for leakage, to avoid false tripping of the rcd your installing.
   

UKspark
Mar 15 2013 07:46

where on earth (haha) would you get bare earthed cable in NZ. Persoanlly speaking I find no problem with bare earthed cable (coming from the UK), but I believe it doesn't meet NZ standards - but don't take my word for it
   

AlecK
Mar 15 2013 08:43

5.2.3.4
   

UKspark
Mar 15 2013 09:12

5.3.2.4.

(numbers swapped round Alec)
   

Russ
Mar 15 2013 09:34

I think we can assume he means extending from an existing bare earth subcircuit with new compliant TPS.
   

AlecK
Mar 15 2013 10:05

Dyslexic fingers strike again

Donald already gave the right advice on that.

I was pointing UKSpark at the appropriate rule - or rather, trying to - that prevents us from re-using old bare-earth cable.

ESR 113 lets it remain in service, but 5.3.2.4 means we can't re-use or even re-route it.
   

TimoW
Dec 20 2017 20:16

Is the reason you can't extend such a circuit because the COC you'd issue applies to the whole circuit being worked on not just the extension and hence the existing TPS wouldn't comply with 5.3.2.4?
And if there's no way to get new circuit back to the board then you're s**t out of luck i suppose.
   

daniel2
Dec 20 2017 20:31

"... but 5.3.2.4 means we can't re-use or even re-route it ..."

Nonsense.
   

AlecK
Dec 20 2017 22:47

Not nonsense at all - just a direct reading of the Wiring rules. And nothing to do with the existing part of the circuit (which is NOT covered by a CoC - CoCs are for the work we do, not for things left in service).

5.3.2.5 requires the PEC of any cable we install to be insulated. Re-routing or otherwise altering the circuit is installation work, not "remaining in service" as allowed by ESR 113 and not "maintaining" as allowed by ESR 59. It's installation, and ESR 59 requires compliance with "3000".




   

daniel2
Dec 21 2017 07:22

5.3.2.4
‘Earthing conductors shall be provided with insulation.

The lists the exceptions that need not be provided with insulation.

It doesn’t mention anything about TPS with a bare earth.
   

zl2aj
Dec 21 2017 08:01

So then the question is - is the outer TPS considered "insulation" for the purposes of 5.3.2.4. Interesting.
   

AlecK
Dec 21 2017 08:09

Now look at 5.3.2.5 re ID, and 5.3.3.1.2 re sizing.

So even assuming the sheath counts as insulation for the PEC core of a TPS (or TRS), use of the cable for installation work cannot comply.
   

zl2aj
Dec 21 2017 08:36

5.3.2.5 - Green sleeving - tick.
5.3.3.1.2 - by calculation 5.3.3.1.3. Depending on fault current should comply.


   

zl2aj
Dec 21 2017 08:37

3.8.2 - "along their entire length" - yep its out.
   

zl2aj
Dec 21 2017 08:39

Or not... its not a single core cable. 3.8.2c would thus allow it?
   

AlecK
Dec 21 2017 09:40

maybe.

And I accept that only single insulation is required for a PEC; and since TPS is regarded as being "double insulated" WRT A & N, a case can be made that the sheath provides the required insulation for the PEC. I'm certain that's not the intent, but what matters is the words.

On the matter of calculation; yes the option is there, but have you actually got enough data to do it? Usually not.

The PSSC can be assessed (and in an old domestic is unlikely to exceed 2kA). So assume the cable is an old 3/0.029 or a 7/0.029; first you need to know the actual size in mm2 of the PEC, then you need a "K" value for the combination of copper + (sheath) insulation. By the time you've worked this out, IF you can work it out (probably have to use approximations); and assuming the answer is favourable; you've already cost the customer more than the cost of the tiny bit of cable you've saved.


   

zl2aj
Dec 21 2017 10:33

No its not worth it - but thats not the question :)