Looking for clarification about Page 47 in IET wiring regulation 18th edition.

Page 46 not 47

The 18th edition of B.S. 7671 on page 39, defines a PEN conductor as, " A conductor combining the functions of both a protective conductor and a neutral conductor."

It is normally earthed at  the supply  end so is not normally at a potential much above earth potential. If it is disconnected at the supply end it can achieve a potential at or approaching the supply Voltage. This may be due to a fault or breakdown. The live supply will travel through loads and liven the PEN up.

In the U.K. the PEN conductor is terminated at the house cutout where it is initially just one conductor. After that the house installation is supplied by a Line and Neutral supply and  separate protective conductors. So inside the house we have three conductors at sockets and most other wiring accessories, Line, neutral and C.P.C. The neutral is then considered to be a live conductor. The neutral conductor is defined as "A conductor connected to the neutral point of a system and contributing to the transmission of electrical energy". It is current carrying.

A line conductor is defined as being: "A conductor of an AC system for the transmission of electrical energy, OTHER THAN A NEUTRAL CONDUCTOR".

So, the LINE conductor runs to loads. Then a NEUTRAL conductor is connected to the other side of the load. Both conductors carry the same current when the load is energised. Touch a bare LINE conductor and you may receive a fatal shock. Touch a NEUTRAL conductor and you may not receive a fatal shock, but it is best NOT to try this test. It may be fatal.

The actual measured Voltage between the neutral conductor in an installation, and true Earth, may be a few Volts due to the resistance of the conductors.

Z.

Well it is not a technical reason, more of an administrative one,  and only really a matter of the way  that a 'live part' has been defined a few pages earlier in the document.

Live part. A conductor or conductive part intended to be energized in normal use, including a neutral conductor but,
by convention, not a PEN conductor.

no mention of carrying operating current (or not).

There is a secondary consideration, that it is only supposed to be possible to get a shock to earth from  'live' parts, and because of that earthed parts do not require insulation ' the earth' is supposed to be safe to touch.  A great many things  are connected  to earth via a PEN such as metal switchgear enclosures. Almost any appliance or similar thing with a CPC has exposed earthed metal parts that are not insulated, and to do so retrospectively would be impractical, and arguably un-necesssary,

To re-define a PEN as a live conductor would require a complete change in the treatment of earthed metalwork on TNC-s supplies.

Note that in much earlier versions of the regulations a neutral conductor was not considered a live part either, so things like concentric cables with neutral armour were considered to be safe against penetration, and metal at neutral potential was not always insulated.

Mike

I agree it's a "deemed" to be not a live conductor.

Generally neutral conductors are treated differently from protective conductors. Neutral can be switched, and may need to be opened for isolation under certain conditions but as a result their connection to Earth is less reliable - a simple fault in a multipole switch for example can leave N disconnected but the corresponding L still connected - so N downstream of the switch may be live by being connected to line by downstream loads. Protective conductors on the other hand usually have no switching but remain solidly connected to Earth - so remain safe under the majority of conditions.  PEN conductors, being a combination of PE and N are then a puzzle - should they be treated as N or PE? The committee has plumped with treating them as PE and the remaining rules match that decision.

     - Andy.

Thank you for helpful answers. 

mapj1 said:

no mention of carrying operating current (or not)

Melad Al Aqra appears to be referring to 18th Edn. In 18th Amd2, we have to look at page 52: 312.1.1 where his quote may be found.

I would add that a PEN conductor is not "BY CONVENTION" not considered to be a Live conductor whereas a N conductor is so considered. Suggest to me there might be some cases where we should think of it as Live despite the convention

Also consider the common sense approach,

We can not under any common conditions consider the PEN conductor as a live conductor because we generally need to connect it to metallic water pipes and to structural metalwork. Best of luck treating those as live parts.

ebee said:

Suggest to me there might be some cases where we should think of it as Live despite the convention

The same could be said of any other type of protective conductor in certain circumstances.They are not always safe to remove ... and not, always, safe to touch (this is acknowledged in BS 7671).