Cryptic Regulations

For openers I'd like to nominate 462.1 (isolating/switching N):

461.2 In TN-C systems and TN-C-S systems, the PEN conductor shall not be isolated or switched.

In TN-C-S and TN-S systems, isolation or switching of the neutral conductor is not required if protective
equipotential bonding is installed and either:

(a) the neutral conductor is reliably connected to Earth by a low resistance to meet the disconnection times of
the protective devices according to the requirements of Chapter 41; or

(b) the distributor declares that either the PEN or the neutral conductor of the supply is reliably connected
to Earth by a low resistance to meet the disconnection times of the protective devices according to the
requirements of Chapter 41.

Two points:

• "if protective equipotential bonding is installed" ... does it really mean installed, or just installed if required. These days with many buildings supplied with plastic services there are often no extraneous-conductive-parts there to need bonding - I'm having difficulty in imagining any situation where an unswitched N is any more hazardous in a building with no extraneous-conductive-parts than it would be in a building with bonded extraneous-conductive-parts. In both cases exposed-conductive-parts would be earthed (via the MET) as usual.

• What on earth is meant by "the neutral conductor is reliably connected to Earth by a low resistance to meet the disconnection times of the protective devices according to the requirements of Chapter 41"? If by "Earth" we are to read the part 2 meaning - i.e. the conductive mass of the planet - then in TN systems then "connected to Earth" presumably means the earthing of the star point at the source (and possibly repeated along the length of the PEN conductor in PME systems) - that resistance can be significant - anything up to 20Ω in the UK (especially in rural areas where separate HV and LV earthing is needed). So how does that mesh with " to meet the disconnection times of
  the protective devices according to the requirements of Chapter 41"? In TN systems the earth fault loop is metallic all the way and doesn't include the general mass of the earth and so our "connection to Earth" is irrelevant to disconnection times. If it were, perfectly legitimate electrode values in the tens of Ohms would make almost all overcurrent devices useless for ADS. I'm guessing they're meant to be two independent requirements - N is reliably earthed (actual resistance isn't that significant) and ADS is up to standard (including EFL impedances, which don't include the system earthing electrode) so even during earth faults the PE can't remain at a significantly different potential to N for any significant length of time. The thinking being that if PE is at an elevated voltage while N remains close to earth (most likely in a TN-S) system there will be a hazardous potential difference in much the same way as if N was hazardous and PE at true Earth. We possibly would want to be sure that L-N faults were cleared promptly as well, otherwise N could remain at an elevated voltage while PE remains close to zero volts.

  - Andy.

AJJewsbury said:

"if protective equipotential bonding is installed" ... does it really mean installed, or just installed if required.

Nice one, Andy. I am happy to address your first bullet point.

I can see two possible interpretations.

First interpretation

In any TN-C-S or TN-S system, you may leave the neutral conductor unswitched provided that the following conditions are satisfied.

(Protective equipotential bonding is installed) AND (condition (a) OR condition (b) applies)

Second interpretation

In a TN-C-S or TN-S installation in which protective equipotential bonding has been installed, the neutral conductor may be left unswitched provided that either condition (a) or condition (b) applies.

It follows that if there is no protective equipotential bonding (e.g. because the other mains services arrive in plastic pipes) the neutral conductor may be left unswitched even if neither condition (a) nor condition (b) applies.

I favour the second interpretation, not least because of 462.1.201.

The second paragraph requires that in a household installation, the main switch must interrupt both L and N of a single-phase supply. Impliedly, if it is not a household SP installation, all that is required is a linked switch or circuit breaker (see page 49 for definition) which opens all three phases together. It need not isolate neutral.

So, there are circumstances in which neutral may be left unswitched and it would be absurd if that were possible only if protective equipotential bonding were required, but there is nothing to bond.

ETA: I think that a subtle change in the words would make the paragraph clearer: If protective equipotential bonding is installed in a TN-C-S or TN-S system, isolation or switching of the neutral conductor is not required provided that either ... [condition (a) or condition (b) applies]. 

AJJewsbury said:

"if protective equipotential bonding is installed" ... does it really mean installed, or just installed if required.

Nice one, Andy. I am happy to address your first bullet point.

I can see two possible interpretations.

First interpretation

In any TN-C-S or TN-S system, you may leave the neutral conductor unswitched provided that the following conditions are satisfied.

(Protective equipotential bonding is installed) AND (condition (a) OR condition (b) applies)

Second interpretation

In a TN-C-S or TN-S installation in which protective equipotential bonding has been installed, the neutral conductor may be left unswitched provided that either condition (a) or condition (b) applies.

It follows that if there is no protective equipotential bonding (e.g. because the other mains services arrive in plastic pipes) the neutral conductor may be left unswitched even if neither condition (a) nor condition (b) applies.

I favour the second interpretation, not least because of 462.1.201.

The second paragraph requires that in a household installation, the main switch must interrupt both L and N of a single-phase supply. Impliedly, if it is not a household SP installation, all that is required is a linked switch or circuit breaker (see page 49 for definition) which opens all three phases together. It need not isolate neutral.

So, there are circumstances in which neutral may be left unswitched and it would be absurd if that were possible only if protective equipotential bonding were required, but there is nothing to bond.

ETA: I think that a subtle change in the words would make the paragraph clearer: If protective equipotential bonding is installed in a TN-C-S or TN-S system, isolation or switching of the neutral conductor is not required provided that either ... [condition (a) or condition (b) applies].