Thoughts on two circuits and two gang switch ?

[JohnW2]

So what if the two circuits come from two different CUs, and there's no isolation switch before them, making the only single point of isolation the cut-out fuse?

But the two circuits are not going to the same terminal, so you isloate in the normal way by pulling the fuse.

Is that 'the normal way'? Would it not be a bit drastic to pull the cut-out fuse in order to work on a two-way light switch? In the situation decribed by BAS, it's surely the case in the real world that one would not worry too much about 'a single point of isolation' but, rather, would isolate at both CUs?

Kind Regards, John.

 

[sparkticus]

It's actually 537.2.1.3 we've been talking about.

Yep, I quoted 537.2.1.1 by mistake and meant 537.2.1.3.

 

[sparkticus]

Would it not be a bit drastic to pull the cut-out fuse in order to work on a two-way light switch?

I suspect holmslaw was generalising about pulling individual final circuit fuses/MCBs etc.

 

[Paul_C]

The only warning label required on light switches is to warn of the presence of 400volts.

It seems that even that requirement no longer exists with the latest amendment, as mentioned here:
http://www.diynot.com/forums/viewtopic.php?t=283088&start=45

 

[holmslaw]

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[sparkticus]

To clarify, that reg applies to any equipment with more than two supplies that cannot be isolated by one switch/isolator. And the warning will not be anything to do with 400V.

Yes and I have seen that / placed such labels in situations where backup generators / PV panels exist.

With backup generators the supply may even go dead for a few seconds while the generators kick in. With PV panels/batteries the supply may remain live from the alternative source (mains or PV)

But are you saying (and I don't disagree if you are) that it is within the scope of multiple generators only (whatever the generators are) That is, it does not refer to two (or more) final circuits having a common single source of electrical supply needing to be isolated to fully isolate (make dead) an enclosure (such as a domestic hallway 2 gang switch plate) ?

 

[ban-all-sheds]

But the two circuits are not going to the same terminal,

No - they'd be going into adjacent terminals of a single accessory in a single enclosure.

so you isloate in the normal way by pulling the individual circuits fuses

That's not a single point of isolation.

 

[holmslaw]

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[holmslaw]

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[OwainDIYer]

But the two circuits are not going to the same terminal,

No - they'd be going into adjacent terminals of a single accessory in a single enclosure.

so you isloate in the normal way by pulling the individual circuits fuses

That's not a single point of isolation.

It's not even isolation; AIUI isolation requires all poles to be disconnected.

For most domestic installations that will be the Big Red Switch, depending on how the RCDs are arranged and whether they disconnect both L and N with sufficient contact gap to be an isolator.

 

[JohnW2]

It's not even isolation; AIUI isolation requires all poles to be disconnected.

That's how I was brought up, too. However, per 537.2.1.1, BS7671 does not require isolation of neutral with TN-S or TN-C-S systems (hence, by implication, it does require DP isolation with TT).

Kind Regards, John.

 

[Paul_C]

However, per 537.2.1.1, BS7671 does not require isolation of neutral with TN-S or TN-C-S systems (hence, by implication, it does require DP isolation with TT).

And if you think about the implications of that, it makes very little sense.

Older editions of the Wiring Regs. made no such distinction, although they did still have the inconsistency of requiring an isolating switch to disconnect both poles of a 2-wire supply but not to disconnect the neutral of a 3-wire or 3-phase supply.

 

[JohnW2]

However, per 537.2.1.1, BS7671 does not require isolation of neutral with TN-S or TN-C-S systems (hence, by implication, it does require DP isolation with TT).

And if you think about the implications of that, it makes very little sense.

It certainly doesn't make much sense to me. Their 'explanation' is that, with TN-S and TN-C-S, the neutral is "regarded as reliably connected to Earth via a suitably low impedance" - but that, of course, assumes that no supply-side faults are present.

Kind Regards, John.

 

[Paul_C]

Their 'explanation' is that, with TN-S and TN-C-S, the neutral is "regarded as reliably connected to Earth via a suitably low impedance" - but that, of course, assumes that no supply-side faults are present.

And exactly the same distribution network can be feeding various houses, some of which are connected to the supplier's neutral for earthing resulting in a TN-C-S arrangement, and some of which use only their own local earth rods resulting in a TT arrangement. So exactly the same distribution neutral somehow needs to be disconnected in one case but not the other?

 

[JohnW2]

Their 'explanation' is that, with TN-S and TN-C-S, the neutral is "regarded as reliably connected to Earth via a suitably low impedance" - but that, of course, assumes that no supply-side faults are present.

And exactly the same distribution network can be feeding various houses, some of which are connected to the supplier's neutral for earthing resulting in a TN-C-S arrangement, and some of which use only their own local earth rods resulting in a TT arrangement. So exactly the same distribution neutral somehow needs to be disconnected in one case but not the other?

Indeed. I'm not sure that I understand their reasoning at all. The 'low impedance path to earth' from a TN-C-S installation's neutral is all due to the supply's neutral conductor itself, and the same is also at least partially (possibly largely) true with TN-S. So why is the same not also true with a TT supply, even if it only supplies TT installations?

Kind Regards, John.