This is probably more theoretical than anything else, since I imagine that common sense probably usually prevails (even if perhaps 'despite the regs'), but I would appreciate some assistance in interpreting the reg's intentions as regards isolation in TT systems:
(1)...537.2.1.1 appears to imply that, with a TT system, there must be a facility for isolating every circuit which includes isolation of the neutral conductor. However, it also says that "provision may be made for isolation of a group of circuits by a common means". Does this mean that an RCD or main switch in a CU would satisfy this requirement for a means of DP isolation for all the circuits supplied by the device?
(2)...5.1.1.iii of the OSG appears to interpret 537.2.1.2 of the regs as meaning that, in addition to the ability to isolate circuits, every 'item of equipment' needs 'a means of isolation', hence seemingly DP isolation for a TT system (per 537.2.1.1). Is that how 537.2.1.2 is generally interpreted? If so, more importantly, is this to be intepreted as meaning that each item of equipment requires its own means of (seemingly DP for TT) isolation - or, as in (1) above, do the regs intend that isolation of a group of circuits (at the CU) including the one serving the item would be adequate? If not, everything permanently wired, including every light fitting, would presumably theoretically require a means of independent DP isolation with a TT system ('plugged' in appliances/equipment are preumably OK, since the plug/socket is an acceptable means of isolation).
(3)...Thinking about the implications, specifically in relation to my installation, I think that any potential issues would probably be mainly in relation to lighting and some fans - since most other things are likely to have plug/socket, DP FCU or DP switch available for isolation. With lighting, it could get silly, so I think it's probably best to stick with common sense! As for extractor fans, my house has several. The ones with timers (i.e. with constant L, as well as switched L, feeds) have the standard 3-pole isolator switches (hence isolating neutral). However, some are simple 'on-off' fans with simple SP switches (mainly pull switches) - seemingly no worse than any light bulb or lighting fitting but obviously without any local isolation of neutral, so I wonder what the regs really intend there.
(4)...In passing, although it's not something I would ever contemplate, am I interpreting 537.2.2.5 correctly? On the face of it, it seems to be saying that physically adjacent, but separate, isolators for L and N could be acceptable - even though that opens the dangerous possibility of interupting N but not L.
Kind Regards, John
(1)...537.2.1.1 appears to imply that, with a TT system, there must be a facility for isolating every circuit which includes isolation of the neutral conductor. However, it also says that "provision may be made for isolation of a group of circuits by a common means". Does this mean that an RCD or main switch in a CU would satisfy this requirement for a means of DP isolation for all the circuits supplied by the device?
(2)...5.1.1.iii of the OSG appears to interpret 537.2.1.2 of the regs as meaning that, in addition to the ability to isolate circuits, every 'item of equipment' needs 'a means of isolation', hence seemingly DP isolation for a TT system (per 537.2.1.1). Is that how 537.2.1.2 is generally interpreted? If so, more importantly, is this to be intepreted as meaning that each item of equipment requires its own means of (seemingly DP for TT) isolation - or, as in (1) above, do the regs intend that isolation of a group of circuits (at the CU) including the one serving the item would be adequate? If not, everything permanently wired, including every light fitting, would presumably theoretically require a means of independent DP isolation with a TT system ('plugged' in appliances/equipment are preumably OK, since the plug/socket is an acceptable means of isolation).
(3)...Thinking about the implications, specifically in relation to my installation, I think that any potential issues would probably be mainly in relation to lighting and some fans - since most other things are likely to have plug/socket, DP FCU or DP switch available for isolation. With lighting, it could get silly, so I think it's probably best to stick with common sense! As for extractor fans, my house has several. The ones with timers (i.e. with constant L, as well as switched L, feeds) have the standard 3-pole isolator switches (hence isolating neutral). However, some are simple 'on-off' fans with simple SP switches (mainly pull switches) - seemingly no worse than any light bulb or lighting fitting but obviously without any local isolation of neutral, so I wonder what the regs really intend there.
(4)...In passing, although it's not something I would ever contemplate, am I interpreting 537.2.2.5 correctly? On the face of it, it seems to be saying that physically adjacent, but separate, isolators for L and N could be acceptable - even though that opens the dangerous possibility of interupting N but not L.
Kind Regards, John
