WC Lighting Options

[securespark]

I look at the situation differently.

To me, it's not about "doing it that way because it is always done that way".

Because my work has always been inspected by at least one other party, I think making my work beyond scrutiny is the more intelligent considered pathway.

 

[JohnW2]

To me, it's not about "doing it that way because it is always done that way". Because my work has always been inspected by at least one other party, I think making my work beyond scrutiny is the more intelligent considered pathway.

I don't want to appear to be taking sides here, but I think what you say in that second sentence is probably the main mechanism whereby "doing it that way because it has always been done that way" gets perpetuated ('for ever and ever'), without those 'inspectors' thinking very much (if at all) about the wisdom/necessity of "how it's always been done". In other words, such practices are essentially self-perpetuating.

Kind Regards, John

 

[JohnW2]

I don't want to appear to be taking sides here, but I think what you say in that second sentence is probably the main mechanism whereby "doing it that way because it has always been done that way" gets perpetuated ('for ever and ever'), without those 'inspectors' thinking very much (if at all) about the wisdom/necessity of "how it's always been done". In other words, such practices are essentially self-perpetuating.

I would add that this phenomenon applies to regulations as well as 'practices', and there are a good few examples I could cite.

One of the most obvious cases is the way in which the Wiring Regs/BS7671 has required 'maximum Zs' to be determined in order to provide satisfactory ADS. For decades, the regs specified a method of calculating this which left a substantial proportion of installations (those with a supply voltage less than 'nominal') under-protected, even though compliant with the regs. The proportion of under-protected installations fell when Uo was bureaucratically reduced to appreciably below the average UK supply voltage, but there were nevertheless a lot of installations that were still under-protected.

This decades-long anomaly was eventually addressed - almost completely, although it still leaves those few with supply voltages between 94% and 95% of 'nominal' under-protected.

As with practices, one can but presume that the anomaly persisted for decades because "that's the way it's always been done", with no-one stopping to think about whether that 'long-standing situation' was actually correct - even though, had they bothered to think, most of those responsible for the content of the regs were more than capable of realising that the 'status quo' was not satisfactory.

Kind Regards, John

 

[EFLImpudence]

Can't argue with that in the worst case scenario if that is what is intended, but I might say that a certain amount of 'diversity' could still apply.

That is - your worries are valid only when the voltage is at the minimum, the conductor temperature is at its maximum and the Ia of the OPD is at its maximum of 5xIn.
I realise we don't know thes things at the time of a fault but for all to occur at once is rather unlikely.

 

[JohnW2]

Can't argue with that in the worst case scenario if that is what is intended, but I might say that a certain amount of 'diversity' could still apply. .... That is - your worries are valid only when the voltage is at the minimum, the conductor temperature is at its maximum and the Ia of the OPD is at its maximum of 5xIn. ... I realise we don't know thes things at the time of a fault but for all to occur at once is rather unlikely.

That's all true, up to a point. I would think that (as with most other regs), the intention probably was to take into account something approaching the 'worst case scenario' but, in any event, I think that there are a few things to consider:

1... For many of the years/decades in question (before Cmin, and at a time when Uo was roughly equal to the average supply voltage), up to half of all installations (those with a supply voltage <Uo) will have experienced the 'calculation anomaly' to at least some extent.

2... Probably the most important time that one considers such things is when testing a new circuit, before it has ever been subjected to any significant loads. One will therefore be measuring Zs on cables which are at ambient temperature - and, particularly, if that is on a not-yet-occupied new build, in the middle of winter, that could be a very low temperature. The Zs measured may therefore appreciably underestimate the Zs that would be seen 'in normal service'.

It's therefore very possible that, at least in the past (whilst the 'traditional' calculation was in operation), supply voltage may have been appreciably below the figure used for calculation (even though rarely 'at its minimum permitted), and with the measured Zs being well below what it would be in normal service. The big unknown (at least for me) is the question of the magnetic trip threshold of an MCB, since that not something that we can realistically measure. In other words, I don't know what, in practice, that threshold is likely to be (other than that it should be no greater than 5 x In for a Type B). However, given that it presumably could be as high as 5 x In for a Type B (and that one can't measure it), I don't think one has much choice other than to assume that 'worst case'.

However, in a sense this is a diversion from my point, since I very much doubt that any thought went into any of this on the part of those who write the regs. I strongly suspect that the specified calculations/limits remained unchanged for so long because "that's how they have always been", not due to any positive decision relation to the consideration of 'sub-worst case scenarios'. For example, although I may be wrong, I don't think they changed the 'maximum Zs figures' when Uo (hence the result of calculations) suddenly changed, did they?

Kind Regards, John

 

[flameport]

However, as far as I can make out, the 'pull switch tradition' is by no means 'rooted in' BS7671

It originated there but isn't a requirement. It's probably due to someone in the distant past 'interpreting' the regulations in a particular way, making some recommendation, and from that point on it became 'the regs' even though it wasn't. A familiar story that is repeated to this day.

This is regulation 405, from the 1955 13th edition, regarding switches and controls in bathrooms:

Nothing there requiring switches to be pull cords, or outside the room. Only that they must be out of reach, which is equivalent to the zones in the regulations today - outside of zone 2 is 600mm from the bath, which for most people is out of reach.
The rest is also remarkably similar to today, with no provision for portable appliances (i.e. socket outlets), and lampholders having protective shields.

Regulation 314 referred to there looks like this:

so for appliances installed in bathrooms, the choices are a fixed pull cord in the room, or a switch outside the door of the room.
Note that this is for appliances only, the whole of 314 being applicable to non-portable appliances.

The definition of 'appliance' in the same regulations is this:

and that definition specifically excludes lighting and motors.

Therefore if a switch was in a bathroom and it controlled something other than a light or motor, a pull cord switch was a requirement. The alternative being a switch outside the room adjacent to the door.
For lighting, normal switches were allowed in the room, and have been ever since, and still are - provided they are out of reach of someone in the bath, or in the more recent editions outside of zone 2, which amounts to the same thing.

If people really want one, there is nothing to prevent pull cord switches being used for lighting in bathrooms or any other location, but it is not a requirement and never was.

 

[JohnW2]

It originated there but isn't a requirement. It's probably due to someone in the distant past 'interpreting' the regulations in a particular way, making some recommendation, and from that point on it became 'the regs' even though it wasn't. A familiar story that is repeated to this day.

Indeed so!

This is regulation 405, from the 1955 13th edition, regarding switches and controls in bathrooms: .... Nothing there requiring switches to be pull cords, or outside the room. Only that they must be out of reach, which is equivalent to the zones in the regulations today - outside of zone 2 is 600mm from the bath, which for most people is out of reach.

Indeed - as you say, not really much different from today - virtually all the pull switches in bathrooms (in the UK) one sees (including, I confess, those in my house) are just inside the door, usually well away from any zones.

Regulation 314 referred to there looks like this: .... so for appliances installed in bathrooms, the choices are a fixed pull cord in the room, or a switch outside the door of the room. Note that this is for appliances only, the whole of 314 being applicable to non-portable appliances. .... The definition of 'appliance' in the same regulations is this: .... and that definition specifically excludes lighting and motors.

Right - that seems clear enough - so, as you go on to say ...

Therefore if a switch was in a bathroom and it controlled something other than a light or motor, a pull cord switch was a requirement. The alternative being a switch outside the room adjacent to the door. For lighting, normal switches were allowed in the room, and have been ever since, and still are - provided they are out of reach of someone in the bath, or in the more recent editions outside of zone 2, which amounts to the same thing. If people really want one, there is nothing to prevent pull cord switches being used for lighting in bathrooms or any other location, but it is not a requirement and never was.

Quite so!

Kind Regards, John

 

[ban-all-sheds]

As with practices, one can but presume that the anomaly persisted for decades because "that's the way it's always been done", with no-one stopping to think about whether that 'long-standing situation' was actually correct - even though, had they bothered to think, most of those responsible for the content of the regs were more than capable of realising that the 'status quo' was not satisfactory.

I don't know if it ever got rectified, but I remember my Chemistry teacher drawing our attention to several alleged 'chemical reactions' (I certainly can't remember which!) that had been perpetuated through generations of textbooks, but were not actually reactions which happened!

 

[JohnW2]

Yes, I almost cited my 'Chemistry textbook anecdote', but I realised that it was actually rather different (and, I would say, probably 'more excusable') than the issue with the regs that I was discussing. In the later case, most, if not all, of those responsible for the content of the Wiring Regs/BS76671 will have had adequate basic electrical knowledge to enable them to have realised (had they thought about it) that there was an anomaly in their requirements.
On the other hand, there was not really any theoretical basis on which the authors of subsequent chemistry texts could have known that the 'reaction which never happened' was not real, and they could not really have been expected to personally verify every reaction mentioned in their book - so that, in some senses, they perhaps had little choice but to accept what appeared to be 'established facts'.

 

[ban-all-sheds]

Tongue maps?

"Have your cake and eat it"?

"Let them eat cake"?