Why the tap?

[JohnW2]

If you were to look into cold cathode neon lighting, you'd find a large number of products made for domestic/hospitality interior lighting which work at a little under 1000V. I've always assumed that's to get it to qualify as LV, instead of the HV normally used by neon signs.

I can believe it.

However, that doesn't alter what EFLI said. To arbitrarily categorise voltages is really not very helpful - one needs to know the actual voltage (or range of voltages) concerned. It is crucial to know whether something is designed for, say, 220-240V or "just under 1000V", just as it is essential to know whether something BS7671 would call 'ELV' is designed for 6, 12, 24 or 48V.

Kind Regards, John

 

[ban-all-sheds]

Indeed.

How many general purpose stores (eg supermarkets), DIY stores, hardware shops or even specialist lighting shops describe anything other than 12V products as "low voltage"?

 

[winston1]

Indeed.

How many general purpose stores (eg supermarkets), DIY stores, hardware shops or even specialist lighting shops describe anything other than 12V products as "low voltage"?

That does NOT make it right though does it?

 

[ban-all-sheds]

Whether it does or not is open to question.

Whether your bleating about it is welcome or useful is not. It isn't.

 

[JohnW2]

How many general purpose stores (eg supermarkets), DIY stores, hardware shops or even specialist lighting shops describe anything other than 12V products as "low voltage"?

Quite so - a point I've made many times. Go into any of those outlets (with the possible exception of some of the last) and ask for "low voltage" products, and it's extremely unlikely that you would be offered 230V products, let alone 900+V ones - so, for the consumer, the 'correct' meaning of "low voltage" is really of no use/relevance, and (if they are 'taught' or forced to use it) seemly results only in confusion.

Whilst we are at it, if you go to any of those outlets and ask for 'lamps', you will very probably be offered fittings, not 'bulbs' - and if you ask for 'a lamp for your lamp' you will almost certainly get some very funny looks!

I know it's controversial, and that there are dissenters, but if one is making and selling 'consumer' products, it seems to me to make a lot of good sense to describe the products in the language which the great majority of consumers use and understand!

Kind Regards, John

 

[EFLImpudence]

It would be just as easy and not open to confusion to label them '12V' as they do for car products.

That excuses are made for doing it confusingly is beyond me.

 

[JohnW2]

It would be just as easy and not open to confusion to label them '12V' as they do for car products.

I agree totally.

I'm no sure that car products really even need the "12V", it really just needs to be made clear that they are products for cars - since, as far as I am aware, no 'ordinary' car these uses anything other than 12V products.

... and there's really no need to even think about describing them as 'low voltage' - since, unlike homes, no products used in cars (other than the ignition!) use voltages a lot higher than 12V. If cars had traditionally always had 250V bulbs but, more recently, a change to 12V had started to happen (such that both voltages were now in use), then I imagine that it's quite likley that we would have seen the 12V ones being described as "low voltage".

Kind Regards, John

 

[EFLImpudence]

I agree to tally.

Well - not totally, because -

I'm no sure that car products really even need the "12V", it really just needs to be made clear that they are products for cars - since, as far as I am aware, no 'ordinary' car these uses anything other than 12V products.

No, but Lorries do (and they look identical) yet they don't call cars low voltage.

... and there's really no need to even think about describing them as 'low voltage' - since, unlike homes, no products used in cars (other than the ignition!) use voltages a lot higher than 12V. If cars had traditionally always had 250V bulbs but, more recently, a change to 12V had started to happen (such that both voltages were now in use), then I imagine that it's quite likley that we would have seen the 12V ones being described as "low voltage".

Not really, 220/240V are marked with the voltage so there was no need to mark 12V ones with anything other than 12V.
Obviously, they would not have called it low voltage had they know what they were talking about.
Particularly as you, yourself, have said it could be 6, 24 or 48V.

 

[JohnW2]

Well - not totally, because - ...

I agreed totally that 'they' (12V lamps/bulbs for use in the home) should simply be labelled "12V". I went on to say that, in the case of "car bulbs" (I don't think I have ever seen them called anything but bulbs in Halfords), even that may not be necessary.

However, lest this turns into yet another lengthy exchange, let me remind you that I have already agreed that any electric product needs only to be labelled with the actual operating voltage, without any statements about whether the voltage is extra low, low, medium high or anything else!

Mind you, when two substantially different voltage 'ratings' exist within a similar context, I don't think you'll ever stop everyday language talking about 'low' and 'high', even if the actual voltage would tell the same story (and more) - just as we talk about 'low' and 'high' fibre/fat/sugar/cost/speed/etc. etc. etc. Indeed, you are probably old enough to remember when radios etc. were powered by LT and HT ('low tension' and 'high tension') batteries - even though could have been called 'just' 3V and 90V (or whatever) batteries!

Kind Regards, John

 

[ericmark]

Back to original first, the design allows a fault on the bell to draw what ever current the transformer can give without blowing a fuse even when non of the bell pushes are pressed, why it uses a PELV supply rather than a SELV supply I don't know? However there is clearly an error.

As to low voltage and extra low voltage, in the USA they call 12 volt low voltage so it is common to have wrong labels on items.

I think some one has got a standard bell transformer and has in error shown it as PELV rather than SELV in the circuit diagram. I would guess that earth wire should not be there or on the centre tap. If the transformer is classed as an isolation transformer then no real need for the earth wire. I don't understand why all the posts seem to centre on the name, rather than the real issue which is the earthing before the fuse so the 4 volt fuse does nothing, that fuse should have been on the centre tap, or the earth should be on centre tap.

 

[bernardgreen]

a PELV supply rather than a SELV supply

The reason is the "what if" approach to designing systems. What if the transformer isolation fails and mains potential comes into contact with the wiring to the door bell push on the front door. Even though the transformer in those days was invariably a split bobbin construction the miniscule risk of mains potential crossing the isolation was considered under "what if" events.

 

[ericmark]

The reason is the "what if" approach to designing systems. What if the transformer isolation fails and mains potential comes into contact with the wiring to the door bell push on the front door. Even though the transformer in those days was invariably a split bobbin construction the miniscule risk of mains potential crossing the isolation was considered under "what if" events.

It would clearly be dependent on the rating of the transformer, however if it needs an earth, then it should have been connected to the centre tap which has no fuse, not the 4 volt tap before the fuse. As shown there is no current limiting device between supply to bell and earth, other than what the transformer can supply.

 

[bernardgreen]

The other reason for earthing the secondary is the "wet door bell tingle". With no Earth connected to the transformer's core the secondary can float anywhere between Neutral and Live potentials due to capacitive coupling between primary and secondary windings. The capacitive coupling at 50Hz is too small ( hence high impedance ) for a hazardous current to flow into the wet finger pressing a wet front door bell push button. But the current can be enough to produce a worrying tingle in that finger.

 

[ban-all-sheds]

I'm no sure that car products really even need the "12V", it really just needs to be made clear that they are products for cars - since, as far as I am aware, no 'ordinary' car these uses anything other than 12V products.

https://en.wikipedia.org/wiki/42-volt_electrical_system

https://www.cnet.com/videos/car-tech-101-the-move-to-higher-voltage-electrical-systems/