Why does the lamp have a coil?

[ban-all-sheds]

No, I am not blocking weserv.

I think it's another case of that poxy weserv site not working properly. Have you really forgotten that there have been times when you couldn't see images which I could?

 

[JohnW2]

Yes, and it still happens occasionally - but at least I then see icons/placeholders in the post (whereas you appear to have seen nothing in this case) and, more to the point, do see the images in the reply window if I quote them.

 

[ban-all-sheds]

If I refresh the page I briefly see two tags (no closing ones), but they disappear almost immediately.

Just checked, and I see them if I quote the post, which does put paid to the blocking theory.

 

[DIYspanner]

Why can a voltmeter detect 11v ac between the transformer output wires but not between either of the wires and earth?

 

[EFLImpudence]

...because neither of the output terminals is connected to earth.

Think of the output like a battery.

 

[ban-all-sheds]

...because neither of the output terminals is connected to earth.

Unless Bernard has got at it.

 

[bernardgreen]

Unless Bernard has got at it

Whether the output should be earthed or need not be earthed depends on the quality of the transformer. Specifically the quality of the insulation barrier between primary and secondary windings.

 

[DIYspanner]

Well there's no earth wire on the transformer!

Double insulated perhaps?

 

[333rocky333]

The case of the light is earthed, that type of transformer is just 2 copper coils that do not need to be earthed.

 

[bernardgreen]

Double insulated perhaps?

Bottom end toroidal transformers depend on nothing more then the insulation lacquer on the winding wire for the isolation of the secondary from the primary.

Better quality transformers have a wound layer of insulation material between windings

 

[JohnW2]

The case of the light is earthed, that type of transformer is just 2 copper coils that do not need to be earthed.

True, but perhaps closer to what bernard was talking about, it is possible to have an earthed screen between primary and secondary of the transformer (and to earth the metal parts of metal-cored/-framed ones) - to pick up what would otherwise be leaks between primary and secondary, rather than protection of people 'outside of the case'.

Other than the trivial case of shaver sockets, I have no experience of 'floating' (230V) supplies in domestic installations, and I'm trying to think about what (if anything) the regs may have to say about them (i.e. are they even 'allowed'?). I'm separated from my BBB at the moment, so am not sure!

Kind Regards, John

 

[bernardgreen]

I have difficulty trusting the insulation between primary and secondary windings. Only when the transformer has a split bobbin or twin bobbins do I trust that the secondary will never become Live.

This is based on an incident where an incoming pair ( similar to a direct telephone line ) on a control room became Live and damaged equipment. It also gave the technician a non lethal shock. This was due to the breakdown of the insulation in a transformer ( single bobbin ) at a remote site.

To prevent a repeat ocurrance an additional safety transformer was wired into the supply at the remote sites until the suspect transformers could be investigated.

 

[JohnW2]

I have difficulty trusting the insulation between primary and secondary windings. Only when the transformer has a split bobbin or twin bobbins do I trust that the secondary will never become Live.

I can understand that view, at least when the secondary is ELV.

However, it could be said that it's really just a specific case of any insulation which exists to protect people from contact with hazardous voltages. The design and manufacture of a transformer obviously should ensure adequate insulation/protection, as should the insulation of cables, accessories, items of equipment or whatever. Do you trust any insulation?

Kind Regards, John

 

[333rocky333]

True, but perhaps closer to what bernard was talking about, it is possible to have an earthed screen between primary and secondary of the transformer (and to earth the metal parts of metal-cored/-framed ones) - to pick up what would otherwise be leaks between primary and secondary, rather than protection of people 'outside of the case'.

Kind Regards, John

I did read up on this stuff years ago, when we looked at a sauna where the alarm people had installed a 24 volt panic alarm, i believe the most safe ones are medical grade units, as for the OPs case id quess they only need to be the basic type, the only risk from failure would be over volting the lamp really, which is totally enclosed

 

[JohnW2]

I did read up on this stuff years ago, when we looked at a sauna where the alarm people had installed a 24 volt panic alarm, i believe the most safe ones are medical grade units, as for the OPs case id quess they only need to be the basic type, the only risk from failure would be over volting the lamp really, which is totally enclosed

Indeed.

To clarify what I wrote before about isolated supplies (and not knowing what the regs had to say about them), I was, of course, talking about transformers with LV (rather than ELV) secondaries - probably in view of Simon35's recent thread about LED lighting fed from a 440V->230V transformer. In the case of ELV, SELV is obviously fully recognised and certainly 'allowed'. I suppose 'SLV' would be the abbreviation for what I was talking about!

Kind Regards, John