Is there a revised lights wiring diagram?

[ericmark]

We have traditionally treated our household AC supply like DC and ignored balanced feeders and transmission line rules.

With twin and earth the line and neutral are nearly the same spacing as 300Ω feeders and if we take the supply to the switch and then to the lamp everything works OK.

But traditionally we take supply to lamp and then only the line goes to the switch and this can cause LED's to remain lit dim and CFL to flash because the capacitance and inductance in the cable no longer match.

I am about to start re-wiring my mothers lights as there are no earths and for simple switching taking supply to switch is OK but for two way it would make it rather complex and also increase the volt drop.

I am a great believer is standard so when some one comes to repair it's wired as expected and also no point in re-inventing the wheel.

So in the view of using LED lighting has there been a change in practice where neutrals are run with live to stop lights coming on dim.

I am aware one can use inductance and capacitance to correct the out of balance and fitting suppressors is now common across the lamp when LED is retro fitted. But what about when starting from scratch is there a new standard method of wiring which keeps neutral with line to stop the dim light caused by ignoring transmission line rules?

 

[bernardgreen]

At 50 Hz the transmission line effect in standard twin and earth cable is negligible and can be ignored in the context of LED lamps glowing when OFF and similar effects.

The problem is 99.9% due to the stray capacitive coupling between Permanent Live and Switched Live in the cables to switches.

When an Earthed CPC is inside the cable and between the Permanent Live and the Switched Live the effect is reduced but not eliminated.

Adding a fourth neutral conductor is not going to significantly reduce the effect as it is electrically almost the same as the CPC

Running a Permanent Live conductor along side a Neutral and / or earthed CPC conductor creates a electro-static field ( for want of a better description ) with potential of 115 volts RMS ( mid point of 0 volts and 230 volt ) Anything close to these conductors will have that 115 volts coupled to it by stray capacity. (*)

One solution would be to screen the Switched Live using co-axial cable with the screen earthed or connected to Neutral ( TV aerial coax is NOT suitable cable for this even as a trial ).

Another would be to run the Permanent Live to switches in a totally separate route. Permanent Live horizontally looped to the switches and Switched Lives vertically up from the switches.

Relays at the lamp location are another option.

But the most cost effective method does seem so far to be the snubber across the lamp. ( snubber = contact suppressor of cap and resistor )

(*) A person close to a switch drop cable and standing on a well insulted carpet would have a voltage induced on them and hence a neon screwdriver held by them and touching Earth or Neutral may glow dimly giving a false "Live" reading or not glow when touching a Live and thus give a false and dangerous indication of not Live.

 

[Taylortwocities]

But what about when starting from scratch is there a new standard method of wiring which keeps neutral with line to stop the dim light caused by ignoring transmission line rules?

Loop in at the switch, then line and neutral (and cpc) are routed together to the LED light(s)

 

[plugwash]

At 50 Hz the transmission line effect in standard twin and earth cable is negligible and can be ignored in the context of LED lamps glowing when OFF and similar effects.

The problem is 99.9% due to the stray capacitive coupling between Permanent Live and Switched Live in the cables to switches.

When an Earthed CPC is inside the cable and between the Permanent Live and the Switched Live the effect is reduced but not eliminated.

Adding a fourth neutral conductor is not going to significantly reduce the effect as it is electrically almost the same as the CPC

Running a Permanent Live conductor along side a Neutral and / or earthed CPC conductor creates a electro-static field ( for want of a better description )

Normally one would just say "electric field".

with potential of 115 volts RMS ( mid point of 0 volts and 230 volt )

The actual potential will dpened on what position you look at. Assuming a normal T&E layout the switched live will see much more coupling to the earth than to the permanent live because it is much closer to it.

3+E tends to be more problematic because in many two way switching layouts you have a conductor connected to the live supply and a conductor connected to the live of the load running next to each other with no earth in between.

What would probablly work well for 2 way switching would be to take permanent live, neutral and earth to the first switch, then use the "end feed" style of two way switching with the strappers on the outer cores of the 3+E and neutral and earth on the center cores.

Coupling between different cables running together is also a possibility but likely to be much weaker than the coupling to other cores in the same cable.

P.S. there is absoloutely no need to invoke transmission line theory to explain any of this. As a rule of thumb you have to move from lumped element models to transmission line models when you start thinking about things that are a substantial fraction of a wavelength in size.

 

[ban-all-sheds]

One solution would be to screen the Switched Live using co-axial cable with the screen earthed or connected to Neutral ( TV aerial coax is NOT suitable cable for this even as a trial ).

No, but BS 8436 cable might be.

And until TTC spoiled the party by being Mr Sensible, I was going to suggest using single-core pyro.

 

[JohnW2]

The actual potential will dpened on what position you look at. Assuming a normal T&E layout the switched live will see much more coupling to the earth than to the permanent live because it is much closer to it.

What am I missing - when I last looked at T&E, it's layout was totally symmetrical?

There is, of course, much more of a potential issue with 3C+E, when used in certain ways.

Kind Regards, John

 

[plugwash]

In a typical T&E switch drop you have permanent live on one side, switched live on the other and earth in the middle.

We don't care (in this context) about the coupling between permanent live and earth, we only care bout how the switched live couples to the other cores and due to the geometry of the fields it will couple much more strongly to the earth (which is next to it) than to the permanent live (which is seperated from it by the earth)

 

[JohnW2]

P.S. there is absoloutely no need to invoke transmission line theory to explain any of this. As a rule of thumb you have to move from lumped element models to transmission line models when you start thinking about things that are a substantial fraction of a wavelength in size.

Quite so - and, if my mental arithmetic serves me correctly, the wavelength of 50Hz is 6,000 km - not all that much less than the diameter of the earth!!

Kind Regards, John

 

[JohnW2]

We don't care (in this context) about the coupling between permanent live and earth, we only care bout how the switched live couples to the other cores and due to the geometry of the fields it will couple much more strongly to the earth (which is next to it) than to the permanent live (which is seperated from it by the earth)

Apologies - I misread your post As a result of too-hasty reading, I thought you were suggesting that there would be more coupling between switched live and earth than between permanent live and earth - which clearly would have made no sense!

Kind Regards, John

 

[ericmark]

So it would seem only problem is with intermediate lighting switches? Looking at a standard triple and earth

if grey always is permanent line then the earth will stop LED's glowing. Only with intermediate switches is there a problem in that L1 and L2 are swapped so permanent line will not always be other side of earth to other two cables.

So have I read your answers correct? on/off no problem and two way as long as grey permanent line no problem only with intermediate switching is there a problem and the suppressor is used only with intermediate switching?

 

[JohnW2]

So have I read your answers correct? on/off no problem and two way as long as grey permanent line no problem only with intermediate switching is there a problem and the suppressor is used only with intermediate switching?

I think that 'no problem' is perhaps over-stating the situation, but the potential problem will certainly be considerably reduced if the live and S/L are as far apart as possible, with an earth conductor between. As you say, with 2-way switching and 3C+E, one cannot achieve that consistently - since the L and S/L will then sometimes be next to one another.

Kind Regards, John

 

[plugwash]

So it would seem only problem is with intermediate lighting switches? Looking at a standard triple and earth

if grey always is permanent line then the earth will stop LED's glowing.

Unfortunately not.

With two way switching you have the permanent live and the switch live but you also have at least one wire that depending on the position of the switches may be connected to either the permanent live or to the switched live. Capacitive coupling to and from that wire must be considered as well.

 

[EFLImpudence]

s 6,000 km - not all that much less than the diameter of the earth!!

Radius.

 

[JohnW2]

s 6,000 km - not all that much less than the diameter of the earth!!

Radius.

True - I'm getting old, and was thinking miles Whatever, whether approaching the radius or diameter of the earth, I reckon it's a fair bit longer than any domestic lighting circuit that eric is likely to come across

Kind Regards, John

 

[ericmark]

Unfortunately not.

With two way switching you have the permanent live and the switch live but you also have at least one wire that depending on the position of the switches may be connected to either the permanent live or to the switched live. Capacitive coupling to and from that wire must be considered as well.

with lights off and we are not worried about with them on the brown wire between the switches is the only line wire. As long as we use the grey wire for this link then in off there will always be an earth wire between line and dead wires.

Unfortunately I would have naturally selected the brown wire as permanent line not grey as I did with drawing. With old cable then blue would have needed to be permanent line which does go against the grain.