Two-way lighting circuits

[D.M. Procida]

At (very near the bottom of the page) there are two two-way lighting circuits represented in diagramatic form.

Is one preferred? The second one to me looks simpler. Between the switches all you need is two-core (plus earth). On the other hand, the default example given in most manuals seems to be the first one, which requires three-core (plus earth).

Why would anyone choose the first one over the second? The only difference I can see is the connection between the two switches, and in the second case it looks more complicated.

Actually having drawn them both out on paper a few times I can see another difference: the first version goes (more or less):

live -> switch 1 -> switch 2 -> light

and the second goes:

live -> switch 1 -> light | | --> switch 2

which might make it easier in some circumstances...

Would a house with older (i.e. cloth-covered) wiring be more likely to use one form over the other?

Many thanks for any advice or answers.

Daniele

[Fash]

Daniele

You should use the version that has the three core cable between switches simply because anyone else who looks at the circuit in the future will understand what is going on. There are no real problems with the second form of the circuit, but why not go the conventional route when the cost and complexity of the two are pretty much the same. Three core and earth in harmonised colours (grey, brown and black instead of red, yellow and blue) is available from screwfix. You do need to check which new colour is equivalent to which new colour as I can't remember off the top of my head, but you should get this easily enough somewhere on the web.

Before anyone else gets the chance to say it, if you have a circuit with cloth covered wire, you should think about how old it is likely to be and consider complete replacement. In my personal experience with old circuits (in my case lead sheathed 1920's wiring with no earth) it is best not to alter them unless by complete replacement.

Fash

D.M. Procida wrote:

[John Rumm]

The second one is the "standard" one, and is very easy to wire - you wire the switch closest to the lamp using L1 and L2 as live in and switched live out - and then run three core and earth to the next switch strapping all three connections to their counterparts in the switch.

This second circuit keeps the current flowing up and down separate conductors in the strap cable, which is preferable for reducing interference with other electronic and electrical systems (particularly things like inductive loop systems for hearing aids).

It actually looks much simpler if you draw it thus:-

N_____________________________________ | L_______________ ______________ | | | | | | | | | | | | | | | \X/ LAMP | | L1 L2 SW1 |\ | | C | | | | | | | | | | | | | | | | | C | SW2 | \| L1 L2

Who cares - it needs rewiring as a matter of urgency!

[D.M. Procida]

Thanks, it does.

Next questions -

1. the old switches aren't earthed - they aren't in a metal box, just in a little cavity hacked in the brick wall, and the switches have no earth terminal in them. Therefore any earth cable to the switches wouldn't actually have anything to connect to... How important is it to have an earth to the switches?

This is an issue because though the downstairs switch is reasonably accessible and could be so earthed if necessary, the upstairs one would probably require a lot of hacking at plaster and black mortar to do that, and likely tearing the loft to pieces as well. So if it's not deemed absolutely necessary it might be much easier not to do it.

1. This is actually a two-gang two-way system - two switches both up and downstairs for hall and landing lights:

L N L N | | | | L1 L2 L1 L2 SW1 |\ | |\ | SW1 | C | | C | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | C | | C | SW2 | \| | \| SW2 L1 L2 L1 L2

Would it be OK to leave out L1-L1 on the right-hand pair of switches, and simply take a little jumper from L1 from the left-hand one in each case? It would save additional cable in some very tight cable spaces if so.

That was what I thought - the electricians were much much more relaxed about it than me, and so apparently was the inspector who came round.

Thanks again,

Daniele

[David Hansen]

On Sat, 25 Feb 2006 09:09:10 +0000 someone who may be [e-mail] (D.M. Procida) wrote this:-

If the switches are plastic, not important at all. However, whether it is important or not is irrelevant, it is a requirement. It is a requirement for the important reason that if someone was to change the switch an earth connection may be necessary.

If the existing cable is going to be replaced then what great difference does the presence or absence of a protective conductor make to the amount of hacking and tearing?

[Dave Plowman (News)]

The big snag with the second way is it requires connection to the ceiling rose from both switches - and only uses one cable from each.

Normally, the switch pair goes to only one switch, so the preferred method is run to the next one in Triple and Earth.

The second way was, however, common many years ago when wiring was run using single core rubber covered wiring.

[Jim Gregory]

"D.M. Procida" wrote in message news:1hbad1g.p94hcmvor9e0N% [e-mail]...

I prefer bottom (2nd) layout and use 3-core + Earth between 2-way switches. Both toggles should be up to make Lamp Off, if/when the pair is wired correctly. One day, you may want to achieve 3-way switching, simply by inserting in the

3+E wiring path an Intermediate switch (4 terminals) whereby L1 and L2 status lines are inverted. Jim

[Dave Plowman (News)]

Makes no difference which method is used as regards later fitting of an intermediate.

[John Rumm]

Not having an earth is ok so long as you have no light switches or fittings with exposed metal parts.

Note that the regulations are not retrospective either - so there is no requirement to change existing installations to match new requirements unless you are rewiring or making significant changes to them.

(Note for completeness - remember that what you have shown as "N" above is still a live - the switch out connection)

The implication of doing that is that you are sharing a live feed between both up and downstairs lights. If there is only one lighting circuit for the whole house then this would be ok. If however you have up and downstairs on two separate circuits then it would be unwise to do this since each floors lamp out to be powered from the appropriate circuit.

The reality would normally be slightly different from the way you drew it with two circuits:

Landing Light Hall Light

L SW out | | L1 L2 L1 L2 SW1 |\ | |\ | SW1 | C | | C | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | C | | C | SW2 | \| | \| SW2 L1 L2 L1 L2 | | L SW Out

So the power feed for the landing light comes from upstairs and the feed for the hall comes from downstairs.

The electrician was probably thinking "not my house" and the inspector would not be in a position to comment unless you just had the circuit installed like that!