* Live feed to Common terminal of downstairs switch
* Light (live) connected to Common terminal of upstairs switch

However, such a method is deprecated as it leads to an induction loop being
formed, which can interfere with audio equipment, especially hearing aids
set to 'Loop' mode.

It is permissible to use the same system provided that the permanent live
and switched feeds are made to one switch. Then 3 core (and earth) cable is
used to link the two switches, taking one of the feeds through to the end
switch down the extra core.

i.e.

(Good)

L SL
| |
| | - 2 core + E
| |
| |
| | o----------------o
| +---o-- --o----+
| o----------------o |
+-------------------------------+
^
|
3 core + E

(Bad, due to induction loop)

L SL ---------------------------+
| |
| - single core |
| single core - |
| |
| o----------------o |
+-----o-- --o----+
o----------------o
^
|
2 core + E

The circuit is the same electrically. However, the routing of cables using 2
or 3 core + E cable and no looping eliminates the induction loop issue. The
use of multicore cable is strongly recommended. However, as an alternative,
if singles are used, they must go by the same route and through the same
piece of conduit.

Christian.