Looped Lighting Circuits – Electrical Installation 101.
The most common method is the ‘loop in’ system. This is where cables loop in and out of each lighting point, which is usually a ceiling rose. The wiring may be a Twin and Earth PVC/PVC cable consisting of a Line, Neutral and CPC (in), a Line, Neutral and CPC (out to the next light) and a Line, Switched Line and CPC (down to the light switch). Additional lighting points can also be looped from each ceiling rose if needed using a Switched Line, Neutral and CPC.
Two-Way Switching.
There are two methods of wiring a two-way switch. The first and most common method is to wire the light switch as usual with a Twin and Earth cable containing a Line, Switched Line and CPC down to the first switch from the lighting point. Then from this first switch run a 3-Core and CPC PVC/PVC cable to the second switch. This 3-core and CPC cable consists of two ‘strappers’ Line (black conductor) and a Switched Line (grey conductor) and also a brown conductor which is called the ‘common’.
The connections at a two-way light switch are C, L1 and L2. At the first switch using the Twin and Earth cable from the light point it’s L1 Line and L2 Switched Line. The 3-Core cable connects to C Brown, L1 Black and L2 Grey. This is the same at both switches. Remember that all CPCs must be sleeved green/yellow and connected to the back box and/or any metal switch plates, also any conductors that aren’t brown will need to be sleeved brown as this is a single phase circuit and they are all line or switched line conductors.
The second method is to run two Single core and Earth PVC/PVC cable down to each switch from the lighting point and link the two switches together using a Twin and Earth cable. The connections at the switches are; single line C and the Twin and Earth conductors, Black L1 and Grey L2 (strappers). Again all CPCs must be sleeved green/yellow and connected to the back box and/or any metal switch plates, also any conductors that aren’t brown will need to be sleeved brown. This method is most often used in conduit installations that are wired in single conductors.
Intermediate Switching.
Additional lighting switches may be installed anywhere between two-way switches but should always be of the Intermediate type. This is sometimes called a cross over switch as what it does is to swap the ‘strappers’ round when switched. As such only the strappers are connected to the switch and the common can be joined straight through using a through crimp if using a 3-core cable as in method one. If you are using method two then it’s just the strappers to connect. Again all CPCs must be sleeved green/yellow and connected to the back box and/or any metal switch plates, also any conductors that aren’t brown will need to be sleeved brown as this is a single phase circuit and they are all line or switched line conductors.
Testing a Lighting Circuit.
In order to comply with BS7671 this circuit must be inspected and tested before being put into service.
Testing
With the power off…
Continuity of Protective Conductor: The result of an (R1+R2) measurement from the distribution board to the furthest light on the circuit should be recorded.
Polarity should be checked to confirm that all switches and protective devices are on the line conductor.
Insulation Resistance: Remove all lamps and close all switches. The test needs to be repeated with the two-way switches in all positions.
With the power on…
Zs should be done by calculation (R1+R2)+Ze.
RCD disconnection if applicable done via a circuit with a socket if it’s an RCCB or completed at the distribution board for RCBOs.
If in doubt seek professional advice 🙂
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