Generally speaking, this can be a relatively straightforward procedure if you follow the manufacturer's instructions carefully. All electrical work must conform to the current IEE Wiring Regulations: BS 7671 and Part P of the Building Regulations 17th edition, see DIY Doctor Project on this. You may have problems when selling your property if it is discovered that you have run electrical installations which include the new colour wiring (blue/brown) without a certificate from a qualified electrician or electrical company.
To fit an exterior lantern of the non-metallic type, with PIR sensor and double insulated, to the exterior wall at the side of the front door, the procedure would be as follows.
Before you start, you need to determine the length of drill bit you will need to penetrate from one side of the wall to the other. If your property is a post-war build, it is likely to be a cavity wall type. The depth of the cavities have varied over the years from 13mm (half inch) to 100mm (4 inches). You will need to measure the thickness of the wall. For solid wall construction, you should choose a 300mm (12 inch) drill bit and 400mm (16 inch) for the majority of cavity walls. A 10mm diameter drill bit should suffice, as you will only need 1.0mm2 TWE wire for this unit.
With the intent of adding this as a spur to the lighting circuit, having determined where you want to position the lantern, hold the unit up to the wall and make a pencil mark on the bricks or render, tracing the shape of the back plate. Remove the lantern, measure and mark where the screw fittings have to be. Some units incorporate machine screws with domed headed nuts, which will secure the unit and leave a nice finish. Also mark the entry point of the wire you are going to use to power the unit.
Now from the inside, with the electric lighting circuit supply turned off, you need to remove the front part of the light switch by unscrewing the two retaining screws and carefully pull away the switch. Observe whether the back box within the wall is made of metal or plastic. Also look for how the switch is wired and any additional wiring in the box. In some houses a double switch may be present the second switch being a two-way type linked to the upstairs landing light. Observe the wiring going in and out of the switch and determine if the cable for wiring is marked to indicate the live feed.
If this is not evident, with a mains tester screwdriver and turning the electrical supply back on, by probing either side of the switch, where the wires are connected and secured with small brass screws, ascertain which side of the switch is the live feed wire. Now switch the electricity off again. With this type of assembly, you may find that there will be a minimum of two neutral wires, commonly black or blue, that may be connected together. They may be enclosed within a terminal block or may simply have insulation tape around the wires themselves. This arrangement of the black/blue wires is known as the 'common side'.
If the back box or pattress is metal, note how the wires come into the box and if the holes where the wires come through have a grommet, which will be a black flexible rubber ring, then it is likely that the wires to the box are run in plastic conduit. However if you observe from within the box large metal nuts, then you can be confident that there is metal conduit, either side of this box. Looking at the top of the box, see if there is only one wire coming in through a central hole. If there are 2 knock out holes in the backing and only one has been used, then it may be possible to chase a slot in the wall, going up to where the light fitting wire will go through to the exterior wall above the second (unused) pattress hole.
Your aim is to judge whether you can cut a slot in the plaster or render of about 12mm x 12mm to house mini trunking of 12mm by 8mm, without affecting the existing wiring. The new wiring to the light needs to be run in the trunking, either buried in the wall, if you can safely do this, or it will have to be surface mounted, to link with a surface mounted 16mm plastic pattress.
You are seeking to create an additional live feed to the lantern. The switch to the lantern is the PIR sensor itself, so at the internal switch you are looking to connect a wire to the live feed and to the neutral blue/black wires. Next, take the mini-trunking, measure the distance from the switch to the height of where the lantern will be positioned on the outside of the wall, add a short length for tolerance and cut the mini-trunking with a hacksaw or cutters.
Offer this up to the interior of the wall and using a spirit level, ensure this trunking is plumb. Mark either side of the mini-trunking with a pencil. So within the two pencil lines you are looking to chase the wall to a sufficient depth to bury the mini-trunking, which will encase the wire, allowing for plaster filler when decorating afterwards.
Having determined the point at which you are going to drill though the wall, with a centre punch or other pointed object tap an indentation in the wall with a hammer, to prevent the drill bit slipping from the mark. Drilling through from outside to inside or vice-versa is not really critical, what is important is that you do not break out a chunk of brickwork or plasterwork, when breaking through. Knowing the thickness of your wall, from the tip of your drill bit, measure this distance and affix some insulation tape minus 10 - 15mm to the drill bit. Whether you use a conventional hammer drill or a percussion drill usually with an SDS type chuck, the idea is to turn off the hammer action when the tape marking reaches the wall edge and the final few millimetres is with rotary action only. This should prevent and significant break out from either side.
With a clean hole, check that your wire will go through from one side to the other. Fix the mini-trunking within the chased wall (if using that method), or use the self adhesive at the back of the trunking, by peeling away the protective film and mounting on the inner wall surface. If you are using the second, spare pattress hole, knock the circular disk out from the top of the box, with a screwdriver and remove the disc with a pair of pliers. Fix a grommet to the hole. Now feed your wire through the hole, carefully pull it through then fix it into the mini trunking and through your drilled hole, to the outside. Leave sufficient cable on either side to allow for final trimming and adjustment.
If using surface mounted mini-trunking then you will need to add a 16mm surface mounted plastic pattress. The lantern is usually fixed to the wall using a metal plate, fixed to the wall with screws and rawlplugs : see DIY Doctor Project on Fixing Items to Masonry. The wiring goes through the plate. The individual wires fitted to the terminal blocks behind the face cover of the lantern. Following the manufacturer's instructions connect the wires, fix the lantern in position and secure with the nuts supplied.
From inside and ensuring the lighting circuit is switched off, prepare the new wiring to be partnered to the existing wire. It may be necessary to extend the live feed wire and common wire using a 5 amp terminal block. Adding the live wire to the switch and the new common wire, which may be taped or connected by other means. Ensure all connections are tight. With the switch still away from the wall, reconnect the electrical supply and use the switch, taking care not to touch any metallic parts behind. if all is OK the new outside light should light up.
The lantern may stay lit, but should switch off after a short time. Switch off the electric supply again and ensuring that the wires behind the switch are not severely cramped or kinked, fix the switch into place and secure with retaining screws. Switch the electricity back on and the light should be lit once more. Now check the switch for correct operation. Some lanterns will have adjustment for the length of time they stay on and the distance from where they operate, which will be covered in the instruction leaflet.