Mr. G Domestic Electrical

When the house was built and wiring installed, there was easy access to cables and the routing through the building fabric (joists, walls and ceiling). Then walls, floors and ceilings were covered restricting access. Modifications and changes over decades can make access even more complicated. The wiring over time will degrade or be inappropriate for current needs, and it's time for it to be replaced.

Depending the age of the property, different methods and types of wiring were used, the most common:

Conduit: with single cables. The conduit was used as the earthing conductor and the cables ran inside it. Over time the metal conduit corrodes and the earthing (and safety) is compromised.

Aluminium: Now potentially dangerous. This cable is no longer used for fixed wiring, as aluminium becomes very brittle when it ages and can fracture and break easily. It cannot be joined well to different conductor material as the aluminium will react to different metals - called Galvanic corrosion.

Rubber: Now potentially dangerous. With a life of 25 years, rubber cable deteriorates very quickly and the insulation hardens and cracks away exposing conductors. There are still many buildings wired in rubber and it is not often realised until modification is done; just touching or moving the cable can cause the covering to fall away. It can be very difficult to test because of this.

PVC: This is robust and has a life in excess of 50 years. However, there were batches of PVC cable produced in the 80s where over time the plasticiser in the PVC leaked in the form of a green slime, which when allowed to continue will drip from light fittings, switches and sockets. Only insulation testing can confirm the wiring is safe to continue in use, but the real solution is to rewire the property.

Terms:

Twin & Earth: (T&E) is usually used for the PVC cable bought today. The cable is flat section rather than round and has the insulated copper conductors running along side a bare earth wire. It's generally solid cores and not very flexible, designed specifically for fixed wiring, embedded in the fabric of the building.

Back box: The metal box fitted inside the wall that a socket or switch attaches to (at the front).

Knock out: A metal back box has several preformed and partly cut access holes for the cable to enter/exit. The metal is still attached to these 'holes'. Once the correct orientation of access is decided, the access holes can be opened by 'knocking out' the metal covering. Once opened the edges of the knock out hole can be sharp and should be covered by a grommet.

Grommet: A protective PVC ring fitting that fits in the hole of the metal back box to protect the cable passing against the sharp edge of metal.

Rewiring a property requires the power to be disconnected, since all cables and fitting will be removed. In many cases the electrician may add a temporary site board that allows minimum power to provide work lighting and power for tools.

A rewire normally includes a new consumer unit. The existing cable routing is found and a new installation is designed, planned and installed. This is the best time to reposition sockets or switches if there is other building work/changes in hand. This work involves removing or cutting many access holes in the floors, walls and ceiling to route the new cable. New accessory back boxes are fitted.

This first stage, called first fix will involve positioning all the cables to the accessory end point.  The first fix is generally tested for continuity and insulation to make sure nothing has been damaged during the installation.

The electrician's work then usually stops while plastering and other works can be completed.

Electrical work recommences with second fix; this involves re-testing the installation to make sure that any previous work by other trades has not damaged the installation (for example a screw fixing plasterboard has penetrated a cable).

Second fix connects the accessories (sockets/switches), dead testing is completed with the power disconnected. Power is connected to the curcuit and live testing is started. Results of testing is recorded on the installation certificate. Once testing is completed, the installation certificate and building control notifications are made.

Rewiring can be a traumatic time for a household. The work is far easier if the house is empty and access is better. If there is no choice but to remain living in the house, it will take longer and be more expensive. Simply because circuits may need to be done in sections and power restored at set points each day to allow living to be manageable (heating, cooking and lighting). It can be a dangerous environment and a risk assessment should be conducted to minimise any injury.

Changes after second fix can be expensive, so planning is crucial. Sockets and switch positioning for a new kitchen need careful thought as the units and cupboards need to be fitted and this could damage new hidden wiring. Units are a set width and height and accessories could be badly positioned where two units join, making them unusable. Tiling will cover a wall and sockets need to have been position before tiling is even considered. Changes to design and installation mid kitchen build can be expensive, better to plan positioning carefully before hand.

If you have any questions about this please send me a message through the contact page.