Lighting Circuits Without an Earth
Many lighting circuits (usually installed pre 1966) are still in use that have no earthing. (Note that the earth wire, is referred to in the wiring regulations (BS7671) as the Circuit Protective Conductor or CPC - so any notes you see in articles etc about "lighting circuits without a CPC" are discussing the same topic).
The ways these historic cicuits are spotted are:
- When changing a fitting or switch, only 2 core cable is fitted
- Historic round bakelite switches
- Only two core cable connected in consumer unit
Even the presence of modern T&E cable at a switch or light fitting does not guarantee the whole circuit has an earth, since it may have been used to extend the circuit in the past.
Generally Metal faced switches switches must not be used on an unearthed lighting circuit. The exception to this are cases where the Metal light fittings are labelled with the Class II symbol.
Old lighting circuits may have other safety issues beyond just the lack of an earth conductor (and the lack of an earth conductor would suggest the circuit is at least 50 years old). See the old electrical installations article for more detail.
If a rewire is not an option then the following steps should be taken for safety reasons:
- Metal (Class I insulated) light fittings and metal accessories should be replaced with (Class II) fittings, which are either plastic, or metal and marked with the Class II symbol.
- Insulation Resistance Test. The circuit should be tested between the line conductor (live and neutral connected together) and the earth terminal of the consumer unit. The resistance should be at least 1 Megohm. If the resistance is less that 1 Megohm the circuit must have additional RCD protection. The test should also be repeated between the live & neutral conductors and any exposed conductive part of the circuit (eg the screws for a lightswitch with a metal backbox). Again if the readings are less than 1 Megohm then additional RCD protection is necessary.
- RCD protection. It is advisable to have 30mA RCD protection on lighting circuits that have no cpc, and essential if the resistance readings are lower than 1 Megohm.
- Any 2A or 5A sockets connected to the lighting circuit that may be used for portable equipment should be disconnected. Dedicated clock connectors can be left in place.
- A warning notice similar to the one below should be fixed on or adjacent to the CU or fusebox:
Extending the circuit
Not having an earth wire on a lighting circuit makes it impossible to extend the circuit for extra lighting points in a way that meets current wiring regulations. Extension can in principle be done safely if all switches, pattresses and lights are plastic, and remain plastic, but
- regulations have demanded an earthed lighting circuit since 1966
- possible later replacement with metal fittings can make them unsafe
One viable workaround to add lighting points, is to feed them via another circuit that does have an earth wire. This can be another more recent lighting circuit, or can be a socket circuit via a fused connection unit (with a 3A or preferably a 5A fuse).
Consumer unit upgrades
Most unearthed lighting circuits are wired with historic cable (often old imperial twin PVC) that can handle the required load of the lighting circuit. But there are also circuits in use with unsuitable cable, such as bell wire and speaker wire. If such wire is found it should be disconnected from the supply immediately.