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Do I need to rewire my ring final?
Former Community Member
Good afternoon, I'm looking for some advice as to whether I need to get someone in to have my ring final rewired. I have recently had a new consumer unit installed and circuits tested. All circuits tested fine according to regulations, but the ring final registered lower insulation resistance readings. live/live 119mohm and live/earth 105mohm tested at 500v. Max Z's 0.52ohm according to the certificate. I'm not sure how old the wiring is but we've been here 20 years and the wire has grey pvc outer sheath, red live pvc, black neutral pvc and 3 stranded earth, which looks like plated copper. Some of the sockets are probably the same age. Strangely, some other circuits with same type of cable registered 999mohm when tested. FYI we live in a 1960's bungalow. Just wondered if you had any thoughts. Many thanks in advance. Simon
To give you a sense of scale, the minimum for an inspection pass is 1Mohm (but should be investigated if < 2Mohm). Readings of 500Mohm+ etc are usually only obtained on pristine dry cable. Once installed, then over time with damp, dirt etc it will creep lower, without indicating any real problem.
The minimum insulation resistance required by BS 7671 (IET Wiring Regulations) for this type of circuit is 1 MΩ.
If the insulation resistance readings for your ring final circuit are 119 MΩ / 105 MΩ the they are over one hundred times the minimum insulation resistance required and are therefore nothing to be concerned about.
I think, more importantly, your electrician will have tested for continuity in the ring and the very satisfactory Zs suggests that all is well with the various terminals around the ring.
Yes as others have said based on the information given nothing wrong with those readings.
Given that your installation is some 50+ years old it is probably non-compliant with the current Edition of BS 7671 unless it has been upgraded.
The common non- compliances with an installation of that age are, the water pipe used as a means of earthing, un-earthed lighting circuits, the absence or undersized main bonding and earthing conductors
, the absence or part absence of RCD protection, absence of RCD protection or supplementary bonding in bathrooms, worn sockets and other accessories, open backed wooden consumer units, etc
If you have not had any upgrades and improvements then for your safety and the safety of your family and loved ones I would highly recommend upgrading to the 18th Edition of the Wiring Regulations.
i am guessing you are unsure otherwise you would have posted an explanation?
Up until 1966 the Wiring Regulations did not require lighting circuits to have an earth connection present at each point of termination on lighting circuits. Metallic Class 1 light fittings and switches require earthing for safety. Also if a circuit protective conductor (an earth wire or metallic conduit) is in close association with the live conductors ( line and neutral) and the circuit is RCD protected it provides a measure of fire protection although this is not recognised in the Wiring Regulations at this time. So if you have all plastic light switches and Class 2 light fittings and the circuit is 30mA RCD and there is a warning label on the consumer unit and the premises are not rented out the installation is not unsafe but remains none compliant with the current edition of the Wiring Regulations.
