Old electrical installations

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There are millions of old electrical installations still in daily use, right across the country. While many of these should be safe and functional, they may not offer all of the facilities and protection mechanisms that a modern install will offer. This not only affects their relative safety, but also they may often prove inadequate for modern usage patterns.

This article attempts to collect together notes on the likely pitfalls of older installations, and give tips for how to live with and improve them.

How old?

How do you tell the age of an installation? The Dating Old Electrics page should give you some clues.

Features of old electrical installations and their significance

Item Significance
Lack of RCD protection A functional and regularly tested RCD will massively reduce the likelihood of being injured should you receive an electrical shock. Modern installations will usually protect all circuits with several RCDs. Of particular importance where power is used outside in the garden for example.
Substandard or missing Equipotential Bonding Equipotential bonding is used to limit the voltage that one is able to indirectly (i.e. by touching something that has become live due to a fault) make contact with during fault conditions. It is of particular importance in areas where people are particularly vulnerable to electrical shock injury, such as bath and shower rooms. Bonding also helps mitigate the risks of conductive parts of a house's structure (such as its pipework) from projecting the effects of a fault from one room into another. Lastly, bonding will help minimise the risk that a fault in the earth connection itself supplied to the property does not manifest as a dangerous situation in the property itself.
Substandard or missing Main Earth cables Some old installations lack basic earthing (either as that is how they were originally installed, or the earth was provided by a gas or other utility pipe that was subsequently disconnected or converted to plastic). Having no earth will make automatic disconnection of the supply in many fault conditions impossible, greatly raising shock and fire risk.
Undersized protective conductors in circuits Undersized protective conductors in circuits can result in slow clearing of faults, or even failure to clear in some cases. This most commonly applies to spurs on older ring circuits that used a 1mm² cpc/earth conductor rather than the 1.5mm² in modern cables.
Unearthed circuits Some circuits either have no earth, or may borrow an earth from another circuit. Pre 1966 Lighting circuits usually have no earth at all and it was common for immersion heater circuits to use a twin cable and borrow an earth from another circuit or use the metalwork of the HW system as the earth. This makes extending these circuits in a compliant way impossible, as well as limiting the use of wiring accessories to Class II double insulated ones DoubleInsulated.jpg only.
Badly perished rubber insulation Creates a high risk of insulation failure. This can cause nuisance tripping of RCDs, as well as increased shock and fire risk. It also makes any alterations that require moving a cable risky.
Lead sheathed and other very old insulation materials Very old installs may still include some now rare cable types such as lead sheathed. Rather like old rubber, this should also be treated with care, and checked for insulation resistance failure. Cables of this age should be replaced at the earliest opportunity.
Old switches Old switches (particularly some light switches) can sit in mid position, and if luck deserts you, sit in an almost on position and arc. This either burns the switch out or causes dangerous overheating and possibly fire.
Dirt build up at connection points This particularly affects old plugs and sockets, and can cause circuit faults, insulation breakdown, and fire risk.
Poor connections Connections can deteriorate due to decades of thermal cycling, wire corrosion, or less than ideal design & construction of historic fittings. This can result in overheating and cable damage.
Inadequate circuit separation and discrimination Lack of adequately separated circuits, may mean that faults cause more inconvenience and consequential risks (e.g. trips and falls in the dark) than they would with a modern installation. For example and old installation may only have one lighting circuit, and hence a blown fuse will mean no lights anywhere.
Flammable wiring enclosures e.g. wooden pattresses behind light switches, and sockets mounted directly onto skirting boards. Old consumer units with wood surrounds. These are unlikely to contain serious overheating connection etc without burning and risking spread of fire.
Tool less accessory covers Mainly switch covers that undo easily by hand, a danger primarily to children. However it also includes rewireable fuse holders that expose live parts when removed.
Old unshuttered power sockets Make it easy for children to poke metal objects into sockets.
Undersized meter tails May not be able to withstand prolonged use of high power loads. May cause the electrical supplier to down rate the main fuse capacity when carrying out meter changes etc. This can in turn reduce the total supply capacity to the house to an inadequate level
TT supplies with no RCD protection Old TT installations commonly used (now obsolete) Voltage Operated Earth Leakage Circuit Breakers which have a number of failure modes that do not afflict RCDs. They were also commonly wired to inadequate main earth terminals (gas pipes etc), risking loss of adequate earthing.
Too few power and lighting points Older installs rarely have enough power sockets for modern patterns of use. They also encourage multiple extensions of existing circuits (with varying degrees of workmanship and design care). Too few sockets also tends to promote excessive use of extension leads and multiway adaptors. Leads can present trip hazards, and adaptors can lead to overheating ageing sockets.
Modern MCBs Modern MCBs offer simpler detection and resetting of tripped circuits. They are also not open to abuse in the way that re-wireable fuses are (you can't "repair" a circuit that is repeatedly tripping due to a fault with tinfoil!) (Note however that MCBs can also introduce new problems, since they are more sensitive to short term overloads, which can result in nuisance trips when tungsten filament lamps fail).
Borrowed Neutrals Borrowed neutral connections expose anyone working on a partially energised system (i.e. with some circuits still connected and others switched off at the MCB / fuse removed), to a higher risk of direct contact shocks from conductors that one might expect to be "safe".
Expansion Capacity Often old fuse boxes will have no spare ways. This makes adding new circuit in a sensible and well partitioned way more difficult and expensive.

Upgrading old installations

Some of the above installation problems can only really be addressed with a full rewire - e.g. a system wired in rubber insulated cables. However many others can either be remedied individually, or at least worked around.

Many problems relating to lack of RCD protection (or inappropriate RCD protection such as "whole house" installations) can be rectified by changing your consumer unit. This can also remedy lack of spare ways for expansion, and provide modern MCBs in place of fuses.

Main equipotential bonding should be checked and upgraded if not to current standards when any electrical work is carried out anywhere in the installation. (Note this may require electricians to carry out additional work beyond that requested).

Installations with ageing wires and possibly suspect insulation. Can be tested using an insulation resistance tester. If the readings are acceptable they they can continue in service. (additional peace of mind can be had from ensuring RCD protection is in place for any such circuits)

One of the most common problems with older installations is a lack of sockets. If the installation is otherwise in good order then it is perfectly acceptable to add more as required.

Lack of power for garden tools etc can also be a problem. Old systems can be safely extended to provide Garden and outbuilding circuits if required in most cases. When an install does not have adequate RCD protection, then plug in RCD units can be used when running extension leads to the garden.

See Also