Difference between revisions of "Historic Mains Cables"
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(→VIR: typo, rephrase) |
(→Copper Clad Aluminium: clarify crack risk) |
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==Copper Clad Aluminium== | ==Copper Clad Aluminium== | ||
* An attempt to improve the properties of ali cable | * An attempt to improve the properties of ali cable | ||
− | * Significantly better than al, surface oxidation is eliminated, creep reduced & the cracking | + | * Significantly better than al, surface oxidation is eliminated, creep reduced & the risk from cracking more or less eliminated |
==Imperial T&E== | ==Imperial T&E== |
Revision as of 05:09, 18 March 2008
Before the 1960s several types of cable were used at various times.
Paper
- Paper insulation
- From the WW1 era
- Very rare now
PBJ
- PolyButyl Jute insulated cable
- Rubberised Hessian appearance to the sheath.
- Commonly used for mains incomer insulation,
- Lots of old PBJ is still safely in service
Lead sheathed
- Common in 1930s for socket circuits
- Used as exterior farm cable well after that
- Lead sheath does not make reliable earth connections
- Rubber inner insulation
VIR
- Vulcanised India Rubber insulated cable.
- The most common historic wiring
- Comes in 2 forms:
- Two layers of rubber insulation, one on each conductor, and an overall sheath.
- Twisted pair, cotton/rubber insulated, with no outer sheath
- Both types usually don't include an earth wire, which is sometimes run as a separate uninsulated single.
- Rubber wiring uses thinner conductors than PVC, since the rubber is higher temperature rated
- Rubber insulation perishes, cracks & falls off
- Properties with VIR cable are usually in urgent need of rewiring, and may represent a significant safety risk.
A good percentage of the remaining old VIR wiring is now in a dangerous condition, especially at termination points. It is common to see insulation that has fallen off, often leaving live & neutral conductors bare, unsupported and in very close proximity. In the worst cases 2 bare conductors can be found twisted round each other with nothing rigid to support them. The rule of thumb with old rubber wiring is
- replace it as soon as possible
- don't move it at all, even small movement sometimes causes shorts.
Aluminium
- Cheaper alternative to copper
- Used from 1950s to 1970s, and old stock sometimes used into the 1980s.
- A known fire risk
- Aluminium cable creeps, oxidises & fractures, all of which can cause fires.
- Requires special connections, do not connect to old ali cable using connectors intended for copper.
- Al requires a larger conductor size than Cu for the same current rating
- Presence of aluminium cable may be considered a material fact for insurance.
Copper Clad Aluminium
- An attempt to improve the properties of ali cable
- Significantly better than al, surface oxidation is eliminated, creep reduced & the risk from cracking more or less eliminated
Imperial T&E
Older T&E wiring having similar construction to today's cables.
- 7/.029 T&E
- imperial 7 stranded version of 2.5mm² T&E
- Ashathene T&E
- Precursor to PVC T&E
- PVC outer VIR inner
- an early T&E cable
- 2 core T&E
- no earth, used for lighting circuits, or power circuits with a separate (usually uninsulated) single run alongside to provide an earth.
PVC and ashathene versions of this cable are usually in good condition, but rubber does not last well long term.