New Wires for Old
Despite the obvious importance of maintaining and updating electrical wiring, faulty wiring remains
one of the most common causes of fire in historic buildings. A surprisingly
large number of buildings still retain wiring installed in the mid 19th
century and, in some cases, the earliest forms of wiring are still in
use, presenting a constant threat not only to the building and its contents,
but also its occupants. It is an unseen problem, and often not even
the owners are aware of the situation. In churches, which are subject
to regular architectural quinquennial surveys but not always to electrical
quinquennial surveys, the risk may be understood but may not be dealt
with due to a lack of funds. The situation is not helped by English
Heritage and the Heritage Lottery rarely providing funds for the replacement
of old, dangerous electrical wiring unless it can be claimed as part
of the building work. However, wiring within a listed historic building
is now not subject to VAT providing that listed building consent has
been granted for the work.
The Electrical Regulations,
BS 7671: 2001 state that electrical wiring should be regularly tested.
In Guidance Notes Number 3, a supplement of BS 7671:2001, a table gives
recommended frequency of testing for various buildings. In ecclesiastical
and secular historic buildings it is generally accepted that if a building
has been rewired, the first periodic test should be after five years;
for wiring older than five years, periodic tests should be carried out
every 2½ years. In practice, however, the periodic testing of electrical
wiring rarely takes place, as it is considered to be a very costly and
disruptive exercise. As a result, the scale of the threat to historic
buildings has never been fully quantified.
The wiring in historic
buildings generally falls into one of three categories:
1. Very old wiring predominates with vulcanised rubber (VIR) cables.
These are often run within steel conduits, or are protected by a lead
sheath and installed within the building fabric. There may also be some
mineral insulated copper (MICC) sheathed cables.
2. Wiring is a mixture between VIR cables, poly-vinyl chloride (PVC)
twin-and-earth cables and MICC cables with or without a plastic sheath;
all of varying ages.
3. All wiring is of new MICC/PVC cable or new PVC twin-and-earth cable.
Where wiring falls
in the first category, the existing wiring in VIR cables and lead sheathed
VIR cables will have long passed the end of its productive life. It
would not be a good idea to carry out any electrical testing of the
circuits, as the rubber sheath would now be so old and perished that
harm would be done to the installation, thus leaving the installation
in a more dangerous state than existed before the insulation resistance
tests were undertaken.
collated by the Ecclesiastical Insurance Group indicate that electrical
faults in churches cost £5 million each year. In terms of historic
value the loss is incalculable.
With the second
category of wiring, a careful survey of the existing electrical installation
should be carried out and certain parts of the electrical installation
tested to establish which parts of the wiring may be reused and which
parts must be replaced.
With the third category
of wiring, the electrical installation will have to be tested in accordance
with BS 7671: 2001, Part 7, Chapter 73, Periodic Inspection and Testing.
Chapter 73 describes the procedure and method of testing the electrical
installation, and gives full details of each test to be performed. Further
details can be obtained from Guidance Notes Number 3. The periodic test
report will show the condition of the electrical installation at the
time at which the tests took place, and will list the rectification
work required for the entire installation to comply with current regulations.
In historic buildings,
where lighting points and switches are to be kept in their original
locations and the old VIR cables are found to have been run in conduits,
new PVC single cables could be run within the existing conduits to the
existing points, thus saving a lot of builders’ work, re-plastering
and redecorations. Care must be taken to ensure that each circuit has
its own live, neutral and earth conductor, as the existing steel conduit
cannot be guaranteed to be continuous. The steel conduit has to be earth-bonded
to make the system complete.
The new PVC cables
should have a ‘low smoke and fume’ (LSF) sheath to ensure that the cables
are not liable, like ordinary PVC cables, to produce corrosive halogens
and copious smoke in the event of a fire, which is not only a life-safety
issue but also reduces the smoke damage to the contents of an historic
Old MICC cables
do not have a plastic sheath, but electrical tests could be safely carried
out. It is often possible to re-use old MICC cables, providing that
cable faults are not shown during the electrical tests and that voltdrop
and earth-loop impedance calculations are carried out to ensure that
the existing MICC cables still comply with BS 7671:2001. Even if the
insulation resistance of the cables failed the tests, all might not
be lost; frequently it is the electrical equipment connected to the
cable which is at fault and not the actual cable.
Once the electrical
equipment has been disconnected, the cables must be retested and even
if the cables fail the insulation resistance tests again, the ends of
the cables could be re-terminated; this nearly always solves the problem.
In some bare-sheathed
MICC electrical installations, failure to pass the insulation resistance
tests is usually caused by a hole in the sheath of the cable. This occurs
where bare-sheath MICC cables have been installed in damp locations,
particularly where lime mortar has been used, as the water and lime
combine to create an electrolytic action with the bare copper. A similar
electrolytic action may also have been caused by installing bare-sheath
MICC cables on new oak. The reaction causes the copper to be eaten away,
making a hole in the side of the cable and letting in water, causing
a short circuit between live, neutral and earth. Often the only sign
that this has occurred is the appearance of green verdigris on the bare
copper sheath. In this instance, the only solution is to replace the
section of cable affected.
cables are not generally recommended for use in historic buildings,
as these cables are physically weak and can be abraded when being drawn
into the building or by building movement. Furthermore, vermin often
attack the PVC insulation, and this material can also become degraded
and brittle under adverse environmental conditions and ultra-violet
On occasions when,
due to cost restraints, some PVC twin-and-earth cables are used, they
should be mechanically protected to prevent damage and vermin attack.
The protection can be in the form of steel conduit or heavy grade, high
impact plastic conduit. The PVC twinand- earth cables should have a
Care must be taken
when re-using existing PVC twin-and-earth cables, as the early versions
had a 1.0 sq mm earth conductor. The small cross-sectional area of the
earth conductor may not comply with the earth-loop impedance requirements
of BS 7671:2001, depending upon the length of the cable.
Having decided if
parts of the electrical installation can be re-used or whether the electrical
installation needs to be replaced in its entirety, careful design of
the new electrical installation can take place.
The designer should
always allow for future expansion, incorporating slightly larger cables,
trunking and conduits so that additions may be made without having to
reopen floors and damage decorations.
bare-sheathed MICC cables to a damp wall can result in the formation
of verdigris and cause the cables to deteriorate.
MICC/PVC cable runs across the surface unnecessarily and is fixed
made into the stone itself, rather than into the mortar joints.
To minimise visual
impact, as much of the electrical installation as possible should be
consigned to non-sensitive areas such as cellars, roof spaces, risers
through the building, ventilation-voids, unused chimneys and floor-voids.
Where this is not possible, problems may be resolved by various methods
of concealment and disguise, and careful selections of materials and
finishes. Each building and situation should be treated on its own merits.
It is most essential
that the client and architect/surveyor decide upon the type of electrical
equipment and locations before the new electrical installation can be
engineered. If the historic building is very large, requiring very long
cable runs, the success of the design will hinge on its volt-drop characteristics.
Most of the volt-drop has to be taken up in the sub-main cables running
between the electrical mains intake switchgear and the local distribution
boards to avoid the circuit cables from the local distribution board
to the lighting and power points becoming so large that they are difficult
to terminate into the electrical accessories.
There are usually
two types of cables used for sub-main cables; MICC/LSF (mineral insulated
with an LSF sheath) and PVC/SWA/ LSF cables (poly-vinyl chloride, steel
wire armour, low smoke and fume sheath). The MICC/LSF cable is rigid
and is only suitable for certain locations, whereas the PVC/SWA/LSF
cable is very flexible and can be more easily woven around the building.
The circuit cables
are usually MICC/LSF cables and should not be less than 1.5 sq mm in
cross-sectional area. A standard range of sheath colours is readily
available off the shelf, in reasonable lengths of 100 metres; in orange,
black, light brown, dark brown, stone, grey, blue, yellow, red and white.
Where a suitable colour of sheath does not exist, the cables should
be painted to match the surface upon which they are being run. Of course,
the cable clips and shrouds should be the same colour as the cable.
When fixing cables,
the surface of historic stone or brickwork must not be damaged in any
way. Where running cables across a brick or stone surface is unavoidable,
the cable should be fixed into the mortar joints between the stones
or bricks using brass fixing screws so that they can be removed at a
later date without damage to the historic fabric. Where the mortar joints
are friable, the existing mortar joint should be raked out and refilled
with a mortar composition specified by the architect or surveyor.
When drilling any
holes for fixing the wiring and electrical equipment or for passing
cables through structures, the electrical contractor should use an electric
drill with an efficient vacuum cleaner attachment to ensure that all
the dust is sucked away and does not settle within the room. Cartridge
fixing tools should not be used.
Where the cables
must pass through the walls, holes should be made using a diamond core
drill (again with a vacuum attachment to remove the dust) so that a
hole is neatly made with the minimum of damage to the masonry. Where
cables pass from one fire compartment to another, the walls, ceiling
or floor through which they pass must be effectively firestopped with
a suitable fire barrier to prevent the passage of smoke and flames as
well as rodents.
Where cables are
chased into the plaster walls, they should be carefully and neatly made
by hand and run within 20mm round plastic conduits so that rewiring
can take place at a later date without the need to re-cut chases in
the walls, thus preventing further damage to the decorations in the
he cables should
have cable-markers at each point at which they are terminated within
the switchgear, luminaires, lighting switches, socket outlets and all
electrical equipment, including accessories and junction boxes. They
should also have cable-markers at every three metres within trunking,
indicating the circuit number, distribution board number and the colour
of the phase.
Cables within roof-voids
should be run on galvanized cable trays above the thermal insulation.
When cables pass through the thermal insulation, they must be at right
angles so that the cables are in contact with the smallest possible
amount of thermal insulation to minimise the heating effect of the thermal
insulation on the cable.
Where cables pass
through floor joists, these joists shall be drilled in the centre rather
than slotted. All cutting of joists must be kept to an absolute minimum.
If notching of the joists cannot be avoided, the cables must be covered
with 3 mm steel plates to prevent penetration by nails and screws. Old
notches should be used where possible. The approval of the architect
and structural engineer must be obtained before any new notches are
cut in the floor joists. Further details regarding chasing of walls
and running cables within floor and ceiling voids can be obtained from
the IEE On-Site Guide (to BS 7671:2001 and the amendments).
The IEE Amendment
No 2: AMD 14905 to BS 7671:2001 details the new colours of cable conductors
to be used.
commencing on site after 31 March 2004 and before April 2006 may be
installed in accordance with Amendment No 2 or Amendment No 1 – that
is to say, the new harmonized colours/marking or the old colours may
be used, but not both.
on site after 31 March 2006 are required to use the new harmonized colours
From now until 31
March 2006, the new cable colours may be used to extend an existing
electrical installation providing that a warning notice is attached
to the particular distribution board stating 'CAUTION. This installation
has wiring colours of two versions of BS 7671. Great care should be
taken before undertaking extension, alteration or repair that all conductors
are correctly identified'.
As many extensions,
repairs and alterations of the electrical installations in historic
properties are carried out over a number of years, all new work should
be carried out using the new cable conductor colours. The new cable
conductor colours are as follows:
|TYPE OF CIRCUIT
|SINGLE PHASE CIRCUITS
L Phase conductor (live)
N Neutral conductor
E Earth conductor
L1 Phase 1 conductor
L2 Phase 2 conductor
L3 Phase 3 conductor
N Neutral conductor
E Earth conductor
The art of a good
electrical installation in an historic building is to ensure it fulfils
all present requirements and allows for expansion, but is unobtrusive
with cables, accessories and switchgear out of sight. By using the right
cables and methods of installation and concealment to suit the building
and maintain its historical integrity, the result will last for many
years to come.
pamphlet 9 Electrical installations, SPAB Lighting and Wiring
of Churches, CIO Publishing, Dean’s Yard, London SW1 3NZ
7671:2001*, British Standards Institute Guidance Notes Number 3,
British Standards Institute
- IEE On-Site Guide (WR241), Institution of Electrical Engineers
- Roger W Moss, Lighting for Historic Buildings, The Preservation
article is reproduced from The Building Conservation Directory, 2004
F WRIGHT is the Senior Associate of Lightwright Associates, Building
Services Consulting Engineers.
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