Electric cables

Wired up to go

By Philippa Martin-King

A surprising number of fires are caused by electrical faults that are associated with wiring or wiring devices. In the United States, fixed wiring heads the list of causes of ignition in home fires. There are more fires due to wiring than there are fires caused by cords and plugs, light fixtures, switches, receptacles, outlets, lamps or light bulbs. It's not that wiring is inherently unreliable and thus causes greater problems. No, it's simply that you can't have a building without electricity and if you want the electricity to be distributed it has to go through the relevant wiring which, by following the guidance of IEC TC 20 and testing regularly, should be safe.

Where electric defects are the most common cause of fire, overheated wires tops the list Where electric defects are the most common cause of fire, overheated wires tops the list

Wired to reduce fire hazards

Electrical fire data for 2010 put together by the European agency Leonardo Energy shows that residential dwellings are ageing and that their renovation rate is slow. However, at the same time, the use of electricity is increasing and with it the danger of fires due to faults in the wiring. Where electric defects are the most common cause of fire, overheated wires come top of that list. With age, electrical installations wear, cable insulation hardens, contacts become loose or corrode. In the UK, says Leonardo Energy, one third of the 9 000 fires each year are caused by inadequate wiring.

According to Underwriters Laboratories (UL), over one third of all U.S. housing is more than 50 years old. Put in the context of a typical situation today where wiring and circuitry are overloaded, it is quite likely that the wires become heated and where cabling has deteriorated and crumbled, so it is unable to insulate efficiently, a fire may follow.

IEC 60364-1, Low-voltage electrical installations - Part 1: Fundamental principles, assessment of general characteristics, definitions, deals with electrical installations for buildings. It specifies that wiring should be verified every ten years to ensure there is no overheating, that the switchgear is suitable, the earthing adequate and so on; however this is not mandatory in many countries

The most important test is that of measuring the insulation resistance of cables. If the insulation resistance is low, there's a risk of current leakage. If the earthing is not correct there's a potential danger of electric shock to humans or animals –and a leakage current of 500 mA can generate enough heat to cause a fire. IEC 60364-6, Low-voltage installations – Part 6: Verification, provides guidance for carrying out periodic verification of this element.

Fire Statistics in the United Kingdom was published in November 2010 by the Fire and Rescue Services. Figures show that the "majority of fire-related deaths occur in dwelling fires (three quarters in 2008)". More than half (54 per cent) of those accidental fires in homes arose from cooking. Other common sources of ignition were: electrical appliances (13 per cent), smokers' materials (7 per cent), electrical distribution (7 per cent) and space heating appliances (4 per cent).

In certain highly populated urban areas of the world, people today are housed and work in high-rise blocks and towers. As well as the obvious structural and geotechnical challenges of such buildings, there is a high additional risk attached to evacuating inhabitants in case of fire. Often these residential tower blocks have been built in areas that needed low cost housing and where there were already other social problems to contend with. As a result, it is highly likely that maintenance of the electrical system will not always have been carried out correctly; the risk of fire is therefore proportionately higher.

Statistically speaking, says the U.S. Department of Homeland Security, when a residential building catches fire, what ignites first is not the structural framing of the construction (17 %), wall covering, insulation, panelling and so on (18 %), but the insulation around the wiring (30 %).

International Standards for testing and assessing wiring and cabling

IEC TC 20: Electric cables, prepares International Standards that are used in designing and testing insulated electrical power and control cables, their accessories and cable systems. End-user recommendations (including current ratings) are also issued for wiring and for use in power generation, distribution and transmission applications.

Test houses and conformity among the largest users

As a result, many of the main users of TC 20's publications are the test houses and conformity assessment organizations that are responsible for checking and testing wiring and cabling. Installations range from those in the domestic sector right up to superconducting transmission cables rated at up to 500 kV in the large installations currently being constructed to deal with UHV (ultra high voltage) power transmission or offshore connections. Other evolving markets include charging stations for EVs (electric vehicles) and dealing with renewable energy generation across the Smart Grid. The number of countries involved in manufacturing cables – currently around 80 – is growing, particularly for the lower voltage cables used in basic infrastructure.

Not only testing but better efficiency

Since wiring has resistance, it uses energy and very often loses it too. Energy efficient cabling therefore offers great possibilities for future development and new areas of standardization. A reduced carbon footprint and significant energy savings are demonstrated by cabling that has been designed to reduce transmission losses and heating effects.   TC 20 is now working on a new version of its energy efficiency publication, IEC 60287-3-2, Electric cables – calculation of the current rating.

IEC TC (Technical Committee) 20: Electric cables, first met as an AC (Advisory Committee) in 1934. Today the focus is as much on the economic aspects of cabling as the technical ones. By considering globalization and the rationalization of cables, the International Standards that TC 20 produces will enable their users to better address the issue of electrical energy efficiency.

TC 20 is divided into four WGs (Working Groups) that deal with:

  • WG 16: High voltage cables (1 kV and above), their accessories and cable systems
  • WG 17: Low voltage cables (below 1 kV)
  • WG 18: Burning characteristics of cables
  • WG 19: Current ratings and short circuit limits.

The group WG 18 holds a group safety function for "Fire hazard testing on cables" that consists of:

  • flame propagation tests
  • fire resistance tests
  • smoke optical density tests
  • corrosivity tests

Basic, group and product safety standards

Safety has to be a fundamental consideration wherever electricity is involved. The IEC publications that deal with safety therefore tend to be horizontal in nature. Rather than expecting each IEC TC to repeat or re-describe safety information, some of the IEC publications are destined to apply to entire aspects of safety that cover many products. These are known as basic safety publications. Others apply to the safety aspects of a specific group of products within the scope of two or more product TCs. These are known as group safety functions. This is the role that WG 18 plays in relation to cable fire hazard testing.

Reciprocal arrangements with other TC/SCs

IEC TCs/SCs do not work in total isolation but maintain liaisons with a number of other TCs/SCs and sometimes external organizations that are dealing with matters common to the work of each. In exchanging information they ensure that their committees remain effective and that their work is not duplicated or repeated. In this respect, TC 20 maintains liaisons with:

Where electric defects are the most common cause of fire, overheated wires tops the list Where electric defects are the most common cause of fire, overheated wires tops the list
The Northeast Asia Trade Tower is presently the tallest high-rise building in South Korea. The Northeast Asia Trade Tower is presently the tallest high-rise building in South Korea.
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