7.11.4 - Surface, floor, soil and road warming installations

Most cables have conductors of very low resistance so that the passage of current through them dissipates as little heat as possible. By using a cable with a higher conductor resistance (typical resistance's for some of the resistive alloys used are from 0.013 to 12.3 Ohms/m) heat will be produced and will transfer to the medium in which the cable is buried, There are many types of cable, a typical example being shown in {Fig 7.19}.

Fig 7.19 - One type of floor heating cable

Some of the many examples of the use of heating cables are:

1. - space heating, using the concrete floor slab as the storage medium,

2. - under-pitch application on sports grounds, to keep the playing surfaces free of frost and snow,

3. - in roads, ramps, pavements and steps to prevent icing,

4. - surface heating cables, tapes and mats, used for frost protection, anti-condensation heating, process heating to allow chemical reactions, drying, processing thermoplastic and thermosetting materials, heating transport containers, etc.

5. - in rainwater drainage gutters to prevent blocking by ice and snow, and

6. - for soil warming to promote plant growth in horticulture.

The heating cables used in such situations must be able to withstand possible damage from shovels, wheelbarrows, etc. during installation, as well as the corrosion and dampness which is likely to occur during use. They must be completely embedded, and installed so that they are not likely to suffer damage from cracking or movement in the embedding material, which is often concrete. The loading of the installation must be such that the temperatures specified for various types of conductor {Table 7.7} are not exceeded. Where an electrical under-floor heating system is used in a bath or shower room, it must either have an earthed metallic sheath which is supplementary bonded or be covered by an overall earthed metallic grid which is similarly bonded.

Table 7.7 - Maximum conductor temperatures for ------------------ floor warming cables
Type of cable	Max conductor operating temperature (°C)
General purpose, p.v.c. over conductor	70
Enamelled conductor, polychloroprene ins. with p.v.c. sheath	70
Enamelled conductor, p.v.c. ins. overall	70
Enamelled conductor, p.v.c. ins. and lead alloy sheathed	70
Heat resisting p.v.c. insulated	85
Synthetic rubber insulated	85
Mineral insulated copper sheathed	*
Silicone treated woven glass insulation	180
* The operating temperature depends on the outer covering material, the type of seal, the arrangement of the cold tails, and so on. Manufacturers’ data must be consulted.

Where heating cables pass through, or run close to, materials which present a fire hazard, they must be protected from mechanical damage by a fire-proof enclosure. Where normal circuit cables are run through a heated floor, they must have the appropriate ambient temperature correction factor applied {4.3.4}. Heating cables may he obtained ready jointed to normal cables at their ends (cold tails) for connection to the supply circuit as shown in {Fig 7.20}.