This article from the NICEIC looks at the requirements and considerations for a contractor when installing electric heating cables and systems within walls, floors and ceilings located within domestic dwellings.
This article does not preclude other similar types of environment or alternative installation methods.
Introduction
Section 753 of BS 7671 contains specific requirements for the installation of embedded heating systems which supplement the general requirements of that standard. Additionally, Sections 701 and 702 contain further requirements applicable to the installation of such systems in locations containing a bath, shower or a swimming pool respectively.
Any requirements relevant to a particular installation must be met before undertaking such work, for example, determining the adequacy of supply characteristics including earthing and bonding (Regulation 132.16). Additionally, the recommendations of manufacturer’s instructions should be taken into account, including the nature of any adjacent materials liable to come into contact with the heating source and the local building regulations.
Types of heating cable elements
Electric heating cables operating at 230 V commonly used in locations such as bathrooms, kitchens and the like are at risk of becoming damaged during installation thereby increasing the risk of an electric shock and/or fire. Suitable precautions must be taken to prevent accidental damage occurring before the final installation of the floor covering, screed or tiles.
Although damage is most likely to arise from the penetration of metallic fixings such as nails and/or screws during final fixing, for example, the installation of carpet grip or bathroom furniture.
Regulation 753.511 recognises two types of heating element:
- Flexible sheet heating elements complying with the requirements of BS EN 60335-2-96 – Household and similar electrical appliances – Safety. Particular requirements for flexible sheet heating elements for room heating, as shown in Fig 1 (a). This allows for greater flexibility and a faster installation time.
- Individual heating cables manufactured to the requirements of IEC 60800 – Heating cables with a rated voltage of 300/500 V for comfort heating and prevention of ice formation, as shown in Fig 1 (b). This type of cable typically incorporates a metallic sheath or braid which is utilised as a protective earthed covering for the circuit.
Other types of heating system having no provision for an exposed-conductive-part (metallic covering) are available; in which a separate conductive covering or metallic grid shall be employed and placed over the floor heating cable, likewise, under the ceiling heating cable. Furthermore, Regulation 753.411.3.2 requires the maximum spacing between the metallic grid to be no greater than 30 mm where it shall be connected to the circuit protective conductor for the circuit, except where the protective measure is of SELV (Regulation 701.753) or the cable is of Class II construction.
Protection against mechanical damage
As required for other electrical circuits, cables must be suitably selected and installed for any external influences that may affect operation during normal usage, such as dampness or corrosion (Regulation 554.4.2). Mechanical stresses impacting on heating cables caused by sharp objects, impact from tools or the effects from tension and compression expected during installation shall also be avoided (Regulation 522.6).
Where a cable is intended to be installed within the fabric of the building, embedded in concrete, flooring screed or similar it must be suitable for burying and have a degree of protection not less than IPX7 or floors or, IPX1 for ceilings. Likewise, for reliability cables must be completely embedded within the surrounding substance which is to be heated; any small voids (air gaps) appearing around the cable may lead to hot spots and subsequent cable failure (Regulation 554.4.3).
Some buildings include joints within walls and floors which are so designed to allow for movement and thermal expansion, Regulation 753.515.101 prohibits the installation of cables crossing such joints where there is a potential risk of exposing the heating cable to tension during structural movement.
Protection against electric shock
Regulation group 753.41 identifies the protective measure of automatic disconnection of supply (ADS), double or reinforced insulation and the method of electrical separation, although this is not permitted for wall heating systems or within certain locations.
Typically, the common form of protection for circuits supplying underfloor, wall and ceiling heating systems will be ADS. An RCD having characteristics complying with Regulation 415.1.1 shall be used for fault protection (I∆n ≤ 30 mA) (Regulation 753.411.3.2). This also meets the requirements of Regulation 753.415.1, the need for additional protection. It is a fundamental requirement of circuit design that overcurrent protection is provided, (Section 430 refers).
Typically, in older installations fuses to BS 3036 or circuit breakers to BS 3871 maybe found. Where this is the case and it is not practical to include RCD protection at the consumer unit a 13 A RCD spur maybe installed locally from an existing circuit as shown in Fig 2. Furthermore, consideration should be given to the power rating of the heating system and the implications affecting the existing circuit (Regulation 433.1.204).
It should be noted that the effects from earth leakage currents using screened heating cables may cause unwanted tripping and is more likely to occur where a number of heating elements are supplied through a single RCD.
This is recognised in BS 7671 where Note 2 to Regulation 753.411.3.2 states that limiting the heating power output to 7.5 kW/230 V or 13 kW/400 V downstream of a 30 mA RCD may reduce such risk.
Protection against burns
Regulation 753.424.201 requires one of the following measures to be applied, limiting the temperature for a specific heating zone to a maximum of 80˚C.
- appropriate design of the heating system,
- appropriate installation of the heating system,
- use of protective devices.
Suitable measures shall be taken to avoid floor and ceiling heating systems overheating where it may lead to burns or ignition. In locations where contact with skin or footwear is possible the surface temperature shall be limited to no more than 35˚C (Regulation 753.423). For this reason many manufacturers will limit their products to around 29˚C.
To read more articles offering technical advice click here.
