Automatic Switching Devices – Assessing Their Switching Capacity

Automatic Switching Devices – Assessing Their Switching Capacity

Best practice advice from the experts at ELECSA.

Many existing businesses and homeowners and those designing new buildings are increasingly turning to efficient and effective ways to reduce energy costs by employing automatic switching devices.

Unfortunately, and as demonstrated by a number of incidents reported by ELECSA contractors, there are still a number of persons failing to adequately assess the applicable maximum rated switching current of an automatic switching device and the type of load to be switched by the device, prior to work being carried out, especially where additions and alterations are made (see later).

It is important that persons who carry out electrical maintenance and installation work are competent to assess the suitability of electrical equipment, such as automatic switching devices so that the applicable maximum rated switching current of the device is not exceeded by the current of the load to be switched by the device.

All the incidents reported related to the use of small-scale, single-phase automatic switching devices having a rated current up to 20 A, that are generally used to switch one or more items of fixed current using equipment. This article therefore focuses on the potential dangers relating to such devices.

The guidance given is common practice and can therefore be suitable for other applications involving switching devices.

It should be remembered that an automatic switching device could be employed to control a load when used in conjunction with a suitable relay or contactor, however, such an arrangement is outside the scope of this article.

Using automatic switching devices – Unskilled Persons

A review of the small-scale automatic switching devices available to the electrical industry reveals that there are generally four types used to provide a means of automatic control for items of current using equipment; proximity sensors, time switches, dawn to dusk photocells and thermostats.

These four devices are suitable for switching a variety of loads, typically expressed on the manufacturers information in amps and/or watts, and manufacturers might attribute more than one maximum rated value of current or power to a particular device. For example, the manufacturer’s information for a particular device might provide information expressed as:

  • Maximum switch rating 13 A resistive load
  • Maximum switch rating 2 A inductive load
  • Suitable for switching immersion/ storage heaters of up to 3000 W
  • Suitable for switching 2000 W of incandescent lamps
  • Suitable for switching 500 W of fluorescent lamps
  • Suitable for switching 200 W of compact fluorescent lamps
  • Not suitable for switching discharge lighting
  • Suitable for switching 3000 W of heating mats

Given the possibility that for a particular automatic switching device a variety of maximum ratings might be expressed on manufacturer’s information, it is important that contractors consult this information when selecting the device, to ensure that it is capable of switching the connected load.

Regulation 134.1.1 requires electrical equipment to be installed in accordance with the instructions provided by the equipment manufacturer.

To take account of the reactive component of loads and their effects on switch contacts when they are either opened or closed, the designer will need to choose appropriately rated switches.

For example, a fluorescent load during warm-up might momentarily draw twice the amount of current on starting in comparison to normal running mode.

Potential dangers of inappropriate assessment

Some ways of reducing energy costs might appear straightforward and therefore not requiring assessment of a circuit’s characteristic, such as replacing incandescent lamps with compact fluorescent lamps. However, such like-for-like replacement might result in malfunction or fire, such as where the maximum rated switching current of an in-circuit automatic switching device is not identified and consequently a load greater than the devices’ switching capability is connected. For example, replacing existing lamps with lamps having different characteristics might result in unwanted in-rush current.

In all electrical installations equipment should be appropriately rated for the maximum current to be carried in normal service, and the current likely to be carried in abnormal conditions (Regulation 133.2.2 refers).

Additions and alterations

A particular problem involving inappropriate assessment of automatic switching devices – that might lead to danger from fire – is where a contractor is requested to make an addition or alteration to an existing circuit containing an automatic switching device, and there is an absence of manufacturer’s information for the device.

In such circumstances the contractor should consult the manufacturer (or their website) and/or take account of labels attached to the device (if any), to ensure the switching capability of the device is adequate for the altered circumstances.

If no information can be sourced, consideration should be given to replacing the device with a new device suitable to accommodate the existing load and additional load, to ensure compatibility between device and load.

Part of Regulation 132.16 requires those persons making an addition or alteration to an existing installation to assess the rating and condition of any existing equipment.

Example of inappropriate assessment

As previously mentioned failure to adequately assess the compatibility of an automatic switching device and the connected load might result in failure of the device or fire.

Consider as an example, where a client requires the installation of two 2 Kw storage heaters, controlled by a single time switch rated at 13 A. If it is not recognised that the total load is 4000 W and when both heaters are simultaneously in charge mode, a time switch that can switch about 18 A of resistive load is required. Fig 1 shows the results from such an inappropriate choice.

Utilisation category

The utilisation category for a switching device or fuse is defined in BS EN 60947-1 as, ‘…combination of specified requirements related to the conditions in which the switching device or the fuse fulfils its purpose, selected to represent a characteristic group of practical applications’.

The specified requirements, for example, may concern the values of making and breaking capacities and other associated characteristics, as well as the relevant conditions of use and behaviour.

An item of equipment, such as an automatic switching device, will be labelled in such a way as to enable the designer/installer to choose an appropriate device in the light of the nature of the load to be switched.

Table 1 is found in BS EN 60947-4-1 Low voltage switchgear and control gear.

Contactors and motor starters. Electromechanical contactors and motorstarters applied to equipment such as AC and DC contactors lists the type of applications that would be applicable for particular utilisation categories.


The effects on an automatic switching device of switching a load greater than the manufacturer’s recommend rating, or where insufficient account has been taken of the nature of the connected load, can be more significant than just device failure; a risk of fire also exists.

Contractors should always consult the manufacturer’s information or the manufacturer before carrying out electrical works that involve the use of automatic switching devices to ensure compatibility between device and connected load.


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