3.2.2 -  Emergency switching

Emergency switching is defined as rapidly cutting the supply to remove hazards. For example, if someone is in the process of receiving an electric shock, the first action of a rescuer should he to remove the supply by operating the emergency switch, which may well be the main switch. Such switching must be available for all installations. Note that if there is more than one source of supply a number of main switches may need to be opened (see 3.2.1). The designer must identify all possible dangers, including electric shock, mechanical movement, excessive heat or cold and radiation dangers, such as those from lasers or X-rays.

In the special case of electric motors, the emergency switching must be adjacent to the motor. In practice, such switching may take the form of a starter fitted close to the motor, or an adjacent stop button (within 2 m) where the starter is remote. Where a starter or contactor is used as an emergency switch, a positive means must be employed to make sure that the installation is safe. For example, operation should be when the operating coil is de-energised, so that an open circuit in the coil or in its operating circuit will cause the system to be switched off {Fig 3.2}. This is often called the 'fail-safe' system.

Fig 3.2 - Two circuit breakers linked to a common stop circuit.
The system is 'fail-safe'

To prevent unexpected restarting of rotating machines, the 'latching off' stop button shown in {Fig 3.3} is sometimes used. On operation, the button locks (latches) in the off position until a positive action is taken to release it.

In single-phase systems, it must he remembered that the neutral is earthed. This means that if the stop buttons are connected directly to the neutral, a single earth fault on the stop button circuit would leave the operating coil permanently fed and prevent the safety system from being effective. It is thus essential for the operating coil to be directly connected to the neutral, and the stop buttons to the phase. Such an earth fault would then operate the protective device and make the system safe.

The means of emergency switching must be such that a single direct action is required to operate it. The switch must be readily accessible and clearly marked in a way that will be durable. Consideration must be given to the intended use of the premises in which the switch is installed to make sure as far as possible that the switching system is always easy to reach and to use. For example, the switch should not be situated at the back of a cupboard which, in use, is likely to be filled with materials making it impossible to reach the switch.

In cases where operation could cause danger to other people (an example is where lighting is switched off by operating the emergency switch), the switch must be available only for operation by instructed persons. Every fixed or stationary appliance must be provided with a means of switching which can be used in an emergency. If the device is supplied by an unswitched plug and socket, withdrawal of the plug is NOT acceptable to comply with this requirement,' such action is acceptable for functional switching {3.2.4}.

Fig 3.3 'latching-off' stop button

Where any circuit operates at a p.d. (potential difference) exceeding low voltage a fireman's emergency switch must be provided. Such installations usually take the form of discharge lighting (neon signs), and this requirement applies for all external systems as well as internal signs which operate unattended. The purpose is to ensure the safety of fire fighters who may, if a higher voltage system is still energised, receive dangerous shocks when they play a water jet onto it. The fireman's switch is not required for portable signs consuming 100 W or less which are supplied via an easily accessible plug and socket.

The fireman's switch must meet the following requirements

1. -  The switch must be mounted in a conspicuous position not more than 2.75m from the ground.

2. - It must be coloured red and have a label in lettering at least 13 mm high 'FIREMAN'S SWITCH'. On and off positions should be clearly marked, and the OFF position should be at the top. A lock or catch should be provided to prevent accidental reclosure.

3. - For exterior installations the switch should be close to the load, or to a notice in such a position to indicate clearly the position of the well-identified switch.

4. - For interior installations, the switch should be at the main entrance to the building.

5. - Ideally, no more than one internal and one external switch must be provided. Where more become necessary, each switch must be clearly marked to indicate exactly which parts of the installation it controls.

6. - Where the local fire authority has additional requirements, these must be followed.

7. - The switch should be arranged on the supply side of the step-up sign transformer.