Isolation and Switching Chapter 53 BS 7671

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Key Points of…

Isolation: 537.2

Every circuit / installation must be provided with a method of isolation from the supply. This isolation device should also be provided with a method to prevent unintentional or inadvertent reinstating (usually lockable).

In a TN-S or TN-C-S system it is not necessary to isolate / switch the neutral conductor providing that it is ‘reliably’ connected to Earth on the distributors incoming supply side.

It’s worth remembering that semiconductor devices can not be used as isolating devices.

Switching off for mechanical maintenance: 537.3

Mechanical maintenance is work that does not involve exposure to electrical connections. As such this work is often undertaken by ‘ordinary persons’.

Examples: Cleaning, adjusting or replacing parts of a machine. Replacing lamps in a fluorescent light.

Isolating devices must be able to switch the full load current and should be local to the equipment. More often than not the main isolating device is also used for mechanical maintenance for machinery as it’s close by and lockable. Other devices may be used such as, fused connection units, double pole switches and plugs and sockets.

Emergency switching: 537.4

Emergency switching should be provided for any part of an installation where it is necessary to isolate the supply quickly to remove unexpected danger.

In installations with rotating machinery this is usually done by using ‘stop’ buttons. Care should be taken to ensure that the stop buttons are…

  • Near to the machine / equipment
  • Should not be capable of being reset remotely, unless with a key switch.
  • If remote resetting is possible then the activated stop button must remain activated at the point of the danger until being reset once the danger is removed.
  • In areas where there are untrained people the stop buttons should lock when operated.

Functional switching: 537.5

The most common type of switching found in an installation. For general use to control any part of an installation that may need to be controlled independently from other parts.

They should be conveniently located and easily operated manually if needed. They include…

  • Light switches
  • Plugs and sockets (below or at 32A)
  • Switched fused spares
  • PIR sensors
  • Time clocks / thermostats etc

Functional switches may not include…

  • Fuses
  • Luminaire connections – plug in type ‘Clix’
  • Unswitched fused spurs
  • Socket outlets above 32A

“These are just the key points. For more detailed information see good old BS 7671 Part 5 Selection and Erection: Chapter 53 Protection, Isolation, Switching, Control and Monitoring”

 

 

 

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