Automatic disconnection of supply (ADS) is the most widely used method for achieving protection against electric shock in an installation.
It works by limiting the size and duration of any voltages between exposed-conductive parts / extraneous-conductive parts or Earth. This is achieved through a combination of factors that must be in place.
Basic Protection:
Provided by basic insulation of live parts or by barriers or enclosures.
Fault Protection:
Protective earthing – main earth, CPCs
Protective equipotential bonding – Gas, Water, Structural Steelwork etc
Automatic Disconnection in case of a fault – Protective devices must be able to meet their maximum disconnection times. This is achieved by ensuring that Zs of any circuit is low enough so that any current that occurs during a fault is sufficiently high enough to trip any protective device (ohms law) within its maximum disconnection time.
Reference: BS7671(2015)
Basic Insulation – Section 416
Maximum disconnection times – Table 41.1 and Regulations 411.3.2.3 and 411.3.2.4
Maximum Zs values – Tables 41.2, 41.3, 41.4, 41.5 and 41.6
Earthing and Bonding – Chapter 54