Protection for Safety: Electric Shock
Two layers, two mechanisms
Protection against electric shock has two independent layers: basic protection (against contact with live parts in normal service) and fault protection (against contact in a fault). Both must normally be present; where one measure provides both, it is called reinforced protection (e.g. double insulation, Class II equipment).
Basic protection measures
Insulation of live parts, barriers and enclosures, obstacles, and placing out of reach. Traps: confusing a barrier (accessible only with a tool/key) with an obstacle (no tool needed, e.g. a rail). IP ratings and enclosure integrity are examined here.
Fault protection measures
- Automatic disconnection of supply (ADS) - the dominant method. Requires a protective conductor, main protective bonding, and disconnection within a specified time. For TN systems this depends on earth fault loop impedance (Zs) satisfying Zs ≤ U0/Ia, where Ia is the current that trips the device within the required time.
- Double/reinforced insulation (Class II).
- Electrical separation - one item, unearthed, isolation transformer.
- Extra-low voltage - SELV (safe, no earth) vs PELV (earthed).
- Residual current devices (RCDs) as additional protection (30 mA) for socket outlets and in special locations.
Worked example
A 32 A Type B MCB on a 230 V TN-S circuit needs to disconnect in 0.4 s. The tripping current for instantaneous operation is 5×In = 160 A, so maximum Zs = 230/160 ≈ 1.44 Ω. If measured Zs exceeds this, disconnection is not assured - remedy by reducing circuit length, increasing conductor size, or adding an RCD.
The classic trap
RCD additional protection does not replace ADS requirements or basic protection; it is a supplement. And SELV must have no connection to earth, unlike PELV.