Protection against electric shock

Protection against electric shock rests on two pillars: basic protection (live parts cannot be touched during normal operation) and fault protection (you are protected if the insulation fails). Fault protection is typically provided by automatic disconnection of the supply.

The principle is that a dangerous fault must disconnect the supply within a set time. This requires the fault current to be large enough to trip the protective device, and the PE conductor and earth connection to have low enough impedance. The specific disconnection times and impedance limits are laid down in DS/HD 60364-4-41 and depend on the system earthing (TN, TT, IT) — look them up in the applicable standard.

A residual-current device measures the sum of the currents in the live conductors. If that sum is not (close to) zero, a fault current is leaking to earth and the device trips. A device with a rated residual current of 30 mA provides protection against electric shock in many situations.

• 01The rated residual current is marked on the breaker (e.g. 30 mA, 300 mA).
• 02The type (AC, A, F, B) is chosen according to the nature of the load — for example, electronics with DC components often require at least type A.
• 03The requirement for RCD/RCBO coverage and the choice of type follow the applicable regulation and standard.

Protective equipotential bonding connects exposed and extraneous conductive parts so they are kept at (almost) the same potential. This reduces the touch voltage in the event of a fault. Supplementary bonding may be required in special rooms, e.g. bathrooms.

The limit for how high a touch voltage may be present for an unlimited time is UL = 50 V AC (or 120 V ripple-free DC) in normal, dry conditions. In wet conditions and special locations the limit is lower. If the limit is exceeded in a fault, the fault MUST be disconnected within a set time — and the higher the voltage, the shorter the time. (Source: DS/HD 60364-4-41.)

In the event of an insulation fault, the dangerous voltage may only be present for a short time. The table applies at nominal voltage to earth U0 = 230 V:

The TT system requires faster disconnection because fault current flows through earth and is normally handled by an RCD. (Source: DS/HD 60364-4-41, table 41.1.)

Automatic disconnection only works if the fault current becomes large enough to trip the protection within the time. This sets requirements for the fault loop impedance Zs:

Example: A 16 A C-type circuit breaker trips magnetically at up to Ia = 10 · In = 160 A (top of the C band). The requirement becomes Zs ≤ U0/Ia = 230/160 = 1.44 Ω. If you measure a higher Zs on the installation, the fault current does not reach 160 A quickly enough, and instantaneous tripping is not guaranteed within the time — so Zs must be reduced (shorter/thicker conductor) or the protection changed.

• 01SELV (safety extra-low voltage): maximum 50 V AC, galvanically isolated from earth and from other circuits, supplied from a safety transformer. No earth connection.
• 02PELV: like SELV, but the circuit or device may be earthed — used where earthing is necessary for functional reasons.
• 03FELV (functional extra-low voltage): low voltage for functional reasons, but WITHOUT full isolation. Therefore still requires the same protection as the rest of the system.

SELV and PELV are independent protection methods, because the voltage itself is kept safe. (Source: DS/HD 60364-4-41, section 414.)