Basic Protection, Fault Protection and Additional Protection

The three-tier protection-against-electric-shock model used by IEC 61140 (and adopted in NEN 1010 chapter 4.1), which replaced the older terms 'direct' and 'indirect contact': basic protection prevents contact with parts that are live under normal operating voltage (for example through insulation of live parts, enclosures or barriers) and must function under all normal conditions; fault protection prevents dangerous touch voltage on conductive parts that are normally not live but can become live under a single fault (such as a breakdown of the basic insulation), typically via automatic disconnection of supply in combination with earthing and equipotential bonding; and additional protection is a third, non-standalone layer of protection applied only on top of basic and fault protection, in practice almost always implemented by a 30 mA RCD, to further reduce the risk if the first two layers fail (for example a non-functioning PE connection). A single measure can in some cases provide both basic and fault protection (for example PELV), but the three layers are always assessed separately in design and risk analysis.