Single fault conditions: a condition in which one means for protection against electric shock is defective or one fault is present which could cause a hazard [defined in the IEC 60050-826-2022].
Note 1 to entry: If a single fault condition results in one or more other fault conditions, all are considered as one single fault condition.
In the fundamental rule of protection against electric shock, which is established by IEC 61140, the operating conditions of an electrical installation are characterized by the two terms “normal conditions” and “single fault conditions”.
That is, under conditions of a single fault, some precautionary measure for basic protection (primarily the basic insulation) is in a faulted state, creating real preconditions for electric shocks. Therefore, in the electrical installations of buildings, protective provisions for fault protection (supplementary insulation, protective equipotential bonding, protective devices responding to earth fault currents, protective screening, etc.) are provided which provide protection against electric shocks.
Single faults shall be considered, if they would:
- cause an accessible, non-hazardous-live-part to become a hazardous-live-part (e.g. due to failure to limit the steady-state touch current and charge); or
- cause an accessible conductive part which is not live under normal conditions to become a hazardous-live-part (e.g. due to failure of basic insulation to exposed-conductive-parts); or
- cause a hazardous-live-part to become accessible (e.g. by mechanical failure of an enclosure).
To meet the fundamental rule under single-fault conditions, fault protection, and in certain cases additional protection, is necessary. This protection can be achieved by:
- a further protective provision, independent of that for basic protection, or
- an enhanced protective provision which provides both basic and fault protection, taking account of all relevant influences.
The requirements for provisions for fault protection are given in 5.3 .
Protection by Independent Protective Provisions
Each of the independent protective provisions shall be designed so that a failure is unlikely under conditions specified by the relevant technical committee.
The independent protective provisions shall have no influence on each other such that a failure of one of the protective provisions could impair another.
Simultaneous failure of independent protective provisions is unlikely and need not normally be taken into consideration. Reliance is placed on the unaffected protective provisions remaining effective.
Protection by an Enhanced Protective Provision
The properties of an enhanced protective provision shall be such that the same continued effectiveness of protection as provided by two independent protective provisions is achieved. Requirements for enhanced protective provisions are given in 5.4 .
- IEC 60050-826-2022
- IEC 61140-2016