IEC 61472-2004 pdf – Live working – Minimum approach distances for a.c. systems in the voltage range 72,5 kV to 800 kV – A method of calculation.
1 Scope This International Standard describes a method for calculating the minimum approach distances for live working, at maximum voltages between 72,5 kV and 800 kV. This standard addresses system overvoltages, and the working air distances between parts and/or workers at different potentials. The required withstand voltage and minimum approach distances calculated by the method described in this standard are evaluated taking into consideration the following: – workers are trained for, and skilled in, working in the live working zone; – the anticipated overvoltages do not exceed the value selected for the determination of the required minimum approach distance; – transient overvoltages are the determining overvoltages; – tool insulation has no continuous film of moisture present on the surface; – no lightning is seen or heard within 10 km of the work site; – allowance is made for the effect of conducting components of tools; – the effect of altitude on the electric strength is taken into consideration. For conditions other than the above, the evaluation of the minimum approach distances may require specific data, derived by other calculation or obtained from additional laboratory investigations on the actual situation. 2 Terms, definitions and symbols For the purpose of this document, the following terms, definitions and symbols apply. 2.1 Terms and definitions 2.1.1 highest voltage of a system U s highest value of operating voltage which occurs under normal operating conditions at any time and any point in the system (phase to phase voltage) NOTE Transient overvoltages due e.g. to switching operations and abnormal temporary variations of voltage are not taken into account. [IEV 601-01-23, modified] 2.1.2 transient overvoltage short duration overvoltage of few milliseconds or less, oscillatory or non-oscillatory, usually highly damped [IEV 604-03-13]
2.1.3 fifty per cent disruptive discharge voltage U 50 peak value of an impulse test voltage having a 50 per cent probability of initiating a disruptive discharge each time the dielectric testing is performed [IEV 604-03-43] 2.1.4 ninety per cent statistical impulse withstand voltage U 90 peak value of an impulse test voltage at which insulation exhibits, under specified conditions, a 90 % probability of withstand NOTE This concept is applicable to self-restoring insulation. [IEV 604-03-42, modified] 2.1.5 two per cent statistical overvoltage U 2 peak value of a transient overvoltage having a 2 % statistical probability of being exceeded [IEV 651-01-23, modified] 2.1.6 required insulation level for live working statistical impulse withstand voltage of the insulation at the work location necessary to reduce the risk of breakdown of this insulation to an acceptably low level NOTE It is generally considered that an acceptable low level is reached when the value of the statistical withstand voltage is greater or equal to the statistical overvoltage having a probability of being exceeded by no more than 2 %. [IEV 651-01-17] 2.1.7 per unit value u expression of the per unit value of the amplitude of an overvoltage (or of a voltage) referred to U s 2 / 3 NOTE This applies to u e2 and u p2 defined in Clause 4. 2.1.8 minimum approach distance D A minimum distance in air to be maintained between any part of the body of a worker, including any object (except appropriate tools for live working) being directly handled, and any parts at different potentials NOTE The “appropriate tools” are tools for live working suitable for the maximum nominal voltage of the live parts. [Definition 2.7.1 of IEC 60743 and IEV 651-01-20, modified]
2.1.10 ergonomic distance D E distance in air to take into account inadvertent movement and errors in judgement of distances while performing work [Definition 2.7.3 of IEC 60743 and IEV 651-01-22] 2.1.11 part any element present in the work location, other than workers, live working tools and system insulation 2.1.12 live part conductor or conductive part intended to be energized in normal operation, including a neutral conductor, but by convention not a PEN conductor [IEV 195-02-12] or PEM conductor [IEV 195-02-13] or PEL conductor [IEV 195-02-14] NOTE This concept does not necessarily imply a risk of shock. [Definition 2.1.2 of IEC 60743 and IEV 651-01-03, modified] 2.1.13 work location any site, place or area where a work activity is to be, is being, or has been carried out [IEV 651-01-08]