IEC 61472-2 pdf – Live working – Minimum approach distances – Part 2: Method of determination of the electrical component distance for AC systems from 1,0 kV to 72,5 kV
This part of lEC 61472 specifies a method for determining the electrical component of theminimum approach distances for live working, for AC systems 1 kV up to and including 72,5 kV.
This document addresses system overvoltages and the working air distances betweenequipment and/or workers at different potentials.
The withstand voltage and minimum approach distances determined by the method describedin this document can be used only if the following working conditions prevail:
workers are trained for，and skilled in，working live lines or close to live conductors orequipment;
the operating conditions are adjusted so that the statistical overvoltage does not exceed thevalue selected for the determination of the required withstand voltage;
– transient overvoltages are the determining overvoltages;
– tool insulation has no continuous film of moisture present on the surface;-no lightning is observed within 10 km of the work site;
– allowance is made for the effect of the conducting components of tools.
NOTEIn some countries，special procedures have been developed to permit live working with surface moisture ontools at distribution voltages (below50 kV).
Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and lEC maintain terminological databases for use in standardization at the followingaddresses:
.IEC Electropedia: available at http://www.electropedia.orgl
Iso Online browsing platform: available at http://www.iso.orglobp
3.1
highest voltage of a systemUs
highest value of operating voltage (phase-to-phase voltage) which occurs under normaloperating conditions at any time and any point in the system
Note 1 to entry: Transient overvoltages and permanent induction from adjacent lines are not taken into account inthe calculation formula
3.2
transient overvoltage
short duration overvoltage of a few milliseconds or less, oscillatory or non-oscillatory, usuallyhighly damped
[SOURCE: IEC 60050-614:2016,614-03-14]
3.3
nominal system voltage
suitable approximate value of voltage used to designate or identify a system
[SOURCE: IEC 60038:2009,3.1]
3.4
per unit statistical overvoltage phase-to-earthle2
phase-to-earth per unit overvoltage that has a 2 % probability of being exceeded
3.5
per unit statistical overvoltage phase-to-phaseup2.
per unit overvoltage that has a 2 % probability of being exceeded
3.6
statistical overvoltageU2
overvoltage that has a 2 % probability of being exceeded
3.7
minimum approach distanceDA
minimum electrical and ergonomic distance in air to be maintained between any part of the bodyof a worker, or any conductive tool being directly handled, and any live conductors or equipmentat different potentials
3.8
electrical distanceDu
electrical component of the minimum air distance between two electrodes which represent liveand/or earthed conductors or equipment, required to prevent sparkover under the most severeelectrical stress that will arise under the chosen conditions
3.9
ergonomic distanceDE
distance in air added to the electrical distance, to take into account inadvertent movement anderrors in judgement of distances while performing work
[SOURCE: IEC 60050-651:2014,651-21-13, modified – the symbol DE has been added.]
4Minimum approach distance，DA
The minimum approach distance, DA. is determined by:
where
Duis the required minimum electrical distance, and
DE is the required ergonomic distance which is dependent on work procedures，level of training, skill of the workers, type of construction, and such contingencies as inadvertentmovement and errors in appraising distances (see Annex B for details).
5 Factors influencing the minimum approach distance5.1Control of system overvoltages
The maximum amplitude of overvoltages in the work area can be reduced by the usual practiceof making the circuit-breaker reclosing devices inoperative, or by using protective gaps or surgearresters.
5.2Statistical overvoltage
The electrical stress at the work area shall be known.The electrical stress is described as thestatistical overvoltage that can be present at the work area. In a three-phase AC power systemthe statistical overvoltage Ue2 between phase and earth is:
where
U s is the highest voltage of the system, and
u e2 is the statistical overvoltage phase-to-earth expressed in per unit.