The article is a supplement of the information regarding protecting devices manufactured by ETI Polam and included in earlier issues of "Electro-installer".
This information is especially useful for numerous fuse cutouts users, i.e. for users, which are dealing with measurement and a diagnostics of electric wirings, designers, constructors etc.
For the purpose of the correct selection of fuse cutouts for protecting electric wirings andtheir installations, it is indispensable to have the basic knowledge about their time-current (t-I) band characteristics. Usually, these curves are generated on coordinates with logarithmic scale. Read-out values of operating currents taken from such characteristics can have great errors, especially that not always those diagrams are accessible in large sizes.
To avoid these inconveniences, in tables mentioned below are given read-out values and calculated maximum operating currents values (burn-through currents) for fuse cutouts for times - 0, 2s, 0, 4s, 5s and also suitable k-factors determining multiple fuse link current.
Designing process of the electric wiring protecting devices requires the use of time-current (t-I) band characteristics of fuse cutouts, reading values of operating currents are taken from upper curve, in other words, these values are maximum switching-off times. Figure1 shows the example of fuse link Band
Characteristic for 63A and also shows the manner of the reading the maximum value of the cutoff current Imax for the required time tz ( 0, 2s, 0, 4s, 5s) and the manner of the calculation k factor, k = Iz/In. Times 0, 2s, 0, 4s, 5s are used in the 230V/400V power networks for protection against indirect touch by means of automatic power supply disconnection. In Table1 are given conventional testing-times- t (h) and k-factors for all kinds of fuse cutouts and their characteristics and ranges of high testing-currents If . The testing-time t (h) is the time value, at which we can read the greatest (maximum) operating current for protective devices before the surcharge (overload) is observed.
Having readout values of maximum operating currents (Imax) of fuse cutouts, it is possible now easily assign the greatest value of the impedance for short-circuit loop Zmax = 230V/Imax.
This will simplify measurements of short-circuit loop, because the most meters measure impedance. For the given cutoff time of fuse cutout the measured impedance Zp cannot exceeds the impedance Zmax.
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