7. MEASUREMENT OF LOOP IMPEDANCE AND PROSPECTIVE
SHORT CURRENT
If there is a RCD or fuse in the circuit, they must be bridged out prior to
testing the loop impedance.
According to IEC 60364, every loop should meet the formula: Ra≤50/la
Ra: loop impedance
50: max of touch voltage
Ia: the current than can make the protection device break down the circuit
in 5 seconds.
When protection device is RCD, Ia is rated residual current IΔn.
IΔn 10 30 100 300 500 1000 mA
Ia(50v) 5000 1667 500 167 100 50 Ω
Ia(25v) 2500 833 250 83 50 25 Ω
According to IEC 60364, every loop should meet the formula: Zs ≤ Uо/la
When protection device is FUSE, Uо=230V, la and Zsmax:
Rated Break time (5s) Break time (0.4s)
Current Ia(A) Zs(Ω) Ia(A) Zs(Ω)
6 28 8.2 47 4.9
10 46 5 82 2.8
16 65 3.6 110 2.1
20 85 2.7 147 1.56
25 110 2.1 183 1.25
32 150 1.53 275 0.83
40 190 1.21 320 0.72
50 250 0.92 470 0.49
63 320 0.71 550 0.42
80 425 0.54 840 0.27
100 580 0.39 1020 0.22
Prospective short current must be bigger than la.
6
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