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
Zs
max:
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
