Chapter A General rules of electrical installation design

N - Characteristics of particular sources and loads
4 Lighting circuits
Assuming an assembly of 50 fluorescent tubes of 36 W each:
b Total active power: 1,800 W
b Apparent power: 2 kVA
b Total rms current: 9 A
b Peak current: 13 A
With:
b A total capacity: C = 175 µF
b A line inductance (corresponding to a short-circuit current of 5 kA): L = 150 µH
The maximum peak current at switch-on equals:
-6
C
x 10
Ic = Vmax
= 230 2 = A
L
-6
x 10
175
350
150
The theoretical peak current at switch-on can therefore reach 27 times the peak
current during normal operation.
The shape of the voltage and current at ignition is given in Figure N42 for switch
closing at the line supply voltage peak.
There is therefore a risk of contact welding in electromechanical control devices
(remote-control switch, contactor, circuit-breaker) or destruction of solid state
switches with semi-conductors.
600
400
200
0
-200
-400
-600
0
300
200
100
0
-100
-200
(V)
(A)
-300
0
Fig. N42 : Power supply voltage at switch-on and inrush current
Schneider Electric - Electrical installation guide 2008
0.02 0.04 0.06
0.02 0.04 0.06
In reality, the constraints are usually less severe, due to the impedance of the cables.
Ignition of fluorescent tubes in groups implies one specific constraint. When a group
of tubes is already switched on, the compensation capacitors in these tubes which
are already energized participate in the inrush current at the moment of ignition of
a second group of tubes: they “amplify” the current peak in the control switch at the
moment of ignition of the second group.
t (s)
t (s)
N31
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