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GE LIGHTING - ENTERTAINMENT LIGHTING CATALOGUE
104
Surge Current
The cold resistance of a halogen studio lamp is approx. 1/17 of its value in normal operation. On switch on,
theoretically a surge current of 17√2 x the normal current would flow and depending on the thermal mass of
the filament* this will fall to the lamp normal current in approx. 1 sec. In practice this maximum theoretical
current does not appear due to (a) switch on does not always occur at the peak of the AC voltage, (b) the
supply has some impedance which is comparable with the cold resistance of high wattage lamps, i.e.
maximum possible surge current where V is the applied voltage and Z is the sum of the lamp cold resistance
and the supply impedance.
Typically supply impedance is the order of 0.3 ohm and lamp life is based on testing with such a supply. In the
rare cases where the line impedance is lower than this figure, an adverse effect on life may be encountered
particularly with high wattage types, due to the then extremely high surge current on switching.
lamp type cold max. surge current (amps) normal
resistance line impedance = operating
(ohms) 0 ohms 0.1 ohms 0.3 ohms 0.5 ohms current
240V 10kW 0.34 1000 774 530 405 41.5
240V 5kW 0.7 486 424 340 283 20.8
115V 5kW 0.15 1085 650 360 250 43.5
240V 2kW 1.7 200 189 170 154 8.35
117V 2kW 0.41 404 324 233 182 17.1
240V 1kW 3.4 100 97 92 87 4.15
GE S&S 2000 9/03/00 9/3/00 1:32 pm Page 106