Surge current – GE SHOWBIZ User Manual
Page 58
GE LIGHTING - SHOWBIZ™ ENTERTAINMENT LIGHTING CATALOGUE
110
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 LIGHTING - SHOWBIZ™ ENTERTAINMENT LIGHTING CATALOGUE
111
Fusing of Tungsten Halogen Studio and Theatre Lamps
A lamp normally fails at end of life by fusing of the filament. Often an arc then forms and as there is
little resistance to limit the current this rises to a very high value which if maintained can result in a
serious overload on the envelope and seals. This might result in the lamp shattering. A quick acting
high breaking capacity fuse must be connected in the supply line in all applications. Suitable types are
given in BS88 (IEC 60269), IEC 60127 or IEC 60241.
lamp
fuse (rated current)
power
(amps)
(watts)
100-115V
115-130V
220-250V
500
6
6
4
650
10
6
4
1000
16 (15 UK)
10
6
1500
20
16 (15 UK)
10
2000
25 (30 UK)
25 (20 UK)
10
2500
35 (30 UK)
25 (30 UK)
16 (15 UK)
5000
63 (60 UK)
50
25 (30 UK)
10000
125
100
50