Specifications
ST900 Family General Handbook
667/HB/32900/000 Issue 12 Page 59 of 265
3.14.5 Selection of Lamp Supply Transformer
In order to select the number of lamp supply transformers, the total average signal
Lamp Power is required. See Table 10 on page 55 for the power taken by the
different types of lamps.
When selecting the lamp supply transformer the peak lamp power is not used as the
transformer can withstand the overload for the 2 second Red / Amber period. Thus
for the average power consumption, it is assumed that only one lamp is illuminated
on each signal head. However, Pedestrian Red signals illuminated at the same time
as the Pedestrian and Wait / Demand Accepted signals must be included.
For the theoretical junction (Figure 13) this would be:
One lamp per signal head @ 12W x 12 signal heads
144W
Wait indicators at 7W x 4
28W
Regulatory Sign at 7W x 1
7W
Total average signal lamp power is:
179W
For the theoretical junction, using the figure obtained above (179W), one
transformer would be required.
A single 20A lamp supply transformer is required to power a 20A controller using up
to three LSLS Cards. Two 20A lamp supply transformers are required if more than
20A or more than three LSLS Cards are required.
The table below summarises the current capability provided by one or two 20A
transformers, the number of LSLS cards each can support and the typical number of
phases and signals heads
†
:
LSLS
Cards*
2A
Output
Switches
Phases
(approx
)
Lamp Supply
Transformer
Max. Lamp
Current
Av. Signal
Heads per
Phase
†
1 32 1 – 8 20A 20A >10
2 64 9 – 16 20A 20A 5 – 10
3 96 17 – 24 20A 20A 3 – 5
2 64 9 – 16 2 x 20A
40A
>10
3 96 17 – 24 2 x 20A
40A
7 – 10
4 128 25 – 32 2 x 20A
40A
5 – 6
5 160 26 – 32 2 x 20A
40A
5 – 6
6 192 26 – 32 2 x 20A
40A
5 – 6
Table 13 – ST900 ELV Current Capability
* Refer to section 3.13 on page 54 for limits on the number of LSLS Cards supported.
†
Assuming 12W 48V Helios ELV Signals, 20A allows for approximately 80 signals which will
normally mean that 80 signal heads can be supported since usually only one aspect of the phase is lit