Specifications

ST900 Family General Handbook

667/HB/32900/000 Issue 12 Page 66 of 265

3.15 Cabling

When estimating cable core and controller equipment requirements for the ST900

ELV Controller the maximum cable lengths defined in the following tables on pages

69 and 70 must be complied with.

The voltage drop in the installation must be no more than 4% of the incoming

supply. The tables should be consulted to ensure that the voltage drop is lower than

this maximum for the selected core size and loading. If the voltage drop exceeds 4%

the cores / conductors must be paralleled up to reduce it.

In addition the following guidance should be observed:

1. Where multiple cores are required due to long cable runs it must be noted

that this may require additional LSLS Outputs (and possibly an additional

LSLS Card) in order to accommodate the additional terminations in the LSLS

Backplanes.

2. If common ground return connections were used it is possible for the failure

of a ground return connection to cause unexpected signal displays, where

one or more signals within a given signal head are incorrectly illuminated

simultaneously. This lack of ground return connection is not detectable by the

controller because the signal voltage presented at the controller terminals

does not exceed the required thresholds for conflict or correspondence

monitoring. It is therefore essential that individual ground returns are used

for each green signal

3. The allowable lamp load per cable run is defined in the following tables.

Refer to Table 10 – ST900 ELV Lamp Load (Watts) on page 55 to determine

the total lamp load connected to each drive cable and each return cable.

For each ‘out-going’ drive cable, determine the total load of all the signals

supplied by that cable. This will typically be a single aspect (e.g. one Helios

ELV signal) but could be higher where a green drive also powers a tactile unit

for example.

A single LED signal return core (equivalent to the neutral in an LV system) is

to be provided for each Red, Amber Signal Head (or Nearside Red / Green

Signal). A separate return MUST be provided for the green aspect. Where a

common return core is used, the highest lamp load that may be illuminated at

any one time needs to be determined.

For a UK traffic signal head, one Helios ELV lamp load 12W is considered

the highest for the return cable since only one aspect is ever illuminated; the

short red/amber period can be ignored.

For a near-side pedestrian signal head, the figure is one near-side signal

unless it shares the same return with a Demand Indicator or a Tactile Unit.

Example, assuming a distance of 180 metres using 1.0mm

cable:

 Near-side ped drive cables: 18W each at 180 metres = 2 cores (each)

 Ped demand indicator drive cable: 6W at 180 metres = 1 core