Specifications

Siemens Mobility, Traffic Solutions
Sopers Lane, Poole, Dorset
BH17 7ER
Version 10 Page 117 of 120 Status Issued
Last Editor harry.smyth Date 03 November 2014
Document
Name
GENERAL HANDBOOK FOR HEIMDALL
DETECTORS
Doc. No. 667/HB/31900/000
Copyright  Siemens plc 2014 All Rights Reserved
12.3.2 24V AC/DC Transformer Supply Feed
In order to use the Siemens Detector transformers it is necessary to perform specific
calculations to ensure that the minimum supply voltage to the Heimdall detector is
maintained under all circumstances.
To assist the reader quick reference ‘look-up tables’ are included in Section 11 of this
document to provide the answer for installations where the same detector type is
connected to the supply cable being considered. If more complicated installations are
being considered then the procedures detailed below must be followed.
First calculate the lowest transformer voltage under worst case conditions using the total
transformer current (i.e. the total supply current for all detectors installed):
V Xmfr (min) = Vs(min) - (Rs x total transformer current in Amps)
Where: V Xmfr (min) = the minimum transformer secondary voltage at full detector load.
Vs (min) = the minimum transformer secondary voltage at no load and minimum mains
voltage.
Rs = the transformer secondary winding resistance.
Then subtract the minimum voltage at which the detector will work (19.2 volts). This gives
the maximum voltage drop which can be allowed in any cable.
Max allowed voltage drop = V Xfmr (min) - 19.2
The maximum allowable length for each cable can then be calculated:
Maximum cable length = __V Xfmr (min) - 19.2 _
total cable current x 0.042
(0.042 is the cable resistance in ohms per metre)
If longer cables are required, the arrangements must be revised. For example by using
two cable cores in parallel the maximum cable length changes to:
_ V Xfmr (min) - 19.2 _
total cable current x 0.021
Running separate cables to each post improves the situation since the calculation applies
to each cable, so that each cable will take less current. All loads on each cable must be
allowed for including pedestrian crossing detectors and kerbside detectors.
Where the situation is more complicated due to sharing of cables it will be necessary to
calculate the voltage at each detector - This voltage must be at least 19.2 volts.
The voltage at the end of any single cable is given by subtracting 0.042 x the length of
the cable in metres x the current in the cable from the voltage at the start of the cable.
This applies to 1 mm
2
cable with a separate return conductor for each run. This is based
on the out-and-return resistance of the cable being 0.042 ohms per metre.