In bedford we have been experiencing a high level of failures of the Helios CLS LV head. The cause of failure has been with the LMF unit within the heads rather than the led aspects themeselves.
The repaired units we have been receiving back always seem to detail the same repair. Which involves the resoldering of the PCB which im my opion is a sign of overheating.
As you know we have a number of these and have problems more with the fuses blowing in the LMF.
Currently Siemens are investigating the issue and undertaking repairs at thier cost with us picking up the TM. The ELV ones don't seem to be as much of a problem so far. We have yet to hear back on the findings so far but if it is PCB, the most likely cause may well be as you suggest overheating.
Incidently, when commissioning a site recently we found the need to have more fuses present as if you test the lamp monitoring correctly by removing the LED aspect from the LMF to simulate a failure, the fuse is blown by the LMF to provide the drop in current for the controller to pick up the failure. When the aspect is plugged back in the fuse needs replacing before it illuminates.
This is interesting, in Oxfordshire Helios CLS LV heads account for less than 10% of our signal heads but well over 50% of our lamp faults. The maintenace contractor is blaming the LMF units, but When we've raised this with the powers that be we are being told we are the only customer with a problem..... Its nice to know we're not..
Would that be the same maintenance contractor that is also the manufacturer? We have been assured that there is no issue with the LMF by siemens and they have only had a small number of minor failures !!!
We are now insiting that the 3 recently installed 900 junctions are modified to with the newer LED lamp switch card that can directly monitor the aspects without the need for the LMF unit.
We (Somerset) have also had the same problems with the CLS LV LED heads and have had to return many of them, which is time consuming. I am afraid I cannot give any further details as to why they are failing.
Overall the Siemens LMF unit has proved to be very reliable, providing lamp monitoring compatibility for LED signals when they are used with controllers whose lamp monitoring is characterised for incandescent lamps.
However, as part of our normal product reliability monitoring we have noted an increase in the failure rate of the LMF unit. As well as random failures there have also been a small number of instances where several units have failed on one site. Although the failures only represent a small percentage of the units in service it has prompted an extensive investigation into their cause. This has revealed that the cracking of a particular solder joint within the unit is the cause of the majority of these failures. They appear to be batch related and only a minority of LMF units in service are likely to be affected.
As a result of the investigation a small design change has been made to the LMF to eliminate any issues with the solder joint. All faulty LMFs returned to us within their 5 year warranty period are also being modified to eliminate the risk that this solder joint might fail in the future