The Siemens trains have been around for 12 years.
What has changed so that the static inverters are (apparently suddenly) now a serious problem? Has Siemens/Metro maintenance managed to buy a job lot of el cheapos or has their maintenance regime changed whereby these things are being run beyond their use by date (assuming that they have one)?
Perhaps one of our good learned electrical people can put together a para or two on 'the life cycle of a static inverter'.
Alright, here goes. Static inverters are a type of power electronics equipment, in this case used to convert the 1500V direct current overhead into alternating current that can be used by the induction motors in the bogies. Like many other kinds of power electronics, they use capacitors for voltage regulation. Judging by the stated public facts
of 'Siemens train static inverters are blowing up violently' and 'it was the capacitors wot done it', I would guess that the capacitors used in this instance are electrolytic capacitors.
Normally when an electronic component dies it will go quietly, perhaps emitting a puff of the 'magic smoke' as it finishes its useful life. Electrolytic capacitors have a different failure mode - they blow up. This happens when the dielectric materials inside the capacitor break down, from too much voltage and too high a temperature. High temperature conditions (such as when you flog a Siemens or send it out on one too many 40+ deg C days) prematurely 'age' the capacitor and assist in dielectric breakdown. What happens during a breakdown, you ask? Why, the material vaporises. A capacitor is a small confined space, so rapid vaporisation + confined space = explosion. Here's an example: The Case of the Exploding Capacitor
Given that the Siemens EMUs were introduced onto the network in 2003, I'd guess that their static inverter capacitors are coming up to the end of their life - in consumer electronics, electrolytic caps have a lifetime ranging from 2 years to a decade or more, depending on how well they were built
. Of course, flogging the units hard doesn't help. In any case, they should've started replacing the capacitors around about now, but they don't seem to have many spares, if any. One might say that the underlying problem could be Metro's approach to maintenance as a whole, but I'll give them the benefit of a doubt until there's a publicly released report on the matter.
Regarding rail worker safety vs passenger safety: the static inverters on the Siemens EMUs are mounted in the undercarriage. When they blow up, they blow outward - the direction that the static inverter box channels them (the doors appear to be channelled along with the rapidly expanding ex-capacitor
gas, hence the heavy duty straps remedy). This isn't a huge hazard to passengers, but is to rail workers walking or standing alongside a Siemens as it passes by or as it stands still but energised.