25V volts to earth from a transformer does sound like a centre-tap firmly connected to earth.
I am not quite sure what you mean by a "filter" but I assume that what you must actually mean is some form of lightning protection from each of the busbars to earth.
If so I guess that one of these had failed short circuit. This might be a gas discharge tube (such as the 26As used with TI21 track circuits etc) or solid state device transzorb etc. If there is a lightning strike on one of the power lines, the device attempts to divert the energy safely to earth- in doing so it inevitably suffers some damage. If there is a lot of energy to dissipate (or indeed if it is the "final straw" for a device previously weakened) then it can end up permanently connected to earth- it has failed "short circuit".
This would of course give a full earth on the busbar, rather than the 25V you report. Somewhere along the line there must be a potential divider. The classic when checking busbars for earth using a voltmeter is to fail to realise that there is already an Earth leakage Detector plugged in and monitoring the busbar; inevitably these do place an earth on the busbars themselves, albeit very high impedance because they connect both the B and the N busbar through their monitoring circuitry to the earth.
So I can't quite answer your question; perhaps your "filter" consists of several components- lightning protection often has some in-line fuses, a gas discharge tube across the lines with a centre tapped earth and a varistor in parallel with it (faster acting but cannot tke as much power) and I think that I might need to understand the actual configuration to see where the potential divider effect come from. I am just wondering what would happen if the connection that was supposed to be the earth actually wasn't but simply acting as a common busbar shared between several devices but actually insulated from earth ......
Interesting queston though- any other ideas from anyone?
(08-11-2012, 05:33 PM)Archie Wrote: Please re-post if in wrong section.
Hi could someone be so kind to answer me this question for ease of mind.
If you have roughly around 25 volts to earth from a 50 volt DC busbar, and by changing one of the filters within the transformer that it derives from rectifies the leakage to earth, can someone just clarify the electrical principle behind what is happening in this instance, many thanks in advance.