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.
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.
Many thanks for the quick reply, the small black filters are mounted on the inside right hand of the transformer NT6631, which I believe each act for all your different bus bar voltages 110bx b120 b50 b24, I believe like you mentioned that they are possibly shorting to the case of the transformer upon where they are mounted. But another colleague of mine mentioned that the B50 showing 25 volts to earth is half wave rectification, I am confused as how that relates if at all.
Any clarification would be greatly received.
(08-11-2012, 07:52 PM)Archie Wrote: [ -> ]Many thanks for the quick reply, the small black filters are mounted on the inside right hand of the transformer NT6631, which I believe each act for all your different bus bar voltages 110bx b120 b50 b24, I believe like you mentioned that they are possibly shorting to the case of the transformer upon where they are mounted. But another colleague of mine mentioned that the B50 showing 25 volts to earth is half wave rectification, I am confused as how that relates if at all.
Any clarification would be greatly received.
This is not strictly relevant to the forum, but it is interesting and I am sure we can make a learning point of it! Can you post a picture of the item?
Peter
Certainly the typical 50V dc signalling busbar is anything but straight dc; half wave rectified peaking at around 80V, so there is a large ac component. I don't quite see how it would work out, but perhaps if the component causing the short to earth was on the transformer side of the bridge rectifier then perhaps that short is only "seen" at the busbar for one of the half cycles. thing is that I'd have then expected for that short to be seen on the other side of the busbar during the opposite half cycle.
As Peter says this is interesting and a better understanding of what these filters actually are may help shed light on the matter.
(08-11-2012, 07:52 PM)Archie Wrote: [ -> ]Many thanks for the quick reply, the small black filters are mounted on the inside right hand of the transformer NT6631, which I believe each act for all your different bus bar voltages 110bx b120 b50 b24, I believe like you mentioned that they are possibly shorting to the case of the transformer upon where they are mounted. But another colleague of mine mentioned that the B50 showing 25 volts to earth is half wave rectification, I am confused as how that relates if at all.
Any clarification would be greatly received.