Is there any smart way to Locate/detect earthy leakage field instruments (such as limit switches) connected to Field input termination units bailey Infi-90 DCS system??

We have DC Floating power Supply that now reads +21/+3VDC instead of +12/-12VDC. This Power Supply gives power to bout 500 digital input DCS signals.

So there is ground fault. the -ve side of the power supply has some leakage to the ground but not clear 100% grounded. this leakage may be coming from more than one field instrument.

We need a way to solve this problem without disconnecting the power to the system so that we can return the power o +12/-12VDC and eliminate this ground fault.

Thanks

 

Hanywepco... the simplest way would be to use a DC clamp-on meter to read the combined currents in the wire-pairs (pos/neg) associated with each output circuit!

The unfaulted circuit will display zero-amperes. The grounded-circuit will measure the difference in current between the pos and neg wire.

Regards, Phil Corso

 

Phil Corso has provided a good tool for troubleshooting ground loops and for detecting ground leakage which can be used to troubleshoot individual sensor circuits (for normally energized circuits or for normally de-energized circuits on the power side). The currents involved can be quite low. For a floating system, however, a single ground will not be detected using this method; there needs to be a second ground in the system for you to detect ground leakage current that creates a ground loop (though you may be able to detect a wiring mistake). For a grounded power system, this method can be an excellent troubleshooting tool.

Troubleshooting grounding problems can be difficult at best. It was not clear what you measured your voltage in reference to. In a floating DC system, if you read a voltage to ground, you have a ground in your circuit. If you do not, then your problem lies elsewhere (a problem with your system reference?). A cross connection to a different system can be a possible cause of this problem as can a bad power supply or a wiring mistake. Also, make sure that you do not have AC in your DC circuit

If you do have voltage to ground, you do have a ground in your power system but it is unlikely that a single ground would cause this problem (though not impossible) and a lot harder to find though there are ways to do it. When strange voltages appear in a system, it is commonly caused by a ground loop of some sort. One thing I would suggest is that you use the method Phil described at the closest you can get to your power supply upstream of any sensors and as much of your power distribution as you can and move outward toward your sensors to see if you can find ground loop current. This can eliminate the power distribution being involved and sometimes find the ground loop current path. Also note that the power supply does not necessarily have to be involved in the ground loop but can still be affected by it. If you still cannot find the ground loop, you can then try the sensors. Once you have found the first ground, you have done the easy part, finding the second ground can be more difficult and you should leave the first ground intact till you find the second ground. Also, be careful in disconnecting grounds as some times you do not know what they are connected to if there is another ground in the system.

It is also good to use your power system drawing or a system map to document as you go because these problems can be quite complex requiring some logic to figure out.

One thing which was not mentioned was did this happen after a change or modification to the system? After a rain storm maybe? Had you regularly checked your DC voltage to ground? (it is easy to put in a detector to detect a ground (an LED/resistor might do or you can buy a detector with an alarm contact)) Do you use shielded circuits as an inadvertent connection to a grounded shield can give you a ground in your system? Water in your conduit system can be another potential source of a ground(s) as can corrosion.

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP
Any information is provided on Caveat Emptor basis.