Ground loops are a result of more than one point of connection to the ground system on the grounded conductor. IE at the main service and downstream from the service ground bond connection. Ground loops could exist now if two SDS are tied at some point with downstream grounds shorts (lot of if's). If you do not clear any (POSSIBLE) downstream connections you will have ground loops.
The easiest way to know if you have a down stream ground from the main service (or any SDS) is:
1. You have to know if the system is suppose to be grounded, if it is not and you have a "0" reading on one phase to ground and 480 on the other 2-phases you have a downstream short, phase to ground.
2. If you are not sure about downstream you can use a LEM/flexible CT and encompass all three phases in a Delta, with a Wye system you could encompass all phase and the neutral or just the phases. The readings with the first two test should be "0" or very *low, With the third method the readings should be close to the neutral readings. It would be necessary to do this at various points in the distribution system to attempt to locate the short.
EDITED On second thought this would only work if you had more that on ground on the distribution system, what would help would work, with a good multimeter, measure the voltage on the "0" to ground phase (grounded conductor) starting at the service, then measure and record voltage on the grounded phase at various locations in the distribution, if the readings (millivolts to 2-4 volts) raise as you get further from the service the system is grounded at the service. If the voltage readings are lower on a particular feeder the ground is on that feeder.
3. With a scheduled power outage you could isolate all the loads and megger each conductor to *2ground. I fell this is the preferred method as #2 could miss a spot if the short is at a branch circuit, unless you test all branch circuits.
* A certain amount of leakage current is normal, in a system 60+ years old I would expect a lot of leakage current.
*2 You say you do not have a ground but even ungrounded services have a "ground" to short too. Building steel, conduit, water pipes ECT. In an ungrounded system it is often said the first short is free, the second short trips the OCP.
In Wye systems we check better than 90% (rough guess) have downstream grounds on the grounded conductor/neutral. If you accidentally ground a phase conductor you will trip the OCP, but a downstream grounded neutral goes unnoticed till someone does some testing. Same thing goes for a ungrounded Delta system.
Some advantages of a ungrounded system is as noted above, the first short is free, the site can continue to function till such time as a schedule shut down can be arranged.
If this system is suppose to be ungrounded at a minimum ground alarms should be installed.
Lastly a system this old should have electrical preventative maintenance performed, and IMO if management was wise it this equipment should be replaced on budgeted scheduled arragement.
. There have be no known problems in the plant to date, but maitainence was concerned do to the lack of equipment grounding. My experience to date have been with mostly Wye set ups. I made a call to the local utlity and was told that it was a strait delta and nothing was mentioned about a corner system so i will have to call back and see if it is possible. If this is the instance, If it is I would just need to bond all of the service quipment and all of the SDS in the plant, are there any concerns with the possibility of ground loops??
