3 phase power help
I suspect this is an ungrounded 480V delta service that is running a bit high of a voltage. 500V on a 480V service is only 4% above nominal, so while it is uncommon, it isn't necessarily a big problem. The unpredictable voltages between each phase and ground suggest that this service is not ground-referenced. In other words, it's ungrounded. That is uncommon, and is only appropriate in specific types of industrial installations where there is monitoring equipment in place to detect ground faults. You should not have an ungrounded service in a typical barn or shop - they are only for industrial plants with proper monitoring. If the service drop has ONLY three conductors, and all three are insulated, then you probably have an ungrounded delta service. The definitive test for this is to apply a load (like a string of 5 100W light bulbs) between each phase and a good actual ground (like a rod), while measuring the voltage. If it drops much at all, then the service is ungrounded. This test is a bit dangerous because the reference ground rod will be electrified during the test.
It is also possible that the service was originally grounded, but the grounding connection has failed over time. This could create a very dangerous condition, electrifying metal parts on the failed grounding system. If there is evidence of a neutral conductor anywhere in the system, then it was probably grounded originally. It may have been center-grounded like a normal 3-phase service, so the phase-to-ground voltage should be 277V. Or it may have been corner grounded, where one phase was bonded to ground and the other two are 480V with respect to ground. That is a common configuration for agricultural installations like irrigation well pumps. If your service drop has four conductors, or three insulated conductors plus a bare one, then you almost certainly have a 277/480V service with a bad ground. If the service drop has only two insulated conductors and one bare, then it was definitely a corner-grounded delta service that now has a bad ground. In either case, watch out! Metal equipment may be electrified.
Pictures of the panel and the service entrance would be very helpful.
Last edited by mpoulton; 09-19-2012 at 09:41 PM.
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