04-22-2012 05:54 AM
I encountered problem in our MCC's. I turn off the main breaker and definitely line voltage(440V) at the load side also cut off.
But, I wonder why that the line to the body ground of the MCC's still has a voltage of 220 Volts?
Can somebody explain why? Having experienced with this?
04-22-2012 02:35 PM
Between test equipment with very low burdens and power electronics in use more and more, the presence of leakage votage is more common. Many times finding the source can be quite a problem, and it can be a safety issue since it skews the results of a de-energization check. Is the leakage voltage at line frequency, or a multiple of it?
When you check, what is the state of the MCC bucket switches, all open or closed? Try opening all the switches/breakers and see if the apparent voltage decays, you may need to leave you meter on for a while to watch it bleed down. Then close the switches/breakers one at a time and see if any or multiple ones cause the leakage voltage, likely suspects are VFD's and softstarts. Do any of the buckets or circuits have PF correction capacitors?
Hope that helps, Mike L.
05-30-2012 04:41 AM
BE CAREFUL!! It sounds like a wiring error that could hurt or even kill someone. Obliviously the ground system is not connected. Be sure to ground the MCC as per manufacturer's instructions. OK, you might blow a circuit breaker the first time, but that is better than shocking someone later.
I suspect a wiring error in some external source, like maybe a heater circuit, battery charger, lighting, or auxiliary receptacle. You don't say if this is an old installation or a new one. Maybe the ground system was installed correctly, only to have problems later. I was on a job site one time that the copper grounds were stolen one weekend for scrap. Luckily we found it before anyone got hurt.
While Mikel is technically correct in his answer, this should not be happening. You have a serious grounding problem resulting in a potential safety issue that should be addressed immediately.
A life could be at stake...maybe yours.