Miswired isolated ground

[drillman]

Found and repaired an isolated ground that was installed wrong in a 15 year old building.

An office worker complained of a buzzing noise from a power strip. Some maintenance techs responded and read 70 volts from ground to neutral at the plug, which was an isolated ground plug. They started flipping breakers off and discovered that the voltage went away when they turned off an AC package unit.

I showed up and first suspected an open neutral but found no neutral problems. Called HVAC guy and we found blower moter shorted to ground at the AC unit.

Pulled subpanel cover and noticed that the
AC unit had a ground wire going to the isolated ground bar along with the ground wire from the plug. The bar was isolated from the panel with fiberglass. Pulled main panel cover and found that the isolated ground bar went straight to a ground bar with no other bonding to any steel, water, or rebar.

What I figure happened was that the blower motor winding grounded out, and since the ground wire for the blower was on the miswired isolated ground bar there was no good return path to trip the breaker. The current ended up returning somehow through a power strip in the office.

Bonded miswired isolated ground bars properly. They are no longer isolated.

Checked the blueprints and they show the isolated ground bar as not bonded to anything else. So we have a design failure and a install failure.

This was a new thing for me, hopefully others can learn something from this post.

 

[mpd]

thankfully nobody got hurt, sounds like everybody dropped the ball

 

[infinity]

I have seen this happen too. An IG bar in a panel with no connection to anything but the branch circuit IGC's. Another time I found an IG bar with a #6 conductor run up to a piece of rebar that had been chopped out of the ceiling slab and terminated with a 1/4" beam clamp and a lug. I couldn't believe it either.

 

[zbang]

One of the bits of PM at the steel mill I worked in was called "playing ground hog". When -everything- was down for maintenance, they'd drop power to all IG panels, pull the IG bond, and meter from the IG bus to the GEC. Usually then they'd have to start lifting & metering individual leads until they found the offending one and trace it back :-D. At least things were labeled, they were -very- good at that.