tester shows open ground in hospital receptacle

[Stevenfyeager]

My simple plug in tester shows open ground in a hospital grade, generator backed up receptacle in a surgery room. It is grounded with a wire to the metal box. But I didn't take time to test hot to ground inside the box, only used my plug in tester. I checked several outlets, all had only the middle light come on, on the tester. ?

 

[kwired]

I have no experience in operating rooms but do know they can use isolated ungrounded systems in such areas and is such practice is covered by 517 part VII, and is touched on some in 517.20 as well.

If such an isolated system were used there would be no ground reference to make such a tester indicate proper polarity.

 

[electricalist]

If the device is making poor contact to the box then the tester could show open ground.
I was at a dr in a box last year and the plugs showed open ground, the inspector had me open one and the conduit was the grounding method so there was no wire type to the device, they were existing but i added a ground to the box.
Some say plug testers are like non contact testers,,a mistake waiting to happen.

 

[kwired]

Device contact with the box shouldn't matter in a 517 application we need both a wiring method that is a stand alone qualifying EGC as well as an insulated copper EGC, and according to 517.13(B)(1)(1) that copper EGC will also be connected to the EGC terminals of receptacles.

An isolated system that is not grounded still has to have the same "redundant grounding" installed - it just doesn't have a reference to ground - until there is a first fault.

 

[iwire]

My simple plug in tester shows open ground in a hospital grade, generator backed up receptacle in a surgery room. It is grounded with a wire to the metal box. But I didn't take time to test hot to ground inside the box, only used my plug in tester. I checked several outlets, all had only the middle light come on, on the tester. ?

There is a very good chance the outlets are supplied by an isolated power system and your reading would be expected from those.

http://www.pglifelink.com/pdf/isolated-power-explanation.pdf