Question about LIM Panel

[joedanzhere]

I work as an electrician in a hospital, and I don't quite understand Line Isolation Monitor panels. I have two legs which read 60 volts each, which means there is no neutral or (grounded conductor). I understand this part. I have heard some people say that you should not be able to see any voltage between either leg and the ground. In the circuits I've seen from the LIM panels I get a reading of 60 volts from either leg to ground. From what I understand the grounding conductor is bonded to the enclosure of the secondary of the LIM transformer. So if I am supposed to get a reading of 0 volts to ground on my receptacles this would mean that my LIM transformer secondary enclosure is not bonded to my mains electrical system. Since I am getting a reading of 60 volts to ground on my LIM system does this mean that my system is not grounded properly? Also from my understanding if you have a ground fault in a LIM system where a hot wire touches a metal enclosure nothing should trip but just alarm my LIM monitor. If I am read 60 volts to ground this would lead me to believe that my ground is bonded to my mains electrical system and that if I had a fault where one hot wire touched a metal enclosure I would trip a breaker, which from what I understand should not happen. Basically my question is should I be able to see a potential of 60 volts from either leg to ground in a LIM system and is this because my system is bonded somewhere where it shouldn't be?

 

[roger]

Try using a solenoid type tester in parallel with your DVM and see what it reads.

 

[garbo]

I retired from a large hospital 6 years ago and forget about the measurement from either side of the OR receptacles to ground. We had a special test device that we plugged our meter in and took at least 10 readings. Had a pot that you slowly turned to measure the milliamps that LIM set off an alarm. We had to record line voltage ( usually around 119 to 122 volts ), each side of receptacle to ground ( usually 56 to 64 volts ). Had to come in at 4 AM or early on a Sunday to perform this JACHO required monthly testing. Each of the over 25 OR'S had two LIM panels and a few had a LIM panel for a think it was a 50 amp 240 volt X ray receptacle. Took most of a day to enter all of these #'s on over 25 PM reports on the computer. Each Lim panel had a built in transformer that put out 120 volts with the center tap connected to ground. In the event of a patient receiving a shock it would be limited to around 60 volts. When they upgraded a dozen OR'S I asked them them to install Arc Fault labels. Three people from this company did not have a clue and asked why I wanted them. Never did get Arc flash labels. Hopefully would not be too high in my thinking the 5 or 10 KVA isolation LIM transformer would limit available short circuit current.

 

[jim dungar]

Never did get Arc flash labels. Hopefully would not be too high in my thinking the 5 or 10 KVA isolation LIM transformer would limit available short circuit current.

Arc Flash incident energy values are not well defined for 120V and single phase systems.

The isolated ground system you have is partly intended to prevent arcing in the first place.

The only issue would be if an arc occurred on the line side of the isolation panel internal transformer which would only be possible when the dead front was removed. I would think some type of label should be applied for these conditions, or have documented work practices requiring only deenergized work.

 

[roger]

Here is a partial Isolation Power System test from one of my projects.

and the report