Isolated Power Specialist
Isolated Power Specialist

How to Troubleshoot a Line Isolation Monitor That Is In Alarm

Setting: During an active operation and without notice, the Sentry 5 LIM alarms. A staff member silenced

                 the alarm. After the operation concluded, an engineer has entered the operating room to

                 troubleshoot and correct the problem.


  1. Silence the LIM, if you have not already done so
  1. Press the “Push to Silence” button on the LIM or the remote annunciator
  2. The audible alarm should turn off, and the Red and Yellow lights illuminated
  1. Locate the defective equipment
  1. Check all equipment by leaving them turned on 
  2. Open the circuit breaker door on the isolated power panel that the LIM is in alarm
    1. There should be a circuit breaker directory inside showing what the breaker is serving
  3. Turn one breaker off and wait 10 seconds while watching the line isolation monitor to see when it drops out of alarm 
  4. If the fault did not clear, continue turning off one breaker at a time until the fault clears
  5. When the fault clears, turn all breakers back on except for the breaker that cleared the fault
    1. Turn the breaker that was causing the fault back on and see if the fault comes back on
    2. If it does, look at the circuit breaker director to see what that breaker is powering
      1. Let’s say it is receptacle #3. While the LIM is still in alarm, go to all #3 receptacles and unplug the equipment one at a time. Let’s say the ESC machine is on receptacle #3, you unplug it, and the fault clears, then it is the cause of the alarm. Tag it so BIOMed can check it out for a fault going to ground and take the ESC out of the operating room. Tell the staff not to use ESC until repairs are made.
      2. If you unplug all known equipment that is on circuit #3 and it is still in alarm
        1. You will have to open each receptacle box to make sure the receptacle wires are not shorting to ground
          1. Turn power off and inspect the wires inside the boxes
          2. With wires and receptacles out of boxes carefully turn power back on to see if the LIM will go back in alarm
            1. If the LIM does go back into alarm and you can't find anything wrong
              1. You will have to remove the front of the panel to ohm out the wires
                1. Turn power off disconnect the wires from breaker #3 and ohm out to see which conductor is shorting to ground
                2. This will take some time to complete. If they need the operating room
                  1. You can lock out circuit #3 and tell them they can use the room except circuit #3
                  2. Then you can come back later to finish the testing after hours 
  6. What to do if you turn all breakers off and you are still in alarm

a.  The only components left is the LIM and the transformer 

b.  To determine which is bad unplug the LIM 

  1. Take a voltmeter and measure the secondary side of the isolation transformer Xl to X2
    1. You should get about 120 volts
  2. Measure Xl to ground
    1. With nothing turned on and LIM unplugged you should get about 60 volts
  3. Measure X2 to ground
    1. With nothing turned on and LIM unplugged you should get about 60 volts
  4. This tells you the transformer is isolated from ground and the LIM is defective
    1. Change the defective LIM if you have a spare
      1. If you don't have a spare LIM and you have the Sentry 5 portable line isolation monitor, plug it into a receptacle that the defective LIM was monitoring. The Sentry 5 portable line isolation monitor will monitor the room until the defective LIM can be changed out.
    2. Call Scott Brockman with Isolated Power Specialist if you need help, 859-640-2959.
  5. However, if either Xl or X2 to ground is about 120 volts, the transformer is shorting to ground
    1. check to see what is causing it to short to ground
    2. The problem could be a winding of the isolated transformer is sitting on a screw and it finally shorted to ground
    3. If you cannot determine the short you will have to replace the transformer
    4. The operating room must remain shut down until a functional LIM is monitoring the room


      C.  If the problem is unsolvable, call Isolated Power Specialist


Isolated Power Specialist

Scott Brockman

3332 Tallwood Ct.

Erlanger, KY 41018

Phone: 859-640-2959




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© 47 Years of Experience with Line Isolation Monitors