The Megohmmeter or Megger is a great tool for verifying wire to ground or wire to wire shorts. It applies a high voltage DC potential across the wire(s) under test. The megger will show shorts that would not be known using a regular Volt/Ohm meter.
The ABSOLUTE first step before using a megger is to verify per the wiring prints that ALL associated components have been isolated from the wire under test. Neither analog nor digital circuitry enjoy 250-1000 VDC being thrown at it and most components will not survive a hit from a megger.
One very important note: Never use a megger to verify a “in tank” fuel quantity harness problem.
Digital meggers do not require any adjustment to use. Older analog units have an open lead (tests leads not connected) full scale adjustment. Adjust this to maximum or infinity (∞) before use. This adjustment will change with the different test voltages.
I like to start at the lower voltage settings to check wires. A megger will supply a high voltage potential, but very little current, it is still good practice to start low and then move higher if needed. A good check indicates as close to maximum (∞) as possible.
Tips for Megger Testing
Wire shorts can occur anywhere throughout an aircraft. I have found that most are outside the pressure vessel, in high vibration areas. Wings, pylons, and the horizontal stabilizer areas are prone to wiring problems.
A megger will show that there IS a problem. Finding it can be difficult at times. Isolation by eliminating wire sections is good practice. If possible, narrow the area down to between two plugs with no other connections inline.
Some shorts require two people for troubleshooting, one to watch the megger and one to shake down the suspect area.
Watch where your fingers are. Most people do not forget a megger blast.