In testing for the absence of high voltage, you have completed the lockout/tagout process. Being a conscientious engineer, you are wearing the right personal protective equipment. You have the right voltage tester and have taken the time to learn how to use it. Now you’re ready to go. You touch the test probes to the circuit and — surprise of surprises — you get a voltage where none should be!
There are several possibilities about why this happens:
You’re on the wrong piece of equipment
The equipment is not really turned off
“Ghost” voltage may be present
You’re getting backfeed
The last two possibilities — backfed voltages and induced voltages — can be confused. Induced voltages are typically much lower than the circuit’s nominal voltage, but backfeeds can be in the same voltage range as induced voltages. Since it is not safe to ground a backfeed, what should you do?
Check out this
article by Jim White for a discussion of how to handle backfed voltage. And check out the Protect test equipment rental inventory for the right test equipment.