Lessons · Electrical · test before touch
Test before touch: live, dead, live
Before treating any conductor as dead, test your meter on a source you know is live, test the conductor, then test the known-live source again; the two live readings prove the meter was working during the dead reading in between.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A meter with a dead battery, a broken lead or the wrong setting reads zero on everything, and zero on a live bus looks exactly like zero on a dead one. The person who trusted that zero is why this test has a name. It takes thirty seconds.
How to think about it
The circuit should already be de-energized and locked out; this test is how you find out whether that worked. Find a known-live source: a receptacle you tested, the line side of the disconnect, a proving unit. Meter to AC volts, correct range. Live, dead, live. Test every conductor to ground and to each other; a neutral can be live, a three-phase feed has three hots, and one dead leg means nothing about the other two.
Worked example
Known live: line side of the disconnect, L1 to ground: 277 VThe meter and the leads are working, right now.
Circuit: L1 to ground 0 V, L2 to ground 0 V, L3 to ground 0 VEvery conductor to ground.
Circuit: L1 to L2 0 V, L2 to L3 0 V, L1 to L3 0 VAnd every conductor to every other. A leg that is dead to ground can still be live to its neighbour if the ground is missing.
Known live again: L1 line side to ground: 277 VStill working. The zeros in the middle were real. Now, and only now, it is dead.
Your turn
Write the first of the three points.
1. Test the meter on a known source
Solve one, graded on the server
The trap
Testing to ground only. On a circuit with an open neutral, or a corner-grounded system, a leg can read low to ground and 240 V to the other leg. Test between every pair, every time.