Department of Labor
Mine Safety and Health Administration
Meter Test Leads Short While Testing Contactor
An electrical near-miss accident occurred on September 29, 2007, at an underground mine in District 8.
This accident occurred as a repairman was correcting a problem on a continuous mining machine. The
previous shift reported that the left-hand conveyor motor was pulling 20-amps more than the right-hand
conveyor motor. The repairman locked and tagged-out the continuous mining machine. He examined the
conveyor motor leads inside the controller and found evidence of heating. After tightening the connection
on the contactor, the repairman decided to check the voltage on the conveyor-motor line starter and
scrubber-motor line starter for voltage drops. The repairman energized the continuous miner and used a
Triplett 310 Meter, rated at 1200 volts, to test phase-to-ground voltage on top of the conveyor-motor
contactor. He then tried to test A-phase on the scrubber contactor and his meter showed no voltage. The
repairman noticed that the red test lead on the meter had unintentionally disconnected from the meter, and
he reconnected it. He then tested between the A-phase and the B-phase on the line starter. When he made
this test, a phase-to-phase fault occurred between the tips of his meter test leads. The tips of the meter
test leads melted, and an electrical arc occurred. The repairman was wearing gloves and was sent to the
hospital for observation. He sustained no injuries in this near miss accident.
New style vacuum contactors are now used in the continuous mining machine's controller, instead
of the older style air contactors. The vacuum contactors have decreased physical separation between
phase-lead connectors. This resulted in the meter test lead tips shorting the two phase conductors.
Test and trouble shoot energized circuits only when necessary.
When testing and trouble shooting energized circuits:
- Only make voltage readings phase-to-ground
- Wear gloves and protective clothing
- Pay close attention to the placement of meter test leads
This information was provided by concerned
miners in an effort to eliminate accidents. Accident or "close
call" information within District 8 may be shared by contacting
the MSHA office at (812) 882-7617.