Drilling machine mystery
Our ½ hp, 3ph, drilling-tapping machine is 10 years old. Recently, the 6A HRC fuse in the R phase blew, followed by tripping of the upstream ELCB.
Could it be an earth fault? Ohm-meter did not indicate any visible earthing. I tried hard but could not trace the fault physically. I just replaced the fuse. The machine operated without problems for few weeks.
Wonderful!
And then the fuse blew again. Well, I reckoned it could be just a transient phenomenon.
Cleaned up the electrical panel. Replaced the fuse. No problem.
After about 10 days the fuse blew again, followed by tripping of the ELCB.
Now I was getting really mad. Opened up the incoming R-phase cable terminals at both ends (about 4 meters). Checked with a “Neon tester’ and it glowed gracefully. Cable must have got shorted inside. Changed the cable. Replaced the fuse. Now the machine was running perfectly. But strangely there was no tell-tell sign of burn mark in the old cable.
After about a week the drama was repeated. This time, without much ado, I replaced the R phase fuse with a 16A HRC fuse.
After about a week, there was a flash at the incoming terminal box of the drilling machine.
The Fuse blew, ELCB tripped.
But I was more than happy. I had found the culprit. The incoming screw holding the Terminal Block had fouled with the terminal of the R phase.
Another lesson. Neon tester had been showing capacitive voltage.
--- P. K. Ray ----
raynar.industries@gmail.com
