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
2 comments:
Only yesterday I joined this blog.There can be a severe voltage dip on upstream side in one phase only probably by a single line to ground fault. Before that fault is cleared, your drill became over loaded because not only two phases taking load of three phases, but also of negative sequence causing a retarding torque.
If this is the problem, fitting a negative sequence relay with tripping settings less than that of the fuse will avoid the trouble of replacing the fuses. But this requires an additional circuit breaker.In any case, fittinng a negative sequence alarm will identify whether the problem is as I posted.
RETIRED DEPUTY CHIEF ENGINEER, KERALA STATE ELECTRICITY BOARD.
This is great info to know.
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