Question { 10896 }

Any one can explain if there is any harm from humming noise
heard from air circuit breaker : we have 415 switch board
having two incomers from transformers 3.3KV/415V , when bus
coupler closed, and open one ACB incomer to work in one of
the transformers, the current started hunting around 50
Amps, and heard humming noise from the air circuit breaker,
although the load current was only 1200 amps while the ACB
capacity is 2500 Amps, and the transformer 1600 KVA. The
same didn't happen when taken the other transformer offline.
Why this humming noise and current fluctuation? is there
any implication or danger, bearing in mind that we just
returned everything back to normal and stopped the
maintenance of the transformer. Shall we go ahead and
ignore this noise?

Answer

Thanks Vijendra Joglekar
The breaker has been serviced already and nothing wrong
found, the load is 1200 Amps, but the current fluctuation
is around 50 Amps (+&- 25Amps)when load increased, the
sound started at load above 800 Amps, while the breaker is
rated up to 2500 Amps.

Question { 10896 }

Any one can explain if there is any harm from humming noise
heard from air circuit breaker : we have 415 switch board
having two incomers from transformers 3.3KV/415V , when bus
coupler closed, and open one ACB incomer to work in one of
the transformers, the current started hunting around 50
Amps, and heard humming noise from the air circuit breaker,
although the load current was only 1200 amps while the ACB
capacity is 2500 Amps, and the transformer 1600 KVA. The
same didn't happen when taken the other transformer offline.
Why this humming noise and current fluctuation? is there
any implication or danger, bearing in mind that we just
returned everything back to normal and stopped the
maintenance of the transformer. Shall we go ahead and
ignore this noise?

Answer

This is an old breaker and has no protection devices, close
and open manually. closes by operating rod and opens by
trip button. Protection in the HV side of the transformer
incomer. I suspect the contacts got wear and tear due to
long time service and intending to change the fix and
moving contacts. Small gaps ( not seen by eyes) between
contacts could cause this sown as far as I know.