machine was originally commissioned in 1990 as opposed to
the other two paper machines, No. 1 and No. 2 that were
commissioned in 1978 and 1980 respectively. Because the
older section of the mill was built in an era when circuit
breaker interrupting ratings were typically much lower, the No.
1 and No. 2 machine areas included thermal-magnetic circuit
breakers with integral current-limiters for each motor circuit.
The current-limiters were required due to the mill system
available fault current exceeding the vintage MCCBs
capability as a stand-alone overcurrent protective device. The
three breakers in the Machine No. 3 area were removed from
service during a scheduled rotational outage, based on what
looked to be elevated temperature readings from thermal
scans during an earlier periodic survey. All breakers removed
from low-voltage MCC’s were serving motor loads and were of
the magnetic-only design. The breakers were returned to the
manufacturer’s Product Integrity Center (PIC) in Pittsburgh,
Pennsylvania. There they were carefully inspected and then
subjected to a battery of tests, similar to the testing required
for new circuit breakers as defined by the UL489 test
standard. By observing the manufacturer’s date codes on the
returned breakers, the three units were found to be
manufactured in July and October of 1990 and in January
1996. The factory test results of the breakers returned to the
PIC yielded some interesting results as outlined in Table I.