Case Study

Harris Technical Services received a weekend phone call from a worried engineering manager of a large automated foundry that triggered a chain of events which was ultimately solved by the use of Protect HV and the dedication of the Technical Support team at AMTECH.

After a maintenance outage the staff were restarting a large automated moulding line: the next instant the process plant was without power - the main 11 kV oil circuit breaker had tripped. No one was aware of any incident which had prompted the OCB protection to operate. Initial investigation showed that the OCB, its protection and the transformer were satisfactory, so the supply was reinstated and the plant set in motion.

It was decided to take a detailed look at the grading of the system. Here Harris's engineers came up against their first obstacle; Protect HV did not include the data for the obsolete devices. With the help of secondary current injection data, they were able to compile basic curves, and AMTECH support staff came up with the rest.

The outcome was that whilst on basic settings there appeared to be positive discrimination: at anything above four times rated current the 11 kV OCB protection would operate first. And the culprit - a large induction motor rated at 250 amps, driving a sand mixer via a fluid clutch - had apparently been left full of sand when the plant was shut down for maintenance. Its locked rotor stalled current had bypassed the motor protection and three stages of LV ACB protection, only to be checked by the 11 kV device. Somehow the sand was cleared out before the HV system was investigated and re set.

Ultimate discrimination studies were carried out on the whole of the foundry using Protect HV, and positive discrimination restored, as was the confidence of Harris's client.