Power interruption is classified into two types: a scheduled power interruption and an unscheduled power interruption.
The former requires specific operational procedure before interrupting the power supply, which is to “open” the switches from the load side (power distribution panel) to the power source (power receiving panel) sequentially. To restart the power supply, “close” the switches from the power source (power receiving panel) to the load side (power distribution panel) one by one.
Make sure that the personnel in charge of the interruption/ restart operation thoroughly knows the configuration of the machinery, the operational characteristics, the operational procedures, the place or position of switches installed, the electrical scheme diagram and the load circuit diagram to avoid incorrect procedures.
During the work of starting, operating and stopping the load equipment, pay attention to meter readings, vibration, heat, and sound of equipment. If some abnormal state is found, report to the related person in charge immediately, investigate the causes and take appropriate measures.
In the latter case, investigate the cause of the power failure first. Power failures can be caused by the following: some failure attributed to the power company (outside power suppliers) and some local fault in the equipment.
To identify the causes, read indicated values or signs on the incoming supply voltmeter, under-voltage relay, earth fault relay, over current relay, etc.
Judgement by incoming supply voltmeter or under-voltage relay
• If the receiving voltmeter indicates “0” and the under voltage relay is “tripped,” it implies that power is interrupted on the power source side (attributed to power company). After confirming that the receiving circuit breaker is “opened,” the contact person or authority prescribed by the power company should be asked about the causes and the estimated recovery time. However, the related substation should try to restore power at the earliest.
• If the reading of the receiving voltmeter is within a specified range, the under voltage relay is “untripped” and the earth fault relay or the over current relay is “tripped,” that implies some failure (overload, short, earth fault, etc., in equipment or lines) has occurred in the STP and the circuit breaker for receiving power is “opened.” By studying the protective relay, circuit breakers, etc., which were tripped or opened, identify the line with fault and isolate the broken down point immediately before recovering from the failure