Within this plant, the electrical distribution system is made up of 11 kV, 3.3 kV and 415 V distribution, with the bulk of the distribution and larger motors operating at 3.3 kV. Typically, motors up to 140 kW are employed on the 415 V system, while the 140 kW - 1.5 MW motors operate on the 3.3 kV system and the 2.2-2.8 MW motors are on the 11 kV network.
A typical motor starter installation consists of a motor, local isolator and Motor Control Centre (MCC). The motor starter utilises standard 110 V control circuits, with protection relays to control a vacuum contactor.
During a routine maintenance inspection of a 3.3 kV motor starter, evidence of a high resistance contact was found in the form of decolourisation on the fixed stab contacts. Following this discovery, two Crystal Thermographic Inspection Windows were installed and a thermal inspection was carried out.
The survey indicated a high resistance contact as suspected, the cause of which (upon further inspection by the equipment manufacturer) was identified as a misaligned contactor chassis. After realignment, the inspection was repeated and a significant reduction in heat was noted.
Subsequently, additional drives were fitted with these 'sightglasses' and similar alignment problems were identified on four other motor starters. As a result of the concerns raised by the detection of these potentially catastrophic faults, the installation of Thermographic Inspection Windows is being rolled out across the facility.
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