2006 Muttrah Incident Report - authority for electricity regulation, oman

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or the transportation. Damage of insulation may also occur due to the interference of the edge of the bar with the internal diameter of the bushing (see point a). Support of the tulip contacts The design of the support of the tulip contacts is correct, as it retains the same shape as in the original circuit-breaker, but the supports assembled on site (400A contact system) are not according to the drawing (800A contact system). The assembled shape tends to reduce the movement of the tulip contacts in comparison with the original solution. Stability of the truck The stiffness in the longitudinal direction of the truck of the retrofit solution is comparable with the original one. Not so for the stiffness in the transverse direction: here the structure is quite flexible while the original truck is more robust. The transverse flexibility could affect the correct position of the system to operate the shutter, particularly in the case where the panel floor was not so well leveled. The design deficiencies noted above are the same as those identified in the KEMA Report. Circuit-Breaker locating device In addition to the above, ABB Italy identified a further deficiency relating to the circuit breaker truck locating device. This device locks the truck in the panel in one of three positions test, service, and earth. On the original Brush OCB trucks this device is interlocked to prevent the circuit breaker being racked in when the truck is not in the correct position; in addition on the Brush OCB truck it is not possible to unlock or move the truck with the circuit breaker racked in. This interlocking arrangement was not present on the trucks supplied by ABB Arab. If the operation to lift the circuit-breaker starts in an incorrect position the shutter could be damaged or not correctly opened. 26
6.3 Cause of the Incident Based on the above findings from the site inspections and evaluations by both KEMA and ABB Italy it is possible to postulate the probable failure mode. FIGURE 9 Hypothesis of the Failure Mode After the VCB/truck was racked into the service position arcing commenced between the contact arm, the bushing, the shutter and the top plate of the cubicle on one of the outer phases on the feeder side.. This established a phase-to-earth fault. Following ionisation of the air a second earth fault was established across the bushing of one of the other phases on the feeder side. With two earth faults on the feeder side a two phase short circuit was thus established. This short circuit arc would be fed by the feeder current and was forced downwards (i.e. away from the source) by the electromagnetic forces until it jumped to the lower terminals of the VCB itself. Extreme ionization now exists in the area around the six copper conductor arms. A three phase fault then occurs on the top terminals of the VCB. This fault is fed from the busbar with much higher power and taking at least 1 second to clear on the back-up protection. 27
Page 1 and 2: REPORT INTO THE INCIDENT AT MUTTRAH
Page 3 and 4: GLOSSARY OF TERMS The terms defined
Page 5 and 6: 1 INTRODUCTION AND SUMMARY OF FINDI
Page 7 and 8: 2 SCOPE OF INVESTIGATION AND METHOD
Page 9 and 10: 2.3 Methodology In preparing its re
Page 11 and 12: outages on the distribution system
Page 13 and 14: FIGURE 2 Brush OCB Truck removed Mu
Page 15 and 16: contracts were subsequently awarded
Page 17 and 18: 5 SEQUENCE OF EVENTS The sequence i
Page 19 and 20: The protection relay indications fo
Page 21 and 22: BUSHING SHUTTER FIGURE 4 Reconstruc
Page 23 and 24: FIGURE 7 Truck - Cubicle Interface
Page 25: the shutter mechanism is mounted is
Page 29 and 30: shortcomings in the trucks supplied
Page 31 and 32: Subsequent to the incident it emerg
Page 33 and 34: • That the design defects identif
Page 35: 10 RECOMMENDATIONS The inquiry make

2006 Muttrah Incident Report - authority for electricity regulation, oman

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