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Static Switch failure during on-site commissioning test.  


A new 3-phase Static Switch was field tested prior to connecting the plant’s critical loads. Normal Power was interrupted to the Static Switch to verify the switch would transfer to the Bypass Source without interrupting the load. The Static Switch had a power interruption of ~23 mSec. The Static Switch must operate in 4 mSec or less.


This switch was not tested properly by the manufacturer. It was discovered during our on-site commissioning test that the test equipment and test method used by the manufacturer was not acceptable. The manufacturer test department o-scope was looking at Line to Line voltage through step down transformers. 3-phase power is phase shifted by 120°; the voltage level of one line is always present while one line is at the zero crossing. This gives the o-scope a false reading of voltage available during the transfer. If on the other hand, the o-scope was connected Line to Neutral for each phase, then each phase could be monitored independently for loss of voltage and the duration of the interruption. 


Because of the test method used by the manufacturer the equipment passed all of the factory tests when in fact the static switch was not working properly. Make sure the test equipment you are using to verify a critical piece of equipment like a static switch is properly connected. This switch was shipped by the manufacturer assuming it was performing properly. Industrial Power and Battery caught this problem and prevented the switch from dropping plant load by discovering the manufacturing defect within the equipment and their test procedure before the plant load was ever connected to the defective switch.