Recently I was taking part in a project at the building where I work as a maintenance electrician. A contractor was hired to install a UPS on an emergency power riser that feeds critical equipment that would benefit greatly from not seeing power bumps, etc. Our electrical staff worked with the contractor at various times during the project including the installation of tie breakers to feed certain panels on one emergency riser from another to avoid interruption of service while the UPS was being installed. On the eve of the tie breakers installation the contractor had 3 groups at the three installation points where one panel from a 600A, 3 Phase, 120/208V riser would feed two panels on the 300A, 120/208V riser where the UPS would later be installed. Myself and another electrician co-worker observed and offered assistance as needed. There was a well laid out document of steps to follow including a contingency plan should certain breakers not reset after the shutdown of power. It appeared that everything went as planned including phase rotation testing and phase sequencing (zero voltage test). Once that stage was completed we would later come back to transfer the load from the 300A riser to the 600A riser and wait 24 hours to observe if any issues developed. To avoid another power shutdown the contractor, engineer, and others involved agreed to a “closed transistion” transfer. On the day of the planned transfer I was not involved but I was told that after the first panel was transferred successfully the second panel failed. Apparently five major breakers opened and a little chaos ensued. What went wrong? Over the next two days there was much speculation as to why the second panel’s load did not transfer. In the end the decision was made that the Teck cable and tie breakers between the panel from the 600A riser to the failed panel on the 300A riser would have to be tested again for proper phase rotation and sequencing. In the end it was discovered that a tie breaker had been improperly placed in the panel that failed to transfer load. Phase rotation was correct according to the meter but a zero voltage test showed the phase sequencing did not match. The contractor felt certain that the testing was performed and documented on the day of the breaker installation. I haven’t heard if this was proven. I was in a different location on the day of installation and wouldn’t be able to verify if it was done or not. I’ve never been involved in this type of power transfer before and despite it not going to plan it was a great learning experience.
How I suspect the mistake happened (I was at the panel on the 600A riser when the mistake was discovered but did not hear the specifics as to why the breaker may have been misplaced in the panel):