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Baker Street Bus Bar Monitoring

Power quality analysis can be useful to assist in the design process of electrical equipment upgrades. The Metropolitan TEGG department helped a central London client during the design stage of a plan to upgrade a tap off from 100A to 160A. The tap off was connected to a non-essential busbar fed from a large switchboard in the basement. It was necessary to monitor the load of the busbar to determine if the busbar had enough capacity to allow for the extra load of the larger tap off. The switchboard PB6 in the basement fed the non-essential busbar BB13 which fed 7 floors via tap-off’s including the tap off to be upgraded on level 6 which served a kitchen.


An isolation was not possible for the connection of the power monitor as there were multiple tenants being fed from the non-essential busbar. This scenario required the Metropolitan TEGG procedures to be implemented so that the monitor could be connected during energized conditions. The TEGG procedures require a set of energized RAMS to be approved and full arc flash PPE to be worn by the technicians throughout the process of works. The technicians connected a Chauvin Arnoux 8335 power monitor to the 800-amp molded case circuit breaker located in switchboard PB6. The current readings were obtained by placing flexible insulated current CT’s around the conductors and the voltage readings were obtained by connecting alligator clips to the bolted terminals. The power monitor was then set up to record for a period of 5 days.

The results showed the loads to be well within the capacity of the busbar and the protective device. The neutral loads were relatively higher when compared to the phase loads, this was possibly due to a load imbalance across the phases which is difficult to correct as so many floors and equipment are being fed. The L3 phase carried the lowest load most consistently, so it was noted to the clients that any sizable single-phase loads to be added in the future should be connected to the L3 phase.