A Sound Policy
This particular campus operates its own 13.8-kV primary distribution system and all of the buildings involved contain medium-voltage fused load interrupter switchgear and stepdown transformers to bring the distribution voltage down to 480/277 V and 208/120 V for utilization. It is the practice of the facility management organization at this campus to require commissioning of the electrical service equipment for every building, whether the building systems are commissioned or not. This practice is justified by their previous project experience and, as you will see, these buildings further proved the case.
The scope of the electrical service equipment commissioning is limited to the medium-voltage switchgear, stepdown transformers, and the secondary switchgear or switchboards served directly by them. Because failures at this level can affect the entire building and involve the largest amounts of destructive energy, this equipment is considered the most critical.
Factory Vs. Field Tests
An objection frequently raised by contractors and manufacturers to comprehensive field testing is that all equipment is thoroughly tested at the factory prior to shipment. On this project, we experienced the following field failures of equipment that had been successfully factory tested:
- Two 15-kV circuit breakers failed the vacuum bottle integrity test, indicating the vacuum depended on for current interruption was lost.
- The electronic trip unit on a 5,000-A main breaker for a chiller substation was found inoperative and was replaced.
- A transformer cooling fan controller was found inoperative and was replaced.
- A ground-fault relay on a 2,000-A bolted pressure switch was found inoperative and was replaced.
Coordination And Breaker Settings
A prerequisite of testing circuit breakers and ground fault relays with adjustable settings is the availability of a protective device coordination study identifying appropriate settings. Although, in my opinion, this is the responsibility of the design engineer, it is often passed to the contractor in the specifications, and that was the case for one of these buildings.Our review of the study prior to testing raised several questions regarding device settings. Because the study was performed by the contractor and submitted as a "shop drawing," our comments had to be addressed by the contractor and responses reviewed and approved by the design engineer, finally resulting in changes to some of the recommended settings.
Due to schedule constraints, testing had proceeded with the original settings and a limited amount of retesting was required. Much confusion and delay would have been avoided had the design engineer prepared the study.
Ground Fault Protection Systems (Again)!
My experience is that the greatest source of problems with electrical service equipment is the system designed to protect against low-current arcing ground faults on 480/277-V services. This project proved to be no exception, as both buildings use "double-ended" substations in which a secondary tie permits the entire load to be carried on one transformer with the other out of service. These dual-source ground fault protection systems require interconnections between the main and tie devices that provide plenty of opportunity for error in installation.In one building, our initial review of the switchgear manufacturer's wiring diagrams determined that the ground fault system would not operate correctly if wired as shown. The manufacturer sent a field service representative to the job site to review the diagrams and the actual wiring of the equipment and concluded that neither was correct! The equipment wiring was revised, the system tested successfully and, as of when this column was written, we are still waiting for corrected drawings from the manufacturer.
In the other building, the ground fault systems on all three 480/277-V substations failed initial testing. It was determined that the neutral current sensors installed on the tie circuits were wired with incorrect polarity. This was corrected and all successfully retested.
