Case History: The government of a Middle East country contracted with DeDad Consulting to conduct a review study of a proposed electrical design for a data processing center. Our study was to include the power distribution system (overcurrent and ground-fault protection), PDU power distribution, uninterruptible power supply (UPS) systems, engine-generator sets, transfer switching, any power quality aspects, and grounding.
Our study concluded that the proposed design could have significant ground-fault sensitivity problems and provided a recommendation for GFEP protection on all four conductors of four-wire wye circuits using GFEP equipment that sensed zero-sequence currents. We also recommended sensitivity levels to be set for all series devices at the same pickup setting. Also, we recommended the use of the lowest available GFEP setting on those instantaneous devices that protected individual loads with high inrush currents (motors and transformers), providing they would not cause nuisance tripping because of these currents. Finally, we recommended the use of the zone-selective interlocking method of coordinating GFEP devices.
Our study highlighted a specific concern regarding GFEP and alternate power sources, where a ground fault occurring on any part of the system would allow the resulting fault current to flow on two paths. Here, we recommended solutions such as single point grounding, four-pole transfer switches with overlapping neutral contacts, and transformer isolation.
Our study also included a review of the proposed make of UPS for the site. Two large UPS lineups were anticipated. We commented on the bypass circuit’s configuration and how it could create wiring errors such as having neutrals grounded more than one time and at more than one place. We also pointed out that the proposed make of UPS, while being termed a parallel redundant system is actually an isolated-redundant system because only one UPS unit is supporting the loads at any time. Should that unit fail, the load is then switched to the other UPS unit. As such there is no sharing of the load and no common bus. Basically, the UPS system under review was a redundant system and not a parallel redundant system.
Finally, our study pointed out some transient voltage surge protection issues, potential problems with the proposed design of the power distribution to PDU’s, potential EMI/RFI problems with the proposed bus-bar power distribution in the computer room, the lack of any lightning protection, and the lack of any electromagnetic pulse (EMP) protection.
Our study’s recommendations and notifications were taken in consideration and incorporated into the final design and installation.
Case History: A large design/build company headquartered in Los Angeles contracted DeDad Consulting, LLC to provide training to the company’s electrical engineering and installation components on conduit system design as it relates to cable pulling tension calculations. This was the result of field problems associated with the pulling of long feeders on a refinery project. We created and presented a two-day educational seminar that detailed the parameters associated with calculating pulling tensions on low- and medium-voltage power cables in various types of conduit systems. Included was detailed analysis of the constructions of the two categories of power cables, with information on the function of each power cable component, such as the semicon, shielding, jacket, and conductor itself. We then examined the actual refinery feeders to determine if the installation problems and resulting damage to the feeder conductors was due to incorrect engineering of the conduit system or incorrect pulling tension calculations and procedures made in the field. Our conclusions were accepted by the company and an engineering directive was initiated that included our guidelines for determining pulling tensions and sizing conduits.
Case History: John DeDad, our president, researched, wrote, and presented a podcast for the Caterpillar Company that detailed the important aspects of sizing engine-generator sets. This was the third in a series of three such podcasts requested by Caterpillar and created by John, who at the time, was Senior Editorial Director for EC&M Magazine. To listen this podcast, click here.