Uptime All the Time

04/08/2002 |

Designing a Safe and Reliable Electrical System - Part 1

By Jill Revolt

Facilities managers often believe that a relatively incident-free record guarantees future reliability of the electrical power system. This can lead to a false sense of security that can have dangerous repercussions, especially for electrical systems where no detailed records are kept about minor or major power disruptions within the facility. A record of such incidents, including location, duration, and extent of an outage, can provide an invaluable indication of a design problem waiting to develop into a more serious mishap.

When designing a new electrical system, it is expected practice for a qualified, experienced electrical engineer to evaluate and coordinate the new system to provide a safe, reliable, effective design. The issues that are often overlooked are in coordinating an existing system with the design of a new expansion. Most importantly, the facilities professional must provide the electrical design engineer with current, accurate information on the existing system and insist that all power studies for the expansion incorporate the capabilities of the existing system.

Power Systems Coordination

The following true-life example* (see Attribution) from a metropolitan hospital illustrates the critical nature of power system coordination:

A hospital added a new wing to its existing 30-year-old facility. A power system study performed on the new electrical distribution system only assured coordination between protective devices within the new system. The new electrical system consisted of a 4160V switchgear, 4160V primary/480V secondary substations, 480V and 208V panelboards, and 480V and 208V motor control centers (MCC). The contract for the power system study did not include the existing electrical system in the study.

At 1:25 a.m. on Oct. 3, 1998, a ground fault occurred in a 25 horsepower (hp) fan motor supplied from a motor control center located on the second floor of the existing building. The protective device in the motor control center did not trip, nor did it trip the upstream feeder circuit beaker that supplied this MCC via an automatic transfer switch (ATS). The main breaker for the existing power center thus operated and removed power from the faulted system. This resulted in a loss of power to half of the existing facility (see illustration, Trip 1, opposite page). All power was lost to the critical care loads, including life support and patient isolation ventilation systems.

Subsequent to the main circuit breaker trip, all automatic transfer switches sensed the loss of power. A signal was sent to start the emergency generators. Once the generators started, the automatic transfer switches operated and re-established a supply of power to the emergency system.  The emergency power was not supplied for long since the ground fault was still present at the 25hp fan. The circuit breaker on the emergency system feeding all the transfer switches tripped, resulting in the complete loss of power to the hospital (see illustration, opposite page, Trip 2).

Fortunately, the medical staff, trained for this type of emergency, reacted without delay. Several patients on respirators were in severe danger of suffocating. The medical staff manually operated bag valve masks that breathed for the patients. After 20 minutes, the fault was isolated and power was restored with no harm to any patients.

This example demonstrates the importance of a properly designed electrical distribution system. The electrical system that distributes power throughout a hospital – no matter if it is a single building or large campus – is the lifeblood of that operation. An unreliable system places the facility, the employees, and the patients at risk.

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