Back to Basics

All of us have experienced the loss of electricity from time to time. In most instances, we just grin and bear it. There are, however, numerous manufacturing, commercial and institutional facilities that cannot tolerate the loss of electricity. In a hospital, for example, power failures can be fatal to patients who depend on electricity to operate life-sustaining equipment in places like the operating room (Figure 1). Although the loss of power to a manufacturing facility might not be a life-threatening, the cost of lost production time can be tremendous. As a matter of fact, one sustained power outage can cost far more than the installation of an entire backup system. Think about other places where the loss of power could create big problems. How about an airport’s control tower? In general, a backup source should be a part of any facility that depends on electricity to:

Figure 1. Typical Hospital Operating Room

You will notice that we said backup power source. The National Electric Code (NEC) defines the terms “Emergency Systems,” “Legally Required Standby Systems,” and “Optional Standby Systems” Other words such as alternate, standby, backup and critical are often used in the industry to describe dual source power systems. In this module, the words “backup power” will be used most of the time as a general term to cover all cases. “ Normal Power” will most often be used in this module as the general term covering the primary power or power source typically used every day. “Emergency Power” will most often be used in this module as the general term for the secondary power source.

If there is a normal power source failure, emergency power can be supplied in two ways:

An additional emergency service from the utility would have to come from a source separate from the utility’s normal supply (Figure 2). This requires additional equipment and lines from the utility. In addition, the normal and emergency lines would probably come from different supply locations to reduce the possibility of losing both power supplies at the same time due to the same condition, like a thunder and lightning storm.

Figure 2. Normal And Emergency Power Provided By Utility

Generation of emergency power using gasoline or diesel engine-generator sets on-site is the most common method utilized for standby power systems (Figure 3).

Figure 3. Normal Power Provided By Utility And Emergency Power Generated On-Site

Whether the utility provides both power sources or emergency power is generated on-site, a means must be provided to transfer the load circuits to the available source (Figure 4). Transfer switch equipment provides a means to quickly and safely transfer the critical load circuits, and is the primary topic of discussion of this module.

Figure 4. Typical Emergency System With Transfer Switch Equipment

Standby power systems are defined by one of the three following application categories:

1. Emergency System - An emergency system is a system legally required and classified as emergency by municipal, state, federal or other governmental agencies having jurisdiction. This type of system is intended to automatically supply power to designated areas and equipment if the primary supply source is interrupted. The emergency system designation usually applies when the loss of illumination or power would be a hazard to safety or human life. Transfer from the normal source to the emergency source cannot exceed 10 seconds.

Some typical instances where emergency systems are required include: facilities occupied by large numbers of people (hotels, sports arenas, health care).

2. Legally Required Standby System - Legally required systems are those that are so classified by municipal, state, federal or other governmental agencies having jurisdiction. These systems are intended to automatically supply power to selected loads, other than those already classified as emergency. Transfer from the normal source to the emergency source cannot exceed 60 seconds.

These types of systems are typically installed to serve loads such as heating, refrigeration, communications, smoke removal, sewage disposal and industrial processes.

3. Optional Standby System - This type of system is intended to protect public or private property or facilities, where life and safety do not depend on the system’s performance. Generally, on-site generated power is supplied to selected loads either automatically or manually. There is no time limit associated with the transfer.

This type of system is typically installed as an alternate source of electric power for facilities such as commercial buildings, farms and even residences. Typical loads served include heating, data processing, and industrial processes that when stopped could interrupt or damage the product or process.

A type of optional system that is increasingly popular is a load sharing system. On-site power generation assets are being installed as a backup to the normal source, or as a way to reduce utility bills. It is even possible for these on-site assets to have surplus generating capacity. This allows for power to be supplied back to the utility grid (cogenerate). For cogeneration to take place, the on-site asset must run in parallel with the utility source.

Especially in certain areas, there is a strong interest in customers providing prime power with on-site generation. This is especially true when demand utility rates are extremely high or reliable utility power for peak demand windows is not readily available.