Full Circuit

The manager of construction services at a large corporation was distressed. He called his electrical engineer to relate that at a recent lunch with an electrical equipment representative, he was informed that the use of circuit breakers as a transfer switch was in violation of the circuit breaker's Underwriters Laboratories (UL) listing and the National Electrical Code (NEC).

06/01/2004


The manager of construction services at a large corporation was distressed. He called his electrical engineer to relate that at a recent lunch with an electrical equipment representative, he was informed that the use of circuit breakers as a transfer switch was in violation of the circuit breaker's Underwriters Laboratories (UL) listing and the National Electrical Code (NEC).

But his company had literally hundreds of installations incorporating this methodology and a well-documented company standard supporting it? Naturally, the manager was sent into a panic. How could his company's standards be wrong? Hence, the phone call with the question, Are circuit breakers acceptable for use as a transfer switch?

A Good Question

As an entree to the question, it's important to understand the following definitions:

  • Transfer Scheme . An overall theory or approach of supporting a facility's load through two or more possible electrical sources.

  • Transfer Equipment . The appliance(s) used to support the transfer scheme. This is a broad category of devices that includes transfer switches, breakers, manual switches, static transfer switches and various other devices.

  • Transfer Switch . Per NEC Article 100, "an automatic or nonautomatic device for transferring one or more load conductor connections from one power source to another."

So, what the client was really asking was, Are circuit breakers acceptable for use in our particular transfer scheme? For the purposes of the discussion, the focus will be on electrical systems of 600 volts or less, and 6,000 amps or less in operation. The client's question is part of a larger discussion regarding source-switching approaches or transfer schemes. There are several codes, standards and regulations that influence various aspects of electrical equipment incorporated in electrical systems to support the overall load transfer process. For simplicity, it's important to focus on two basic areas: testing and application.

Testing the Equipment

The NEC requires testing to be performed by an approved testing organization such as UL. Generally, the testing standards focus on the safety of the electrical equipment in operation. These standards include the ones shown in "Test One, Test Two" on Page 20):

  • ANSI/UL 1008 — Transfer Switch Equipment

  • UL 489 — Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures

  • UL 1558 — Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear

  • UL 508 — Standard for Industrial Control Equipment

  • UL 891 — Dead-Front Switchboards

  • ANSI C37 — Low Voltage Power Circuit Breaker Standard

  • IEEE C37

  • CSA C22.2/No. 178 — Automatic Transfer Switches

  • ANSI/NFPA 110 — Emergency & Standby Power Systems

With regards to application, the National Fire Protection Association (NFPA), the Institute of Electrical & Electronics Engineers (IEEE) and the International Building Code (IBC) govern, either directly or indirectly, the application and installation of electrical and electrical transfer equipment. Some of these standards are:

  • IEEE 241 — Electrical Power Systems in Commercial Buildings

  • IEEE 446 — Emergency and Standby Power Systems for Industrial and Commercial Applications

  • IEEE 1015 — Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems

  • NFPA 70, 2002 Edition — National Electric Code 2002

It may seem confusing, as one scans the list of applicable codes and standards, to be met by equipment as that equipment travels from its construction to its installation. However, these codes and standards are not mutually exclusive.

NEC, in recent updates, has broadened its definition of listing and labeling requirements for electrical products. In earlier NEC editions, UL was specifically named as the testing and listing agency. Today, however, NEC no longer specifies UL as the sole approved testing agency for electrical products. In fact, NEC implies, but does not require, that the testing agency be a nationally recognized organization.

But it should also be noted that in some states the local state codes provide for a more stringent testing requirement. For example, New Jersey Administrative Code requires that the testing organization be a more nationally recognized testing laboratory and restricts the latitude of the authority having jurisdiction.

The Transfer Scheme

To address the issue at hand, it's best to begin with the specific project application and work backwards. The application or transfer need of the project drives the transfer scheme. As an example, mission-critical data centers may have a reliability need that drives them to secure several independent sources of electrical power to choose from in the event of an interruption of the primary commercial power source. Or, a commercial high rise may have several tenants with data closets having a less onerous reliability need. As the design professional working with the client, the electrical engineer must identify the need and provide solutions to meet it.

Examples of transfer schemes include a main-tie-main switchboard supported from separate commercial utility sources. Perhaps a multiple-source switchboard with priority switching between multiple utility and multiple generator sources with load transfer prioritization is required. Or maybe a simple transfer switch between a single utility source and a generator is all that is needed to satisfy the application. The important thing to keep in mind is that the application drives the transfer scheme, and the transfer scheme must drive the transfer equipment selection, not the other way around.

NEC 2002, as one would expect, has quite a bit to say about this issue. Chapter 5 "Special Occupancies," Article 517 "Health Care Facilities," provides explicit requirements for "Essential Electrical Systems for Hospitals" in Article 517.30. This code article requires transfer switches to be used specifically as the means of transferring load from one source to another, or in other words, as the transfer equipment. This means that as a minimum, the transfer switch must comply with UL 1008, "Transfer Switch Equipment."

Often when one hears the words "transfer switch," the image that comes to mind is the standard knife switch configured three-pole, double-throw, open-contact switch. However, several manufacturers produce transfer switch products constructed of molded-case circuit breakers or switches that are also UL 1008 listed, and these products are also acceptable for use under Article 517. These molded-case products generally carry a UL 489 listing as well. The important thing to note is that for applications that fall under the NEC code requirements of Article 517, a listed transfer switch must be used.

NEC Chapter 7 "Special Conditions" contains several sections that reference transferring of electrical loads. Article 700 "Emergency Systems," Article 701 "Legally Required Standby Systems" and Article 702 "Optional Standby Systems" are the three articles that define the classification of generator systems and reference transfer equipment in each of their sections. These sections further extend the scope and definition of transfer equipment to Article 705 "Interconnected Electric Power Production Sources." Articles 700, 701 and 702 each have a subheading entitled "Transfer Equipment."

Specifically, Article 700 "Emergency Systems," paragraph 700.6 "Transfer Equipment" states:

(A) General. "Transfer equipment, including automatic transfer switches, shall be automatic, identified for emergency use, and approved by the authority having jurisdiction. Transfer equipment shall be designed and installed to prevent the inadvertent interconnection of normal and emergency sources of supply in any operation of the transfer equipment. Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705."

The wording in Article 701 "Legally Required Standby Systems," paragraph 701.7 "Transfer Equipment" is slightly different:

(A) General. "Transfer equipment, including automatic transfer switches, shall be automatic and identified for standby use and approved by the authority having jurisdiction. Transfer equipment shall be designed and installed to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of the transfer equipment. Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705."

And the wording in Article 702 "Optional Standby Systems," paragraph 702.6 "Transfer Equipment" is significantly different:

"Transfer equipment shall be suitable for the intended use and designed and installed so as to prevent the inadvertent interconnection of normal and alternate sources of supply in any operation of transfer equipment. Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705."

The difference in these articles is quite naturally application driven. However, take note that each of the articles refers to "transfer equipment" with "transfer switches" being a subset of that equipment. The NEC has not limited the mechanism or method of load transfer specifically to one electrical switching product option for these applications.

Second, all of these articles direct the reader to Article 705, whereby NEC provides further explanation in paragraph 705.22 "Disconnect Device":

"The disconnecting means for ungrounded conductors shall consist of a manually or power operable switch(es) or circuit breaker(s) with the following features..." In summary, through these articles NEC permits the use of circuit breakers and other switching devices as an acceptable means of electrical load transfer, provided the requirements of the articles are met.

Making the Transfer

In IEEE Standard 241, Chapter 5.18 "Load Transfer Devices," the standard establishes reliability classifications for loads to be transferred to alternate sources of power. These classifications parallel the NEC discussed earlier and form the foundation for NEC code requirements.

The American National Standard Institute (ANSI) formally recognizes IEEE 241 as a standard. The NEC 2002, also known as NFPA 70, is a consensus standard developed jointly by the NFPA and ANSI. Here one begins to see the relationship between the codes and standards mentioned earlier.

Why is this important? IEEE 241 Chapter 5.18 provides the details regarding automatic transfer switches and automatic transfer circuit breakers and discusses their application in general terms.

IEEE Standard 446, a companion recommended practice to IEEE 241, provides the engineer with application examples for the use of both transfer switches and interlocked breakers in load transfer applications.

As industry recognized standards, these IEEE documents provide the foundation for the application and use of equipment, including transfer switches, to support the overall electrical load transfer scheme.

Now, it's been established that NEC allows circuit breakers and switches to be used to transfer electrical loads. It has also been shown that IEEE and ANSI support this, as well as providing guidance in their application. But, where does UL fit in? When the UL writes testing standards for electrical equipment, the organization often writes with consensus and contribution by outside agencies, including CSA, NFPA and ANSI. In fact, in today's environment of globalization, there is an effort underway by various testing organizations to "harmonize" their standards for worldwide acceptance. Examples of this are UL 1008 and UL 489.

UL 1008 "Transfer Switch Equipment" is the UL standard by which transfer switch equipment is tested and listed. In fact, UL 1008 has been revised to indicate approval as an ANSI standard as well.

UL 489 "Molded-Case Circuit Breakers, Molded-Case Switches and Circuit Breaker Enclosures" is the UL standard by which molded-case circuit breakers and switches are tested and listed.

If a manufacturer desires its molded-case switch-based transfer product to be listed for use as a transfer switch, it must be tested and listed under UL 1008. Those same molded-case switches, which comprise the transfer switch, may also be individually listed under UL 489 for stand-alone use in other applications.

Do circuit breakers used in a transfer scheme need to be UL 1008 listed and labeled? The answer is no. Unless the application specifically requires NEC Article 517 that a transfer switch be used, UL 1008 does not necessarily apply.

On the contrary, circuit breakers can be employed in an electrical system to perform a number of different tasks including over-current protection, shunt-tripping and switching. They are therefore tested and listed under a separate UL test procedure. Molded-case circuit breakers are tested and listed under UL 489. The switching function, if available, is also tested and listed, permitting the breaker to be used in load switching applications.

In a transfer scheme application where multiple sources can support a load, can UL 489 listed circuit breakers be used in lieu of a UL 1008 listed transfer switch? Most certainly!

As we reviewed in NEC Article 700, 701, 702 and 705, provided one adheres to the requirements of the code, breakers can be combined and used, in a controlled manner, to transfer electrical loads between distinct separate electrical sources.

Application Considerations

With all of this said, in what applications does one use circuit breakers and in what applications does one use a transfer switch? As with all answers, this one is not too straightforward.

Transfer switches, by design, provide ready access to the knife blades and contacts making field maintainability practical and possible. Transfer switches could also provide greater durability in applications requiring frequent load transfers.

Molded-case circuit breakers, on the other hand, may require a smaller installed footprint. Molded-case circuit breakers have contacts that are sealed within the breakers' case making them more desirable for use in harsher environments. However, molded-case circuit breakers cannot be field maintained; they must be replaced.

In the end, the selection and application of transfer equipment in support of a specific transfer scheme must be carefully considered as part of the engineering in support of the project.

As previously discussed, it's important to let the clients' needs dictate the transfer scheme and then let the transfer scheme drive the transfer equipment requirements. It's not a matter of choosing one technology, approach or methodology over another. Each has distinct merits in given applications. Instead, it's best to select the electrical equipment to fit and answer the given need.

Remember, however, if one specifies a transfer switch, it must be listed under UL 1008 or another recognized standard. Circuit breakers used in a transfer scheme must be listed under UL 489 or another recognized standard. And most of all, UL 1008 is specific to transfer switches and does not apply to breakers used to support load transfer in a transfer scheme.



Test One, Test Two

The National Electrical Code (NEC) requires testing by an approved organization such as Underwriters Laboratories (UL), focusing on the safety of the electrical equipment in operation. The relevant standards include the following:

ANSI/UL 1008 — Transfer Switch Equipment

UL 489 — Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures

UL1558 — Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear

UL 508 — Standard for Industrial Control Equipment

UL 891 — Dead-Front Switchboards

ANSI C37 — Low Voltage Power Circuit Breaker Standard

IEEE C37

CSA C22.2/No. 178 — Automatic Transfer Switches

ANSI/NFPA 110 — Emergency & Standby Power Systems

The National Fire Protection Association (NFPA), the Institute of Electrical & Electronics Engineers (IEEE) and the International Building Code (IBC) govern, either directly or indirectly, the application and installation of electrical and electrical transfer equipment. Some of these standards are:

IEEE 241 — Electrical Power Systems in Commercial Buildings

IEEE 446 — Emergency and Standby Power Systems for Industrial and Commercial Applications

IEEE 1015 — Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems

NFPA 70, 2002 Edition — National Electric Code 2002



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