Key Components of a Mission-Critical Facility – Automatic Transfer Switches

The generator is clearly running…  you can hear the engine humming along.  But why is the building still dark?

The failure to automatically transfer a building’s electrical load to an emergency generator is a nerve-racking scenario that any facility manager would prefer to avoid.  For this reason, automatic transfer switches (ATS) are considered one of the most critical components of an emergency power system.  There are several elements of a transfer switch’s design that should be of special interest to electrical design engineers and facility managers.  In a previous article, I wrote about the importance of short-circuit withstand ratings, and proper coordination of transfer switches with other elements of a power distribution system.  This article will focus on two other subjects that I feel also deserve some attention.

The first topic relates to the use of “open contacts” vs. “enclosed contacts” in transfer switches.  This is a subject that can generate strong arguments between manufacturers.  My goal here is not to suggest that one is better than the other, but rather to point out some facts that may show you how this subject is often exaggerated, and perhaps not all that relevant as a criteria for the selection of a transfer switch.  Secondly, and more importantly, I would like to alert you to a basic transfer switch design feature that may not be in your current specifications – the ability for the switch to be operated manually under load.  You may be surprised to find that this functionality is not present in some transfer switches, and this can have negative consequences for a facility’s owner.

So, perhaps with a copy of your typical transfer switch specification in-hand, please take a moment to consider the following points:

First, some basic definitions…

I think it is important to highlight these technical definitions, as they form the basis for understanding what a transfer switch is, and what it is expected to do.  In the United States, these definitions are set forth by the National Fire Protection Association, under its NFPA 70 “National Electrical Code” (NEC):

What is a transfer switch?  A transfer switch is “an automatic or nonautomatic device for transferring one or more load conductor connections from one power source to another”.

What is a circuit breaker?  A circuit breaker is “a device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating”.

What is a switching device?  A switching device is “a device designed to close, open, or both, one or more electric circuits”.  Also found under this definition, a circuit breaker is “a switching device capable of making, carrying, and interrupting currents under normal circuit conditions, and also of making, carrying for a specified time, and interrupting currents under specified abnormal circuit conditions, such as those of short circuit”.

… and a brief mention of the applicable safety standards

What are the applicable safety standards for transfer switch equipment?  In the United States, the applicable standard is UL-1008, which is prepared and maintained by Underwriters Laboratories (see scope of UL-1008).  If you have access to the contents of this standard, I encourage you to study the performance and testing criteria for transfer switches.

More than you ever wanted to know?

Isn’t it ok to simply specify that any proposed transfer switch meet the applicable standards?  Sure, pointing to the standards is one way of stating your basic requirements for compliance with national codes.  However, there is a good chance that your project needs more than just the basic requirements.  It is also possible that your specifications include additional wording that might in effect contradict the standards, or the basic NEC definition of a transfer switch.  If you use vendor furnished specifications, or even third-party “master” specifications, a close look might reveal that they include “requirements” that are driven more by a manufacturer’s marketing strategy than by technical merit (a common example is described below).  I would suggest that, armed with an understanding of the true requirements that increase the safety and reliability of an electrical power system, you can put forth a stronger set of specifications, and a much better facility for your client.  Consider this example…

“Open-contact” vs. “enclosed-contact”?  Does it matter?

Most automatic transfer switches base their designs on one of two switch mechanisms, referred to as either an “open-contact” or an “enclosed-contact” design.  These terms are intended to describe the basic components used for “making” and “breaking” the electrical connection between the load and the selected power source.  With very large initial investments in development and certification, some manufacturers will spend a great deal of effort touting their chosen design as being superior to another.  But is it really all that relevant?  Here are some facts:

  1. When defining its test criteria, UL-1008 does not differentiate between enclosed-contact and open-contact.  The safety and endurance tests are what they are, regardless of how the transfer switch is designed.
  2. The endurance criteria is comprehensive and very demanding.  After certification is achieved, it should be clear that a transfer switch “listed” to UL-1008 can be expected to operate as intended (i.e.: as a transfer switch).

I am of the opinion that marketing people have made this subject one of needless controversy.  Most claims of one design being better than the other are without any basis in technical facts.  However, over time, marketing campaigns have had some success within the engineering community.  I often meet design engineers that have a very strong opinion for or against a particular design.  Some positions may have formed from experience with a particular design (and should be respected for what they are), but others may be just the result of sustained marketing campaigns by one manufacturer or another, using misleading information and inaccurate technical arguments.

If you agree with the NEC definitions above, you may question why some specifications for transfer switches include the following statement:  “Designs utilizing components of molded-case circuit breakers, contactors, or parts thereof, which are not intended for continuous duty, repetitive switching or transfer between two active power sources are not acceptable”What is the purpose for this statement?  If a given product has obtained certification (having met the applicable UL standard) which confirms it to be suitable for the intended use, why is this statement necessary?

This topic has been the subject of many white papers and technical discussions.  Rather than re-state what has been said before, I recommend reading the following documents:

This manufacturer’s Technical Brief includes important historical clarifications on the composition of the “contactors” that are often used in open-contact designs.  It also makes an important point pertaining to enclosed-contact transfer switches: they do not necessarily utilize “circuit breakers”.  The correct terminology is either molded-case switches, or insulated case switches.  Only when these non-automatic switches get fitted with optional overcurrent protection do they become a true circuit breaker.

Confusion, Standby Power and the NEC is a white paper from a Professional Engineer and Senior Member of IEEE.  The author discusses the importance of UL-1008 as the only applicable standard for transfer switches in the US (almost identical versions are used in Canada and Europe).  He also highlights the fact that “UL-1008 does not preclude the use of double throw manually operated safety switches or interlocked circuit breakers from being integrated into transfer switch equipment”.

Can your transfer switch be operated manually, between active power sources?

Let’s move on to our second subject – transfer switch operation by manual means.  Why is this important?  As you know, an ATS is equipped with an electronic controller.  This controller is responsible for monitoring the normal and emergency power sources and, when needed, executing the automatic operation of the transfer switch.  Adequately isolating and protecting this controller from damaging power surges has always been a tough engineering challenge for transfer switch manufacturers.  A lightning strike near the equipment, or a power surge from the electric power utility, can cause extensive damage to the ATS controller.  While surge protective devices  may help, it is a fact that they are not always effective in supressing the sudden high currents that can be seen at the transfer switch’s power terminals.  Of course, without the controller, an expensive ATS reverts to a “manual mode only” device.  So, when the controller fails, what follows? 

In facilities that lack continuous monitoring of the emergency power system, damage to a transfer switch controller may not be apparent until an outage is experienced.  So how does the transfer switch behave without its “brain”.  You may know that an ATS is designed to have its “engine start” relay contacts default to the “start” position, thus ensuring that a generator receives its “start” signal when there is no power available.  When the “engine start” relay contacts are sufficiently isolated from the main control board, you may still see an engine start following the failure of the ATS controller.  A common scenario might see a building operating under normal power, while the generator starts and runs unexpectedly.  This could point to a damaged ATS controller that is no longer sensing the normal power source, and has defaulted to the position that starts the generator.  Here, the engine will start but, without a functioning controller, the transfer switch remains connected to the normal power source.  This is likely the scenario of least concern, since the building’s loads remain under normal power.

Now, envision a power surge that damages the ATS controller, and this is immediately followed by a full utility outage.  This scenario would now have the engine starting (if the engine start contacts remained functional), but there would be no transfer of the load.  A facility manager facing this situation may have to deal with a “dark” building, a generator that may or may not be running, and the loss of the automatic operation of the transfer switch.  All the while, the critical processes conducted by the facility are interrupted, and every minute that passes could represent thousands of dollars. 

This leads to one of the most important safety features of a transfer switch: its ability to be transferred manually under load and between active power sources.  Once the engine has been started (if not already running), the facility manager may then proceed to assess the condition of the transfer switch.  If the damage is limited to the loss of automatic operation (due to the failed controller), the next step might be to perform a manual transfer to bring the facility back on-line.  But wait, can the operator perform a manual transfer with generator power present on the emergency terminals of the ATS?

Consult the manual!

The answer to that question requires that the operator be familiar with the pertinent section of the ATS’ operation and maintenance manual.  Hopefully this has been considered in advance, and the operator can proceed to safely bring power back to his critical facility.  Whether a transfer switch can be operated manually between active sources is dependent on the design of the switching mechanism.  Safety is a key concern here because, during manual operation, as a switch breaks (or makes) its contact with a live power source, arcing can occur.  Arcing between contacts can generate extreme levels of heat, which can cause damage to the contacts, catastrophic damage to the entire equipment, and very likely, serious injury or death to personnel.

Some transfer switch designs do not allow manual operation between active power sources, and in these cases, technical literature will include cautionary statements referring to manual operation as a maintenance-only function, to be performed only when all power sources have been disconnected.  Other manufacturers do permit manual operation, and address the issue of arcing by employing arc chutes, pre-loaded contacts and other devices in their switching elements.  The ability for the transfer switch to be operated manually between live power sources may be important for the following reasons:

  1. It would allow the operator to quickly get his facility connected to an available emergency power source, without having to open the generator main line circuit breaker.
  2. Almost as important, it would allow the operator to manually “return” to the normal power source (once it has been restored), without the disruption caused by first having to again disconnect all power sources to the ATS.

This may or may not be an important concern for some facilities, but it should be understood by specifying engineers charged with product selection, and certainly by the facility operations personnel that may need to interact with the equipment during an emergency situation.

My advice… Form your own opinion

As a design engineer, you may want to select a transfer switch product that goes beyond the minimum requirements of NFPA, NEC and the UL-1008 safety standard.  It is clear that some equipment, although meeting applicable standards, may be inferior to others in terms of technical features, quality of workmanship, or capabilities in the areas of service and support.  You should absolutely search for the best possible solution for your application, and I encourage you to do your own research, making an effort to sort out what is important and what is not.  Product sales representatives can be a valuable resource.  I always encourage my clients to seek out expert advice, but don’t be afraid to ask tough questions and challenge any recommendation that appears to be unfounded.  Relying on your own research and a thorough evaluation of competing products will surely improve your design specifications, and ultimately benefit your client with the best value for his investment.

What is your experience with transfer switches and critical facilities?  Post a note below… I would love to read your comments.

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