Ecmweb 7504 Nec Article 720 Pr
Ecmweb 7504 Nec Article 720 Pr
Ecmweb 7504 Nec Article 720 Pr
Ecmweb 7504 Nec Article 720 Pr
Ecmweb 7504 Nec Article 720 Pr

Article 702: Optional Standby Systems

June 19, 2015
For some facilities and operations, certain standby power systems are required by business concerns but not by law.

People sometimes confuse various systems that supply alternate power on site. You may hear a production manager refer to the optional standby system as “the emergency generator” or something else that it’s really not. An example may help clarify how these systems can be very different.

Suppose the facility has an emergency power system and an optional standby power system. The emergency power system uses batteries and a type of generator that quickly comes up to operating speed. Loads automatically transfer to their respective systems upon loss of power.

Photo 1. Notice the starting batteries on this standby power system gen-set (lower left of photo).

The standby power system has only starting batteries (Photo 1). It’s there to cope with peak load periods or power special equipment as operationally needed. The generators are the type that come up to speed slowly and run for a long time.

The standby power system in this example is only one of many possible variations. This particular configuration is different enough from an emergency system to illustrate that standby power isn’t emergency power. Many optional standby systems do have batteries and do transfer the load automatically upon loss of power. The generator(s) may be fast start and short run, slow start and long run, or a mix of the two.

You often can’t tell whether a system is optional or legally required, just by looking at it (Photo 2). If a standby system isn’t legally required (see Articles 700 and 701 of the NEC), then it’s optional (Art. 702). But that’s in a legal sense. For business or operational concerns, it might be mandatory.

For example, a plant has a contract to provide 100 units a day for a specific customer. If the plant failed to perform and subsequently lost that contract, it would need to close and lay off the workers. To ensure a utility interruption doesn’t produce this outcome, the plant has a standby power system.

Photo 2. Can you tell if these generators are legally required or optional? While you’re looking, notice the panel with the local indicators.

The fact a system is legally required often has to do with the fact that people’s lives may depend on it. The fact that it’s legally optional can mean people’s livelihoods depend on it.

Other reasons, of course, can have a bearing on why a company deems it necessary to invest in the purchase and ongoing maintenance of a standby system. The NEC requirements are the bare minimum. There can easily be reasons to exceed these.

Which loads to serve?

The purpose of an optional standby system is to provide on-site generated power to selected loads. And you size the system to support the loads you’ve selected [702.4]. The decision on which loads to select isn’t a public safety one, but is often an operational one.

Here are three examples of using an optional standby system for something other than backup power:

• Operations rarely runs a particular process and, by design, the equipment for that process is supported not by utility power but by the optional standby system.

• In the summertime, the chillers add enough load to put the facility well into the peak load penalty box. Transferring selected loads to the standby power system for 4 hr each day keeps the facility out of that box.

• Two loads seldom run at the same time, but occasionally the production schedule doesn’t allow enough time to run them alternatively to each other — so one load is transferred to the optional standby power system.

Testing

The NEC requires witness testing for legally required standby systems but not for optional ones. So does this provide a cost savings when the system is legally optional? Yes, in the short run if you forego this testing. However, that can be an expensive way to save money. Saving a few bucks here isn’t worth it.

The legal incentives to provide witness testing and acceptance testing aren’t there, but the operational incentives are. Consider the reasons the system is being installed. You want this system available when it’s needed. For that reason, exceeding the NEC testing requirements is usually advisable.

However, just adding the Art. 701 testing requirements isn’t enough. Add in functional testing of the system, too. In one facility, an optional standby system didn’t provide power when needed. All of the components had been tested. Because nobody had functionally tested the system, everybody missed the fact that the transfer switch wasn’t actually connected.

Adequate testing ensures your system will do as intended once it’s installed. But it also helps ensure it will do as intended well after it’s installed. The initial pre-operational testing should include all of the tests that will be done for maintenance purposes to establish baseline test data. Waiting until the first maintenance window to establish baseline data makes that first maintenance effort an exercise in working blind.

Feedback

The NEC presents minimum requirements for visual and audio indicating feedback [702.6]. Any “off-the-shelf” packaged system should easily meet these requirements. Prepackaged systems save you money; the engineering, testing, rework, re-engineering, retesting, and re-rework has already been done for you. These systems usually begin as standard designs for specific types of applications, and you can customize from there by selecting options that are already designed and tested.

Installing a standby system is a capital-intensive undertaking, and sometimes, in the case of an “optional” one, the mentality adopted by management is one of “what can we get by with” to save money. If you’ve ever submitted a carefully crafted capital proposal based on sound engineering only to get an answer, “Cut the cost by 10%,” you’ve seen one manifestation of this mentality.

Alarm and notification options are prime candidates for short-sighted cost-cutting. Get there first with the question, “How can we ensure the standby system will reliably support our operational goals?” Ask it on page one of your proposal, and answer it with your design.

The first time an “optional” alarming mode prevents a system shutdown at a critical time or averts some other disaster, the managers you persuaded will be patting themselves on the back.

Mixing is OK

You can mix other system wiring or equipment in with that of the standby system [702.10]. Whether to do so is an engineering decision that considers factors such as ease of testing, repair, and maintenance.

Stand your ground

To ground or not to ground, that is the question. Article 100 defines ground as a connection to the earth. There’s almost never a reason to make a ground connection on the load side of a power source disconnecting means (you bond, instead). And so it is with generators. If a generator is a separately derived source, you ground it [702.11(A)]. Otherwise, bond it to the equipment grounding conductor and then bond that to the system grounding electrode or electrode system [702.11(B)].

You don’t want “separate” grounding systems. Always bond all of the grounding electrodes (size the bonding jumpers per Art. 250 requirements). This prevents dangerous differences of potential that produce hazards of shock and flashover.

If you’re installing an outdoor generator less than 15kW, do you need a disconnect where the conductors serve or pass through the building? Yes, but not if it’s a cord- and plug-connected unit [702.12(B)].

If you’re installing an outdoor generator greater than 15kW and the conductors serve or pass through a building, you don’t need an additional disconnect if there’s already a disconnect located within sight of the building [702.12(A)].

Don’t let failure be an option

The NEC requirements for optional standby systems are less rigorous than those for legally required standby systems. However, exceeding the NEC minimums can result in higher reliability, lower maintenance costs, and more operational flexibility. Or you may need to exceed the NEC minimums to satisfy the reason for installing a given standby system in the first place.

Consider what you can do with wireless notifications rather than just the mandatory panel indicators. When developing a capital project proposal for one of these systems, consider which people in the organization would benefit from automatically receiving specific notifications. You will dramatically improve the chances of approval by tying the notifications to specific job responsibilities and the facility’s mission.

Never design the system for the lowest cost to get it installed. Always design it for the right cost to do the job that justified installing it in the first place. Follow this up with the appropriate testing — not the NEC minimums — to lower the total cost of ownership and engender rock-solid reliability.                                          

Lamendola is an electrical consultant located in Merriam, Kan. He can be reached at [email protected].

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

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