National Electrical Code Tips: Article 702, Optional Standby Systems
People sometimes confuse standby systems with emergency systems and with backup systems. It might help to use an example. A plant might have an emergency power system and a standby power system. The emergency power system typically uses batteries and a type of generator that quickly comes up to operating speed. The standby power system typically doesn't have batteries and is there for such things as coping with peak load periods or running special equipment for which it is the designated power source. An emergency system comes on when it needs to, a standby system comes on when you want it to.
Standby systems include legally required systems, optional standby systems (the subject of this article), and backup systems. All of these except backup systems are defined in the NEC Chapter 7 Articles 701, and 702, respectively. The definition of a backup system depends on what you're backing up. Is it backup power? Data backup? Some other type?
Our focus here is on Optional Standby Systems. As noted, those are covered by the NEC, Article 702.
Let's address some
highlights of the Article 702 requirements:
- As the name implies, these systems are optional; they aren't legally required. But they might be "required" by the owner for a long list of reasons. They're just not required by some governmental agency having jurisdiction. Nor they required by any other entity, such as the facility's insurer. This doesn't mean they aren't important, just because they are "optional." For the continued existence or viability of the business that owns them, they may be quite mandatory. Legally required standby systems are required because of implications for life safety. That onus does not fall upon optional standby systems, though they may bear the onus of the implications of the facility's continued viability.
- The purpose of an optional standby system is to provide on-site generated power to selected loads. Whether this is done manually or automatically, it's still being done when you want it done rather than in response to an emergency or loss of power. The decision on which loads get supported is often an operational one. For example, operations is going to run a particular process and by design the equipment for that process is supported not by utility power but by the optional standby system. Or perhaps there are two loads that seldom run at the same time; however, a need arises to do so and one of the loads is selected to be supplied by the optional standby power system.
Unlike legally required systems, these systems don't need to be witness-tested upon installation (or periodically afterward); at least, not according to any legal requirements. But if it's an expensive system and there are serious consequences to failure to start or stay running, then for operational purposes the same level of quality assurance makes sense. And, of course, proper maintenance of any asset is not something a responsible owner needs legal pressure to do. The financial incentives alone are usually more than enough.
- It's an optional standby power system, not an emergency system or legally standby system. Nor is it a backup power system. Don't let various factions start making demands based on their confusion over what kind of system this is. If the IT department wants it to do double duty as a backup power system, explain to them why it's not suitable for that purpose and how it's in their best interests to obtain a separate system for that purpose. Remember the mission, and select the supported loads accordingly. Size the system to support those loads [702.4].
- Ensure the system provides at least the minimum requirements for visual and audio indicating feedback [702.6]. Packaged, predesigned systems come with the proper alarms and signal devices and they nearly always cost less than the home brew variety by the time you factor in the engineering, rework, re-engineering, and re-work that has already been done for you.
Prepackaged systems begin as standard designs for specific types of applications. Customizing for your particular application probably will involve very little change. As a bonus, these systems usually exceed the NEC minimums because they must meet warrantability, performance standards, and other issues the NEC does not address but the market expects.
So don't go down the path of cost-reduction games that involve design degradation based on a "what can we get by with" mentality. Instead, ask the question, "How can we ensure the standby system will reliably support the load and that the transfer always prevents inadvertent connection of normal and alternate sources of supply?" Chances are, something already out there answers this question at a reasonable price.
Let's not forget that we live in the age of apps and smartphones. Look into that functionality, as well. Even if it costs extra, it's probably worth it because it will likely shrink response time while providing responders with the information they need to respond properly.
- You can mix other system wiring or equipment in with the standby system equipment [702.10]. Whether you do or not is an engineering decision that takes many factors into consideration, including ease of testing and maintenance. Generally, you want to minimize this if not avoid it altogether.
- There is no code requirement for an optional standby system to produce power within some time limit. Remember, it's not compensating for sudden loss of power. You may factor the total starting time into your final buying decision, but operational concerns will probably make that a minor issue compared to other concerns. Also remember there are trade-offs between acceleration and sustainable run time.
- Do you ground or not ground? 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 (you bond, instead). If a generator is a separately derived source, you ground it [702.11(A)] which typically means you drive a ground rod and bond the generator to it. If a generator is not a separately derived source, you bond it to the equipment grounding conductor (which you then bond to the system grounding electrode or electrode system) [702.11(B)].
Don't inadvertently create a bunch of separate grounding systems; make sure you bond all of the grounding electrodes using an adequately sized bonding jumper. That way, you prevent flashover due to differences of potential. But obviously, if your SDS generator is 200 feet away from the main building and the nearest grounding electrode is way over on the other side of the building, flashover isn't a concern. So don't go nuts running needless bonding jumpers and drilling tunnels under sidewalks and parking lots to connect everything.
- If it's an outdoor generator less than 15KW, do you need a disconnect where the conductors serve or pass through the building? Yes, but that requirement is waived if it's a cord and plug connected unit [702.12(B)].
- If it's an outdoor generator greater than 15KW, do you need a disconnect where the conductors serve or pass through the building? Not if there's already a disconnect within sight of the building [702.12(A)]. For example, the generator is on a skid next to the warehouse and is used to power the packaging grinder that was added last year. This was cheaper than upgrading the utility service, and the grinder is needed only about once a week. There's a big disconnect right on the generator, and you can see it as you walk out the warehouse door. No additional disconnect needed.