To really understand what the NEC means by “legally required standby systems,” we need to back up a bit and get a handle on what a standby system is. Chapter 7 starts off with a series of Articles covering various types of onsite power generation systems:
- Article 700. Emergency Systems
- Article 701. Legally Required Standby Systems
- Article 702. Optional Standby Systems
- Article 705. Interconnected Electric Power Production.
Articles 703 and 704 do not exist, but they are not in that series. The last item in the series operates in parallel with the main power source. A standby power source doesn’t supply power until it needs to carry the load dropped by the main power source. An interconnected power source may or may not be set up to be on standby.
A site might have, for example, an interconnected solar panel system to supply a portion of its power. Since solar is an intermittent power source, it’s often used (as in this arrangement) to supplement the main power instead of being an alternative to it. The same site might also have a bank of gas turbines that are on standby to assume the load should main power be lost. Those turbines would constitute a standby system.
The other three systems types of standby systems. Let’s look at them in terms of the breadth of their safety effects.
- An optional standby system helps the owner protect an investment in equipment, material, data, or something else of value. The facility owner can choose whether or not to have such a standby system and also which loads it will support.
- An emergency system provides power to emergency systems, such as lighting, process shutdown equipment, emergency communications, and fire alarms to protect mostly the people in the facility. It’s generally intended to help people safely exit the building. The facility owner may install them to meet insurance requirements or local fire codes, among other reasons.
- A legally required standby system is typically intended to protect people and property outside the facility in the event there’s a loss of main power. It might be required to protect employees in the plant, as well. But something about the nature of what the plant is producing means the government requires this system. So if a company wants to build a plant that produces something that can be harmful if the power goes out, the authority having jurisdiction will require standby power to prevent the formation and/or release of whatever that is — or, in some cases, the theft of whatever that is (e.g., by terrorists).
Article 100 defines legally required standby systems as, “Those systems required and so classed as to be legally required standby by the municipal, state, federal, or other codes or by any governmental agency having jurisdiction. These systems are intended to automatically supply power to selected loads (other than those classed as emergency systems) in the event of failure of the normal source.”
The legal requirements for a legally required standby system go beyond simply installing it. There are commissioning and maintenance requirements that don’t apply to optional standby systems; you will find these in Sec. 701.3 (and for emergency systems, they are in Sec. 700.3).
