At one time, the National Electrical Code (NEC) contained two Articles you won’t find as of the 2020 revision: Art. 280 (surge arrestors) and Art. 285 (transient voltage surge suppressors or TVSS). While the two Articles made sense when this arrangement was created, further thought and deliberation along with significant input from the industry led to an examination of what these devices are actually doing — providing overvoltage protection.
And since that’s the case, shouldn’t this topic immediately follow overcurrent protection? Overcurrent protection is covered in Art. 240, so Art. 242 was created to address overvoltage protection. Article 280 and Art. 285 were essentially moved into Art. 242.
To make the distinction between surge protection devices (SPDs) and surge arrestors clear, the voltage level is stated. Part II of Art. 242 is for voltage protection at 1,000V or less (which means SPDs) and Part III is for voltage protection above 1,000V (which means surge arrestors).
In each case, the voltage being referred to is not the nominal system voltage, nor is it the voltage of the spike or transient. It is the maximum continuous phase-ground voltage at the power frequency available at the point of application, also called the maximum continuous operating voltage (MCOV).
SPDs, 1,000V or less
These are typically installed indoors but can be installed outside [Sec. 242.22]. You would use an SPD outdoors where the MCOV is 1,000V or less, rather than use a surge arrestor for that purpose. For example, you have an outbuilding that is supplied by a 480V feeder from another building or it’s a residential application (120/240V).
You can’t use SPDs [Sec. 242.9]:
- For circuits more than 1,000V.
- On systems that are ungrounded, impedance-grounded, or corner-delta-grounded unless listed specifically for such a system.
- Where the SPD rating is less than the MCOV at the point of application.
Every SPD (other than a receptacle SPD) is marked with a short-circuit rating. Don’t install it where the available fault current exceeds that rating [Sec. 242.10].
SPDs are Type1, Type 2, Type 3, or Type 4 [Sec. 242.12, 14, 16, and 18 respectively). The installation requirements differ, and each is designed for a different location in the power distribution system. You install:
- Type 1 SPDs before the main OCPD.
- Type 2 after the main OCPD.
- Type 3 on the load side of the branch circuit OCPD.
Type 4 you don’t actually install; only an equipment manufacturer can install a Type 4. It’s a component-type SPD; for example, one that is integrated into the power supply of a piece of equipment.
Surge arrestors, more than 1,000V
These are typically installed outdoors but can be installed inside [Sec. 242.46] where the MCOV is over 1,000V. For example, you may have a motor that operates at 2,400V (called a 2,300V motor, oddly enough).
To protect against lightning-induced transients on 2,400V overhead supply conductors, the surge arrestors would be installed outside where those conductors are.
You can’t use surge arrestors where the device rating is less than the MCOV. Notice the NEC doesn’t bar you from using one where the MCOV is below 1,000V. That’s not a safety problem, as would be using an SPD where the MCOV is above 1,000V. But doing so is wrong from an application standpoint.
Some of the requirements for SPDs and surge arrestors are the same. For example, avoid excess length and unnecessary bends [Sec. 242.24 and Sec. 242.48]. Others are very different. When working with overvoltage protection, always be clear on where in the distribution system you are installing the device and whether the MCOV is more than 1,000V. Apply Part II or Part III requirements accordingly.
