National Electrical Code Top Ten Tips:
Article 285 -- Transient Voltage Surge Suppressors: TVSS
by Mark Lamendola
Please note, we do quote from copyrighted material. While the NFPA
does allow such quotes, it does so only for the purposes of education
regarding the National Electrical Code. This article is not a substitute
for the NEC.
These are the 10 NEC Article 285 items we deem most important, based
on the pervasiveness of confusion and the potential costs of same.
- NEC Article 285 addresses transient voltage surge suppressors (TVSSs), while NEC
Article 280 addresses surge arresters. These are not
the same, though they both provide surge protection.
- Surge arresters apply to (in most cases) just the supply side
(line side) of the meter (280.22). TVSS devices apply to just the
load side (285.21). Some devices are listed for use in either
location, but typically surge protection devices are designed for
use in only one or the other.
- TVSS devices (SPDs) reduce potentially damaging
short-duration spikes on main power sources in a facility. They are not a substitute for lightning protection--they address
voltage levels that lightning protection does not, and therefore are
complementary to it.
- The general hierarchy is this: lightning protection (service),
TVSS (service and feeders), surge arrester (feeder and branch
circuits), point of use surge protection (provided by plug-in
devices, UPS systems, line conditioners, and so on).
- When you use a TVSS device, you must connect it to each
ungrounded conductor. The logic here is pretty obvious.
- Don't get confused over TVSS ratings. More isn't necessarily
better. While the NEC doesn't require you to think out the system,
reality does. A two-stage system is almost always the only way to
- You can put TVSSs indoors or outdoors. It simply is not
true that the NEC prohibits locating them outdoors.
- TVSS has strict wiring requirements, to deal with high frequency.
Do not violate these and expect protection.
- Connecting your TVSS to a ground rod driven into the
dirt is not sufficient. The electricity is always trying to get back
to the source--that's why electricity works. You must use one of the
four types of "ground sources" specified in Article 280.
If you don't understand what these are--and most people do
not--refer to Article 250.
- TVSS is essentially useless without good grounding and
Check out this grounding