UFC 3 -520-01
June 10, 2002
protection against e xternally-induced transient surges; however, there will always be
some amount of surge let-through at the service entrance. For this reason, install surge
protectors on downstream distribution panels to provide additional protection against
surges. The locations marked with the symbol
are typical surge protector locations.
The surge protectors downstream of the service entrance also limit internally-induced
surges as well as any let-through from the service entrance.
Figure 11-1. Simplified Facility Layout
Lighting or Other Loads
HVAC (or Large Load)
11-2.2.2 Critical loads downstream of the distribution panels should have an additional
level of surge protection as shown above. Switching loads such as MCCs should have
surge protection to limit the transmission of switching transients to the rest of the facility.
It is not necessary to install a surge protector on every panelboard in the facility; the
selection of which panelboards should have surge protection depends on the
importance of the loads served by each panelboard. HVAC equipment usually contain
electronic controls that are sensitive to surges. Lighting electronic ballasts often are
equipped with internal surge protection; however, once installed, there is often no easy
method of confirming that the internal protection is either present or functional.
11-2.2.3 The surge protector at the service entrance must have a minimum surge
current rating of 80,000 amperes per phase or per protection mode. Downstream surge
protectors must have a minimum surge current rating of 40,000 amperes per phase or
per protection mode. Refer to Appendix D for additional equipment ratings.
11-2.2.4 By the arrangement shown in Figure 11 -1, protection is provided within the
facility for both internally and externally induced transient surges. Proper operation of
this cascaded design requires that the installation criteria specified in paragraph 11-3
are met. For example, excessive lead length on a surge protector could mean that it
would never respond to a surge event; the other surge protectors would respond first.