UFC 3 -520-01
June 10, 2002
Near commercial utility systems with routine substatio n capacitor switching.
The design criteria in this chapter apply to permanently installed, hard-wired
surge protectors and should not be applied to smaller plug-in type surge protectors,
although the principles of surge protection are similar, regardless of the size and
location of the surge protector. Specific end -use equipment might still warrant a small
plug -in type surge protector even if the facility is protected in accordance with the
criteria of this chapter.
Point-of-use only surge protection (smaller plug -in types) can be applied by
the facility occupant for those facilities with little or no critical equipment. Examples of
these types of facilities include residential housing, barracks, warehouses, and other
facilities in which equipment damage by surge transients will not interfere with the
This section does not apply to electromagnetic interference (EMI) filters
installed on, or connected to, 600 V or lower potential circuits and listed in accordance
Appendix D provides recommended performance evaluation criteria for surge
protectors. The recommended performance evaluation criteria are based on NEMA
LS 1, Low Voltage Surge Protective Devices, and UL 1449, Transient Voltage Surge
SURGE PROTECTION DESIGN .
Parallel Versus Series Protection.
11-2.1.1 Surge protectors within the scope of this manual should normally be of the
parallel type rather than the series type.
11-2.1.2 Parallel surge protectors are connected in parallel with the circuit and operate
when a transient voltage exceeds a preset limit. Parallel surge protectors have little
interaction with the circuit under normal conditions.
11-2.1.3 Series surge protectors are connected in series with the circuit and must be
capable of carrying the circuit's full load current. Also, loss of the series surge protector
will mean loss of power to all downstream equipment. For this reason, series surge
protectors are usually used only to protect individual loads and usually include some
level of filtering also.
Multiple Layer Protection Design.
11-2.2.1 Apply surge protection in a multiple layer approach. Figure 11-1 shows the
preferred design approach with surge protection installed at the service entrance, and
additional layers of surge protection provided at load centers, panelboards, and MCCs.
Referring to Figure 11-1, a protector installed at the service entrance provides some