Table 1-19. Transient Surges Projected to Occur in 10-Year
Period on Externally-Exposed Landlines
(Voltage and Current)
Number of Surges
Operational upset level. The operational upset level is the transient energy level that causes a
change in the equipment operating state. Since a change in the equipment operating state will normally create
an intolerable change in associated system operation, transient protection must ensure that transient energy
levels appearing across protected equipment do not cause operational upset.
To establish the equipment withstand level, compare the transient energy levels that cause immediate
component destruction, component overstressing, or equipment operational upset. Select the lower of the three
transient energy levels, and establish the withstand level at 10% below the lowest transient energy level.
220.127.116.11.4 Determination of Need for Transient Protection. Power supplies (5 to 48 V) operating from ac inputs
and supplying operating power for solid-state equipment always require internal transient protection. Other
equipment that directly interfaces externally exposed lines, including commercial ac inputs, may or may not
require transient protection designed as an integral part of the equipment. To determine whether transient
protection is required, compare the equipment withstand level with the transients of Table 1-17, 1-18, or 1-19,
as applicable. If the equipment withstand level is above the transient amplitudes provided in the tables,
equipment-level transient protection is not required. When the transient amplitudes are above the equipment
withstand level, equipment-level transient protection is required, either at the ac input, other externally-
exposed line-equipment interfaces, or both.
18.104.22.168.5 Minimizing Transient Damage. When equipment requires protection against lightning generated
transient damage, transient suppression design must ensure that transients are attenuated to the equipment
withstand level prior to entering any equipment component. Therefore, the transient suppression must be
effective at the external line-equipment interface.
(1) AC inputs. The most feasible method for providing transient suppression is to design the
suppression as an integral part of the equipment.