220.127.116.11.2 Minimizing Damage.
Damage can be minimized, and in most instances eliminated, by properly using the generally field-
proven protection methods detailed in this section. In order to be cost effective and to provide effective
protection, allocation of protection must be divided into three general categories which are:
(1) Transient suppression (metal conduit and guard wires) for outside lines that interface
equipment to be protected.
(2) Installation of transient suppression devices on both ends of exterior lines immediately after
equipment building penetration or at exterior equipment termination, and on incoming ac service entrance lines
at the facility main service disconnect means. On shielded facilities, transient suppression devices (TSD's)
should be installed in an entry vault or inside the main service disconnect box.
(3) Including transient suppression as an integral part of protected equipment at the exterior line-
If realistic transient protection is to be designed, frequency of transient occurrence, amplitudes and
waveforms of transients, and the withstand level of protected equipment must be defined. The withstand level
is the short-duration voltage and current surge levels that equipment can withstand without overstressing or
immediate destruction of components occurring, and without equipment operational upset occurring. The
information required for effective protection is provided in this section. The most susceptible components are
identified together with typical withstand levels. Frequency of transient occurrence is also provided. Because
of the large physical size of incoming ac service conductors, less impedance (resistance and inductance) is
presented to transient surge current flow. As a result, amplitude and waveforms of transients appearing at ac
inputs are quite different from those appearing at control, status, data, signal, and in-system powerline inputs.
Therefore, protection for incoming ac power service conductors is discussed separately from that for other
externally exposed lines.
18.104.22.168.3 Susceptible Components. Integrated circuits, discrete transistors and diodes, capacitors, and
miniature relays, transformers, and switches used in the design of solid-state equipment are very susceptible to
damage from lightning-generated transient surges. Other components are not immune to damage but are
susceptible to a much lesser degree. Standards do not exist for specifying the withstand level against lightning-
transients for most equipment and components. Therefore, accurate information must be obtained from
manufacturers, laboratory testing performed or conservative engineering estimates made. Typical withstand
level limits for some common types of equipment and components are:
Integrated circuits: 1.5 times normal rated junction and Vcc voltage.
Discrete transistors: 2 times normal rated junction voltage.
Diodes: 1.5 times peak inverse voltage.