ERICO Practical Guide to Electrical Grounding - Manual (Page 123)

Practical Guide to Electrical Grounding 116
present. (Fig. 7-1) Power line faults, power line equipment
problems and lightning are the greatest threat at this
location. In many cases the local electric power company
provides surge protection on the high voltage side of the
supply transformer. The transformer itself is usually
provided by the local electric company. SPDs at these
locations are designed to limit overvoltages to a value
sufficiently less than the transformer’s basic insulation
rating (BIL) and to protect switchgear and main breakers
from internal flashover. These SPDs must be large enough
to absorb the high energy available from transients at the
service entrance.
Location Category B is the level of protection at the
branch circuit level. Phase to neutral protection plus neutral
to ground protection is recommended. These SPDs can be
somewhat smaller than those at location category C
because the peak voltage and energy will be less.
Location Category A is the level of protection at the point
of equipment utilization level. Location category A
protection can be built into:
1. The load equipment itself - such as an uninter-
ruptible power supply.
2. A separate enclosure containing SPDs of proper
design for protecting loads whose needs are
known to the installer.
3. Panels serving the above loads
4. Circuit breakers
Line to neutral, line to ground and neutral to ground
protection must be applied on single phase and three phase
systems. Neutral to ground voltage rises of more than a few
volts can cause misoperation of electronic equipment.
The coordination of the SPDs for location categories A, B
and C is important,
otherwise the benefits needed for
proper protection may not be realized. If a location category
A device “sees” a surge large enough to have operated the
larger location category C device then the location category
A device and its associated load may be damaged or
destroyed. Proper coordination depends on knowledge of
surge magnitudes, as well as number and location of the
various branches of the power system circuits. While
computer simulations are possible, they are time
consuming and expensive. Easier “cook book” methods can
also be employed. Design and installation assistance is
often supplied by the manufacturers of SPDs. Table 7-1 and
7-2 should be of help in selecting SPDs for the different
location categories.
If a Power Center is used, then it should also have its own
separate SPD protection. Its Wye secondary neutral should
be connected to building steel if possible, to form a
separately derived ground. Then, its SPDs should be
bonded to the output of the Power Center through the
shortest possible lead lengths.
Each piece of equipment also should be protected at or very
close to the point of entry for all data and power conductors.
Data line surge protection also should be considered,
especially where data lines are long or separated by one or
more floors up or down in a multistory building. These
specialized devices are not discussed in detail in this book.
Typical data lines that should be protected include RS232
or RS485 computer serial data interfaces, PLC signaling
connections, LAN cabling and RF coaxial cables. In
particular, telephone lines are often exposed over long
distances and adequate SPD protection is essential. Having
installed both power and data/telephone protection, it is
essential that the ground connections on the protective
devices be connected to the same ground point to avoid
potential differences.
The Tables which follow are derived from ANSI/IEEE Std
C62.41-1991. They may be used by the contractor or
engineer to define location of SPDs and the severity of
expected transients. From this information, it is possible to
select an appropriate SPD for the majority of applications.
Grounding Book 4/14/99 10/5/99 6:02 PM Page 116 (Black plate)