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before they enter the house. A service-entrance surge protector makes the
protection by plug-in protectors easier but, as discussed on page 8,
installation at the service entrance generally requires an electrician,
unless you are a do-it-yourself person and your city code allows it.
Q - Why single out two-link appliances as requiring special protection ?
A - A first answer to that question is given on page 12, explaining the voltage
difference problem for these two-link appliances. A more technical explanation
is also given on page 18, intended for your electrician.
Q - I have heard about surge protectors degrading over time: What about it ?
A - Many, if not all, electronic components will age and have a limited life. The
question is really how long a useful life can a surge protector have. Today's well-
designed surge protectors might reach their end of life prematurely if exposed to
some exceptionally high and rare stress but, for those tested according to UL
1449 Second Edition, the way they fail should not be a hazard. The
prevailing opinion among specialists on surge protection is that most of
the observed (and quite rare) catastrophic failures of surge protectors
are caused by excessively high line voltage that can occur when
there is a fault on the power system. Failure from very large
surges that might exceed the surge-handling capability of the
protector is less likely than failure from high line voltage.
And now, the "bottom line" question:
Q - How much money should I spend on surge protection ?
A - It depends on too many factors to give a simple answer. Technology can
change, additional features beyond basic surge protection vary, stores offer
"specials", and how much margin makes you feel comfortable is an intangible
factor. This booklet is not oriented toward rating product performance or
prices, but rather toward explaining the principles, so that you can make
an informed decision that will give you confidence.

Installation hints

As discussed on pages 7 and 8, there are several locations in your power
system where you can connect (install) a surge protector, depending on the
type of protection you desire and your inclination toward do-it-yourself
or hire someone. These possible locations are shown in the sketch
below, with appropriate hints on how to go about the installation.

A very important point to keep in mind is that
your surge protector will work by
diverting the surges to ground
(see "What's in a name" on
page 6) The best surge
protector in the world
can be useless if
grounding is not
done properly.

À Meter-base adapter
This is the most involved installation, requiring cooperation of the power
company if they allow it to be done by a licensed electrician. They might also
do it as part of their own program .

Á Service panel, upstream
Also a job for a licensed electrician since the power cannot be turned off on
this side of the service panel .

‚ Service (breaker) panel, snap-in
First opportunity as a do-it-yourself project, provided that the hardware is
compatible and that local authorities allow the owner to do it.

à Service panel, downstream
Another possibility for do-it-yourself. The protector should be connected on
the load side of a pair of dedicated (spare) breakers in the panel .

„ Receptacle, built-in
Provides same protection as a simple plug-in protector, but stays in place and
cannot easily be moved to another receptacle, should that become desirable.

… Plug-in (with cord or directly into receptacle)
The easiest of all for anyone to do. The only question is "Which to choose ?"
Pages 15 and 16 have given you some elements for making that choice.

It would be a good idea, to ensure compatibility, to ask the electrician to look into the ratings of the
device and the power system available fault current.
The protector should be located close to the service panel (less than about 30 cm or one foot),
otherwise the voltage-limiting effect will be degraded.

(But it’s OK to take a peek)

The problem of shifting reference potentials
According to insurance company statistics, two kinds of appliances are at the top of the
list as the most frequently damaged during a lightning storm: video systems (receivers
and/or VCRs) and computers with a modem connection. There is at least one
explanation for that very unhappy situation: if one of the two systems - power or
communications - brings a surge to the house, the intended operation of surge
protectors creates a difference in the potentials of the references across the two input
connections of the appliance, causing failure.

In the set-up of Figure 1, a personal computer (PC) is connected to both the power
system and the telephone system, and we assume that a surge is coming along the
telephone wires. The surge current ("Surge I" in the figure) flows from the telephone
system toward the common grounding point via the network protective device (NID) at
the point of entry, and the grounding conductor mandated by the National Electrical Code
(NEC ®). This current produces a magnetic field that couples into the loop formed by the
power branch circuit, the telephone premises wiring lines to the PC, and the bonding
conductors. A voltage (Vdiff) is induced in the loop and appears across the power input
and telephone input of the PC, with upsetting or damaging consequences. The presence
or absence of surge protection on the AC side (Arrester or SPD) has no effect on the
coupling. One effective solution is to have the consumer install a combined protector, as
explained on page 12.

Figure 1
Shifting of reference
potentials between
power and telephone

Even though this installation complies with the NEC, the situation is made worse if the
point of entry for power and the point of entry for communications are at opposite ends
of the house, with a large loop separating the two cables. The 1999 NEC now limits the
separation between points of entry to 20 feet (7 meters) for new installations. The
situation is even worse yet when the incoming service, cable TV in particular is not
bonded to the power service ground. That is a clear violation of the NEC but experience
has shown that it is not so rare, and the result can be severe damage to the appliance.

NEC® is a registered trade mark of the National Fire Protection Association


Recommended integrated bonding
The recommended bonding arrangement shown in Figure 2 is applicable for
new construction or for existing homes where an opportunity occurs to relocate
the point of entry of the cable TV or the telephone NID. It is compliant with the
NEC minimum requirements for safety, and will reduce the problem of shifting
reference potentials just described by inter-system bonding of all utilities serving
a residence. The usual components for each service connection are simply
installed next to each other.

Another possibility for reducing the shift in reference potentials is to install an
integrated, multi-utility surge protection at the breaker panel, as shown in Figure
3. This device includes appropriate surge protection for all three services: AC
power, telephone, and cable TV, with the minimum length for the bonding

Figure 2

Figure 3
integrated multi-protection
device for service panel

There are many sources of information on surge protection, including codes,
standards, handbooks, and many technical papers. Most of these are
written for technically-oriented people rather than typical consumers,
with the exceptions of the bill-stuffers
from your utility, occasional
consumer-oriented articles,
or the present booklet.

On the other hand, with the popular
and ever-increasing use of the
Internet, useful information
can be obtained on line.

The sources identified below are listed as a starting point for a Web search,
but with the understanding that new ones can appear and existing ones
can disappear. Furthermore, these sources include some
commercial entities as well as non-profit organizations. Such
identification is not intended to imply recommendation or
endorsement by the National Institute of Standards and
Technology, nor is it intended to imply that these
sources are necessarily the best available for the

Weather and lightning information:
¨ http://www.accuweather.com/wx/services
¨ http://www.lightningstorm.com
¨ http://www.lightningsafety.com
¨ http://www.nfpa.org

Surge-protective devices
¨ http://www.nema.org

Wiring practices
¨ http://www.necdirect.org

Power quality
(Surges, sags, and outages)
¨ http://www.epri-peac.com

* NEC® is a registered trade mark of the National Fire Protection Association

Donald L. Evans, Secretary
National Institute of Standards and Technology
Karen H. Brown, Acting Director
NIST Special Publication 960-6
May 2001
Electricity Division
National Institute of Standards and Technology
Gaithersburg, MD 20899-8110
National Institute of Standards and Technology
Special Publication 960-6
Natl. Inst. Stand. Technol.
Spec. Publ. 960-6
20 pages (May 2001)
For sale by the Superintendent of Documents
U.S. Government Printing Office
Internet: bookstore.gpo.gov
Phone: (202) 512-1800 Fax: (202) 512-2250
Mail: Stop SSOP, Washington, DC 20402-0001

Co-Sponsors: EPRI
Illinois Power Company
State Farm Insurance Companies
Text by Fran§ois Martzloff based on Special Publication 0545.R,
Recommended Practice for Protecting Residential
Structures and Appliances Against Surges,
EPRI PEAC Corporation, Knoxville TN.
[Available at http://www.epri-peac.com]
Cover and illustrations by Susan Spangler


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