SURGE PROTECTION FOR LOCAL AREA NETWORKS

The purpose of this Guide is to describe the application of surge protection
to local area networks (LANs), and to assist the reader in answering the
following questions:
What is surge protection?
Is my network at risk - do I need surge protection?
What are the costs and benefits of surge protection?
Assuming I were to invest in surge protection, how would I
apply it to my network?
How do I choose surge protection products?
Atlantic Scientific's principal area of expertise is in surge protection.
This Guide does not attempt to be a treatise on networks, the subject being
so vast, and some knowledge of networks and basic electrical theory is
assumed. For those new to the subject, the further reading section
contains some helpful references.

The reader primarily interested in networks and coming new to surge
protection may well discover that there is a need for surge protection at
his site which extends beyond network protection. An example might be fax
machine and modem connection to copper telephone cables.

WHAT IS SURGE PROTECTION?
Electronic systems can be damaged or disrupted by what we refer to here
as surges. These are voltages which are much greater than the normal
working voltage and which appear in a system such as a local area network
for a short period of time, and hence are also sometimes referred to less
succinctly as transient over-voltages.

These surges can arise from switching of nearby heavy electrical
equipment or from the clearance of an electrical short circuit fault (e.g. by
a fuse blowing), but the most potent source is lightning. We shall be
covering this in more detail, but it is important to appreciate that although
catastrophic damage can indeed result from a direct lightning strike to
one's building, this is relatively rare. Far commoner is the substantial
damage to electronic components inflicted by a strike to ground within a
distance of the order of a kilometre or so. As we shall see, this can produce
a surge on cables feeding vulnerable electronic equipment, resulting in
damage. Typical damage to a circuit board consists of such items as
scorched and vaporized copper track, burned and open-circuit resistors,
integrated circuits with part of their package blown away, and
semiconductor junctions failed short-circuit.

At a lower level, but more insidious, is latent damage to semiconductors
which subsequently fail perhaps months later, as can happen with
electrostatic discharges.

Surge protection consists of the use of hardware devices, increasingly
termed surge protection devices (SPDs, see glossary for other terms),
which, correctly positioned and installed, limit surge voltages reaching
protected equipment to a safe level. The operation of SPDs is covered
briefly later for the interested reader.

How big are surges?
We can characterise lightning-induced surges by:
a) Open-circuit voltage: the peak voltage which would be
measured on a cable assuming no breakdown occurred
b) Short-circuit current: the peak current flowing once breakdown
has occurred, as commonly happens.
c) Time: the time taken for the voltage or current pulse to rise
to the peak and then decay. Lightning impulses have a fast
rising edge, occupying a few microseconds, and a relatively
slow decay of tens to hundreds of microseconds.
Application layer
Presentation layer
Physical layer
Data link layer
Network layer
Transport layer
Session layer
Communications channel
Application layer
Presentation layer
Physical layer
Data link layer
Network layer
Transport layer
Session layer

RISK FACTORS
As with insurance, the only certainty with surge protection is the cost of
obtaining it! The risks must be described in terms of probability only. It is
entirely possible that in a given location, damage due to lightning may not
occur for twenty years, and then twice in the same week.
A risk analysis can be undertaken using British Standard BS 6651: 1999,
Appendix C, General advice on protection of electronic equipment within or
on structures, against lightning, which is the best reference of which we
are aware. This publication identifies risk factors which affect the
probability of equipment suffering damage.
Frequency of lightning strikes to ground.
Size and exposure of building.
Soil resistivity (the higher the resistivity, the greater the risk).
Number and length of copper cables entering a building.
These include mains power, telephone and data cables.
Vulnerability of equipment.
Sample calculations are given in the standard.