Re: Surge Protection?

[Mike Sokoly <secure15@xxxxxxxxxxxxx>]

 From Mike Holt Today
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Basics of Surge and Transient Protection ? Part 2

In this multipart series we will cover surge and transient protection
for all types of signal, control and power lines. My goal is to help the
reader understand the basic principles and be able to assess the
capabilities of the types of protection available on the market today.
One of the most important skills you should learn from this series is
the ability to ask the right questions and evaluate the answers received.

In part 1 of this series we looked at the NEC requirements for surge
arresters and transient voltage surge suppressors in particular articles
280 and 285. In this segment we will look at the nature of surges and
their impact on equipment and systems.

The modern home or office will typically have several connections to
power and communications services. These include the AC power, telephone
and cable TV system. Residential broadband internet access is
accomplished via the TV cable or a DSL telephone connection. Commercial
internet connection is generally either T-1 or DSL. Any or all of these
utility service connections can be a potential surge entrance. Surges
can be due to a direct lightning strike to the utility network, an
induced current, load switching, power factor capacitor switching and a
direct lightning strike to the building among others.

Surges and transients are terms that are frequently used interchangeably
to describe events of very short duration (significantly less than 1
cycle or 16.66milliseconds (mS)). Transient events are normally measured
in microseconds or 1/1000 th mS. A long transient event would be one
that lasted for a full millisecond. The Institute of Electrical and
Electronic Engineers (IEEE) Standard C62.41.1?-2002 is a guide that
describes the surge voltage, surge current and temporary overvoltages in
low voltage (up to 1000v AC rms) AC power circuits. The term, temporary
overvoltage, is one that addresses a rise in voltage for a significant
period of time from a few milliseconds to perhaps hours. A temporary
overvoltage generally presents a threat to the facility that cannot be
addressed through the use of Surge arresters or TVSSs.

Surges resulting from a lightning strike will generally have the highest
potential current. Because the source of this current is the difference
in potential between the sky and earth, this energy must be returned to
the earth. For surges that are a result of load or capacitor switching
the source is the power system itself and the energy must be returned to
the power system. Externally generated surges are most likely to be
common mode, that is, elevated voltages with reference to earth. A
direct lightning strike to a single power line conductor near the
facility would however generate a surge between the conductors or normal
mode (meaning at right angles) surge. Surges due to switching and load
variations are far more frequent than those due to lightning. These
surges do not normally carry the power of surges due to lightning but
they can cause a disruption of the proper operation of equipment.

There are three basic impacts that a surge can have on equipment or
systems. These are:

?  Immediate damage to the equipment or system that stops its proper
operation until repairs are made. This applies to redundant systems
because proper operation is defined as all redundancy available.

?  Damage to the system is not immediately apparent but the system will
eventually stop proper operation at some time in the future as a result
of the surge.

?  Circuit interruption. This is becoming increasingly common with the
advent of smart equipment with microprocessor controls. Sometimes
systems will restore themselves and other times human intervention is
required such as a reboot of the system.

The goal of proper surge protection is to prevent all of these outcomes.
A more likely experience is that we will prevent the first two and limit
the third outcome. The reason for this is that surge protection will
affect the normal function of power, control and communication lines and
may actually cause the type three outcome.

In our next segments we will look at the basic operation of surge
arresters and TVSSs and the individual components they employ. We will
also consider the impact of inductance on suppression operations.

Ed Roberts