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

Visit our website at www.efrobertsassoc.com

E. F. Roberts & Assoc.

Lightning and Transient Protection, Grounding,

Bonding and Shielding Education

Copyright © 2005 by E. F. Roberts and Assoc.

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