Surge Protection Components
A number of components are available which can be used to prevent excessive energy reaching sensitive parts of equipment or systems. These operate by diverting surges to earth or disconnecting signal lines. An ideal device is fast in operation and capable of carrying large currents for short periods while limiting the voltage across or the current through protected equipment to levels below which damage can take place. Maintenance-free and self-resetting devices are normally preferred where interruptions to service should be avoided.

Lightning Protection Systems C H E C K L I S T This checklist is designed to help guide users through a brief visual check to establish whether a site is effectively protected against the effects of lightning both with respect to structure and electronic computer networks, telecomms, and process and control equipment. If the answers to the questions raise doubts, a specialist should be consulted to offer advice. Telematic Ltd operates a lightning protection consultancy service staffed by experts qualified to provide sound advice from design to implementation. Please see the Departments area of this web site for Telematic contact details.  Protection for internal equipment Is there a lightning surge protection device (SPD) installed on the main power distribution board/ incoming power board? Is an SPD installed on the telecommunication lines feeding modems and telemetry equipment? If the controls section of switchgear cubicles contain sensitive electronic equipment (e.g., flowmeters, PLCs, computers, etc.) is the power feed into this section protected by a locally connected (i.e., within 1m) in-line SPD? Are data/signal/network cables installed outside the building over distances of more than 10m (either underground or overhead) equipped with SPDs at the controls section end of the cables? Is any field-mounted equipment that is critical for the process or expensive (e.g., magflows, ultrasonic instrumentation, etc.) provided with locally-mounted (less than 1m distance) SPDs?

Air or carbon spark gaps
Air spark gaps are generally connected between line and earth in locations where a high voltage transient can 'flash over' to earth. The protection level is a function of the gap distance, but is affected by environmental factors such as air humidity. They are inexpensive but their insulation resistance can fall significantly after several operations and frequent replacement may be necessary. Carbon spark gaps operate similarly to air gap protectors except that very high current levels can literally vaporize the carbon electrodes and then either reset to a much higher striking voltage or generate a fairly high resistance to earth. For modern SPDs, these 'components' are not practical and are, therefore, not used.

Gas discharge tubes
Gas discharge tubes (GDTs) seek to overcome some of the disadvantages of air or carbon spark gaps by hermetic sealing, thereby eliminating environmental effects. Gas filling enables spark discharge conditions to be quite rigorously controlled since the breakdown voltage of such a device is related to gas pressure and electrode separation for a particular set of materials. Typically, low voltage protection devices have electrode spacing of 1mm or so in an argon/hydrogen mixture sealed within a ceramic envelope at about 0.1 Bar.

Zener diodes
Semiconductor devices such as Zener diodes are fast in operation and are available in a wide range of voltages that provide accurate and repeatable voltage clamping - albeit with limited surge current withstand capability. Standard Zener diodes cannot usually handle surge currents, however, modified 'surge suppression' diodes are available with power capabilities of up to several kW for pulses of less than 1ms. Surge diodes with a capability of several kW can be rather large and expensive so indiscriminate use is not common.

Metal oxide varistors
A varistor is a voltage-dependent resistor in which the increase in current with voltage through the device is non-linear. Varistors are made from metal oxide particles (usually zinc) and are thus generally known as 'metal-oxide varistors' or 'MOVs'. The zinc oxide particles are compressed together so that inter-particle contact acts as a semiconductor junction. Millions of these particles mimic millions of diodes at various voltages, so, as voltage across the MOV increases, more and more junctions become conducting. Excess current is then bled off through the component, with power being absorbed through the mass of the MOV. The power handling capability per unit-volume of varistors is much higher than that of the surge suppression diodes since the varistor effect is a feature of the total material of the component, not just the semiconductor junction area. However, the millions of junctions in a MOV lead to a much higher leakage current at low voltages. Response time to impulses is as fast as a Zener diode and varistors are mainly applied to ac load protection where networks for single-phase and three-phase supplies are easy to construct. Their characteristics of ‘soft’ voltage clamping and high leakage current at nominal voltage (together with a tendency for both characteristics to deteriorate with temperature changes and repeated pulse diversion) mean that MOVs are rarely used for the accurate and repeatable protection needed for instrumentation and communications equipment.

Fuses
Fuses can be used to great effect in protecting equipment from over-currents. However, as they rely upon thin sections of wire melting, they take a significant time to operate and the current passing through while this occurs can still be sufficient to damage sensitive electronics. Fuses also have the major disadvantage of being usable only once, leaving lines disconnected until the blown fuses are replaced.

Surge relays
Surge relays are designed to disconnect the signal lines in the event of high current surges, thus protecting the associated equipment. Modern designs can handle high power levels and both operating level stability and sensitivity are good. Speed of response is their major disadvantage, the physical movement of the relay contacts together with the generated arc taking milliseconds to interrupt the current flow. The majority of lightning induced surges are less than 100µs in duration and hence surge relays are too slow. Maintenance is also needed to keep the relay contacts clean and to prevent cold welding of contacts which can prevent the disconnection of lines under surge conditions. When the relay does operate, signal lines are disconnected and reset, so contact bounce can prove a problem in data communications lines if the bounce sequence is inadvertently interpreted as valid data. Surge relays are primarily used to disconnect power surges created by failures in the power system which are of a significant duration.

Circuit breakers
Circuit breakers are normally designed for power systems and though energy handling capability can be increased to whatever level is considered necessary, speed of response is of the order of tens of milliseconds, generally too slow to be effective against transients of short duration.

Multi-stage hybrid circuits
It is generally necessary to use more than one type of the above components in a protective network to obtain the best possible combination of desirable characteristics. The most common combination forming a 'multi-stage hybrid circuit' incorporates a high-current relatively slow-acting component with a faster acting but lower power rated component in such a way as to minimize voltage and current output. The design of such a circuit should also take into account the possible consequences of surges below the operating point of the high power component but above levels at which the lower power device can be damaged. (Please see the -Product Area- of this web site for descriptions and pictures of many different SPDs.)

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