An electrical transient is a temporary excess of voltage and/or current in an electrical circuit which has been disturbed. Transients are short duration events, typically lasting from a few thousandths of a second (milliseconds) to billionths of a second (nanoseconds), and they are found on all types of electrical, data, and communications circuits. Your power distribution system and attached load equipment is under constant attack from various types of power line disturbances. The result is an estimated $26 billion-per-year cost to U.S. companies in lost time, equipment repair, and equipment replacement. Transient voltage surges comprise the most severe and immediate danger to sensitive electrical and electronic equipment, and are often a neglected aspect of facility design.

Studies have shown that approximately 80% of transient activity at a given facility may be internally generated. From the normal on-and-off switching of copiers, heating and ventilation, capacitor banks, and air conditioning systems to robotic assembly and welding machines, practically every industrial machine or system causes or is adversely affected by transients. Surges and transient power anomalies are potentially destructive electrical disturbances, the most damaging being over-voltage occurrences and short duration over-voltage events, and the cumulative effect of these transients is a major source of semi-conductor degradation and failure.

The installation of Surge Protection Devices (SPD) are crucial for all facilities where microprocessor based electronics and electrical machinery is in use. This protection is essential to reduce the risk of personal injury, physical equipment damage, and loss of operations. Although lightning can cause the most visible damage, it is not the primary cause of transient voltage surges.

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"I have a UPS, why do I need a TVSS?” For over 15 years this has been a common objection dealt with on a regular basis. Following are a facts about UPS systems and its susceptibility issues as wel as some comments from IEEE.

A UPS provides secondary or backup power in the event of loss of utility power. A UPS protects electrical and electronic systems, process controllers, and data against the loss of the primary power source, which is the utility company.

Many UPS' incorporate relatively low energy and low pulse life Category A or B surge suppression. This low energy built-in surge suppression may protect the UPS and its connected load from a limited number of small hits, but it should not be relied upon as a transient solution. The Institute of Electrical and Electronics Engineers (IEEE) recognized this problem and stated in Section 9.11 of IEEE Std 1100-1992 that a networked Transient Voltage Surge Suppression device and UPS TVSS protection is required.

There are two basic types of UPS systems. First is the rotary UPS which has a motor connected to the primary power source, which is used to turn an alternator which generates the AC output to power selected loads, along with a battery bank. The other type is the electronic or static UPS. This type of units relies upon inverters and rectifiers to generate the AC sine wave and is supported by a bank of batteries to achieve the desired run times. Within this category you find stand-by, line interactive and double conversion types of UPS'.

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Transient Susceptibility of the UPS

• Protecting the Rotary UPS Transient protection is recommended at the input to the rotary UPS to protect the motor windings, (and rectifier/charger and inverter when present), static switch, and bypass path to the loads. Additionally, protection at the output is recommended to protect the sensitive loads from the inductive transients developed in the generator inductance and/or inductive loads served by the rotary UPS as these loads are cycled on and off.
• Protecting the electronic UPS Transient protection is recommended at the input to all electronic loads and the UPS is no exception. The AC-to-DC converter, the static bypass switch, and to a lesser extent the DC-to-AC inverter, are all susceptible to transient damage. Sensitive and transient susceptible electronic control circuitry constantly monitors the status of the UPS and the input and output AC power of the UPS. This circuitry is used to determine if the UPS should be delivering power from the batteries and DC-AC converter: or, if the UPS should switch to the bypass mode and deliver AC from the secondary source which is usually the utility AC power. Additionally, when the UPS is in the bypass mode for maintenance or due to failure, the sensitive loads should also be protected against transient disruption, degradation, and damage from the transients appearing on the secondary AC source or utility power.

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The IEEE Standard 1100-1992 states the networked TVSS protection is needed, and UPS TVSS protection as follows:

• Service entrance protection Section 9.11 of IEEE Std. 1100-1992: "... A listed and properly rated TVSS should also be applied to each individual or set of electrical conductors (e.g. power, voice, data) penetrating any of the six sides forming a structure."
• Premise electrical system lightning/surge protection Section 9.12 of IEEE Std. 1100-1992: "... it is recommended that additional TVSS be applied to downstream electrical switch boards and panel boards if they support electronic load equipment."
• UPS surge protection Section 9.11.3 UPS surge protection: "Lightning and other transient voltage producing phenomena are harmful to most UPS equipment and to sensitive electronic load equipment (e.g. via an unprotected static-switch bypass around the UPS). Therefore, it is recommended practice that both the rectifier-charger input circuit to the UPS and the associated UPS bypass circuits (including the manual maintenance bypass circuit) be equipped with effective Category B TVSS protection as specified in IEEE Std. C62.41-1991..."

IEEE standard 1100-1992 assumes that a TVSS protection network is in place when discussing UPS surge and transient protection.

The requirement that the service entrance and down stream switchboards and panel boards are properly protected with TVSS is assumed to be met. Additionally, Category B TVSS protection is required for the rectifier-charger input circuit and the associated UPS bypass circuits.

• Built-in UPS transient voltage suppression: Today, many UPS manufacturers provide low level lightning and surge protection. A typical UPS designed to power a computer may claim the UPS passes ANSI/IEEE C62.41-2002 Category A and/or Category B test waveforms. In general, one would assume that the UPS survives these transients, since there is no pass or fail in the ANSI/IEEE C62.412002. The adequacy of this built-in surge suppression to protect the connected load(s), the let-through voltages, the energy handling capability, and the number of hits of a given amplitude for the various transient waveforms that the built-in surge suppression will survive are not in general provided by the UPS manufacturer.
  

If a protected load is of sufficient value to an organization to protect against power outages with a UPS, the UPS and the vital load should also be protected against transient disruption, degradation, and damage.

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Conclusion

A Transient Voltage Surge Suppression unit placed on line in front of the UPS will protect the investment in the UPS and the connected computer equipment. During maintenance and downtime on the UPS, the TVSS will filter out the transients and surges from the line during the static bypass mode or maintenance by-pass mode of operation, thus protecting the connected computer equipment from damage. The UPS itself will benefit with fewer maintenance problems and less downtime.

With the investment in sophisticated computer equipment and a UPS unit, the customer has already committed to keeping the system operational on a continuing basis. The additional investment in the TVSS is minimal by comparison. The additional protection can be significant in terms of preventing even one downtime incident, which at today's costs can run some companies as much as $78,000 per hour according to studies conducted and published by Computer Technology Review magazine.

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Automatic Transfer Switch (ATS) surge protection is critical for the operation of the switch. The modern ATS has circuit boards, electronic controls, relays and sensors that are sensitive to transients. Transient damage to an ATS switch and/or controller can result in failure of the ATS to perform during an electrical crisis. Cumulative damage level transients can cause minor problems in the programming or logic controls for the switch resulting in the false operation of the transfer switch and operation of the generator when no problem exists, or the failure to make the transfer in the event of an actual crisis. Catastrophic damage can destroy the switch and/or controller and take the entire system off-line.

There are four main sources of potential transient damage that can impact an ATS. The first, and most common, is the utility power line. Lightning, generator switching at the utility power station or sub-station, capacitor bank switching on the grid, or nearby facilities producing their own large transients can cause catastrophic as well as cumulative level transients at the ATS. A properly installed Surge Protective Device (SPD) at the power utility line side of the ATS can prevent the damage from this source.

The second source of transients is the generator, or back-up power source line. The quality of the power produced by a back-up generator is dependent on many factors. The age of the generator and motor, the quality and frequency of the maintenance performed on the motor and generator, the load to capacity ratio of the generator, how frequently the generator is run and for how long, the quality and maintenance of the connections from the generator to the ATS, and more all have an impact on the ability of the generator to provide clean quality power to the electrical system during a power utility outage. Problems in any of these areas can result in the generator producing transients with the capability of causing everything from a minor nuisance to catastrophic failure. A properly installed SPD at the generator load side of the ATS can prevent the damage from this source.

The third source of transients is the down line electrical system. Internally generated transients can travel back up line to the ATS and cause cumulative as well as catastrophic damage to the switch and controller. A properly installed SPD at the load side line of the ATS can prevent the damage from this source.

The last source of transients is the actual physical switch within the ATS itself. The blades that switch from the power utility line input to the generator line input will cause arcing and transients when they disconnect from one and make contact with the other. A properly installed SPD at the blades of the ATS can prevent the damage from this source.

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