From computers to copiers, fax machines to printers, today’s ubiquitous electronic office equipment undeniably speed up productivity and output tremendously . But despite their adeptness, they all have an Achilles heel - sensitivity to power fluctuations from lightning strikes and utility and facility electrical surges.
To guard against potentially damaging or destructive spikes, many installations use Transient Voltage Surge Suppressors ( TVSSs) which are highly effective parallel surge protective devices ( SPDs) intended for electrical connection on the load side of the main over-current protection in circuits not exceeding 600 volts rms. (Parallel SPDs typically contain components that clamp across two conductors during a voltage surge that exceeds the unit’s clamping threshold and divert most of the transients from the load.)
A transient voltage surge suppressor limits transient voltages by diverting or limiting surge current and prevents flow of following current . It is designed to tolerate multiple exposures to surge events . Project specifiers who want to select TVSSs that work as well as possible without compromising the functionality of their systems, and have the potential to survive as many events as possible should consider several factors that can affect performance.
When selecting a device for a specific installation, it is vital to make sure it offers the appropriate level of protection for the size of the electrical load . The load level depends upon where within the facility the TVSS will be installed and on the equipment it aims to protect . There are three standard locations for installing a TVSS: at the service entrance, at a branch panel, and at each point of use . The needed capability of the TVSS at each location varies . The size of the main and the size of each panel are crucial considerations guiding selection.
At each location, it is important that the short circuit current rating (SCCR) of the TVSS matches the SCCR of the branch panel or building service it is designed to protect . (The SCCR is the suitability of a surge protection device for use on a circuit capable of delivering not more than a rms symmetrical current at a declared voltage.)
For example, with an 800 amp three-phase building service, a specifier might need a SCCR up to about 65,000 amperes to match the fault current capability of the electrical panel . Further downstream, on a 200 amp branch panel, a TVSS with a SCCR that is only 10,000 amps capable could suffice . And you would need even less capability locally, at, for example, a workstation.
External or Internal Circuit Breakers with TVSSs?
Once the SCCR is determined, the next decision is whether the TVSS will contain its own circuit breaker internally or will connect to a dedicated external circuit breaker within the panel.
If the circuit breaker will be external to the TVSS, the specifier should check to ensure that the breaker has an Available Interrupt Current (AIC) at least equal to the SCCR of the TVSS it will service.
One of the main advantages of an internal circuit breaker or fuse in the TVSS rather than an external circuit breaker in the panel is assurance that the two products have been approved by Underwriters Laboratory (UL) to work together . One cannot guarantee that the external circuit breaker, which can be from any manufacturer and have differing characteristics, will match the I2T rating of the TVSS . If the external circuit breaker trips before the TVSS, then the TVSS survives . However, if the circuit breaker does not trip, because the I2T exceeds the capability of the TVSS, the result could be destruction of the TVSS and a potential hazard to personnel and equipment.
This scenario is a possibility because UL requires that a TVSS manufacturer submit a sample TVSS and a sample third party external circuit breaker only during the initial product investigation . After the investigation, there is no requirement for subsequent testing of TVSSs with any other circuit breakers . Therefore, if a circuit breaker manufacturer changes the I2T of a breaker or someone uses one that has a different I2T rating or trip time, there is no assurance that during a fault condition the TVSS will disconnect in a way that does not damage ancillary equipment or personnel.
However, when an internal circuit breaker or fuse is used, UL requires that samples of TVSSs with internal breakers be submitted for testing and the manufacturer and part number noted in the UL applicants report . Also, manufacturers that have developed proprietary fuses are required to resubmit yearly for testing . The annual follow-up requirement provides the end user with continuing assurance that the short circuit current capability of the TVSS is as advertised . To avoid a potential weakness in the system, use a TVSS with its own matched internal over-current protective device.
External or Internal TVSSs?
Depending upon the location of a TVSS installation, specific code requirements apply . There are two possible locations for parallel TVSSs: external to circuit panels, which is covered by the 2002 National Electrical Code Article 285; and integrated within panelboards, which is covered by standards from UL and the Institute of Electrical and Electronic Engineers (IEEE) C62.72.
When the TVSS is located external to the panel board, NEC 285.6 requires that the TVSS be marked with a SCCR and not installed at a point in the system where the available fault current is in excess of that rating.
UL has its own marking system . Currently, all external TVSSs that are stand-alone products are UL Listed and all internal TVSSs, which are integrated into circuit panels, are UL Recognized.
A UL Listed product is one that has passed every safety test in the relevant UL standard and that can stand on its own from the UL perspective . For TVSSs, the applicable standard is UL 1449, which requires that the unit have appropriate protection from the environmental world, has a sound mechanical enclosure, and is constructed so that no one can touch live (full voltage) parts.
A ‘Recognized’ product is a UL approved component designed to go inside an enclosure of a Listed product, as a component, behind a cover that is accessible only by appropriate service personnel . It is not approved for use as a stand-alone product . Recognized parts that are designed as components of surge suppressors, power supplies, power conditioners or industrial control panels do not have to go through every test that is defined for external products of the same type . Therefore, a UL Recognized TVSS can obtain a UL Mark without being evaluated to all of the safety tests required for a UL Listed TVSS.
Normally, when a Recognized product is installed as part of a Listed product in the factory, it is assumed that all appropriate tests were performed on representative samples by UL before the UL mark is applied . However, with UL Recognized components, this is not the situation as the final product does not undergo the abnormal testing, such as the SCCR, like a UL Listed TVSS . Also, if the UL Recognized TVSS is installed in the field by an electrician, no testing is performed either . This absence of testing raises the question of the safety of the installation.
Potential Troubles
Some undesirable conditions can arise within a Recognized internal TVSS that are not currently discussed by UL or NEC but that are addressed in the IEEE’s Guide for the Application of Surge Protective Devices in Low Voltage AC Power Circuits, C62.72, Draft 19, section 9.25, Interrupting and Short Circuit Current Ratings of an Over Current Protective Device.
As stated in IEEE C62.72, over time, some components of a TVSS lose the ability to remain at a high impedance state at normal system voltage and begin to conduct current continuously . This can cause the TVSS to conduct during steady state conditions resulting in carbon and soot build-up, which in turn reduces the spacing between phases . This build-up may result in destructive phase-to-phase arcing that can lead to destructive arc flashes.
One way to avoid the possibility of damaging arc flashes is to require a protective barrier that would separate TVSS products from the internal electrical distribution.
The IEEE guide addresses this by requiring separation between the over-current protective devices (circuit breakers) of a panelboard and a TVSS . By putting the Recognized TVSS in its own enclosure and requiring the product to be evaluated to the full complement of UL Listed tests, the Recognized TVSS has the same safety protection from arc flashes as would a Listed TVSS . Any contamination that develops in the TVSS would be contained in a separate enclosure and could not migrate to create the arc flash hazard across the buss bars.
At present, because no manufacturer makes a Listed TVSS that can go inside the panel board, the only way to get that level of protection and assurance is to use an external Listed TVSS outside the panel, so that any failure due to contamination is contained to the SPD cabinet.
For maintenance servicing, having the TVSS outside of the panel (where it must be physically close to the panel to maintain conformity to operational specifications) is preferred to having it inside . Maintenance on an internal TVSS requires the lockout of the circuit panel by shutting off the main as the dead front panel must be removed. This shuts down every circuit off that main until the servicing is complete . However, servicing of an external TVSS requires only turning off of the power at the breaker or by the switch prior to performance of the service, limiting disruption of service.
Conclusion
Because the intended function of any installed TVSS is to preclude destruction or damage of electrical equipment, taking care in the specifying process is likely to pay off in spades down the line, when the products put into place provide all the intended proper protection .
Bryan Cole, PhD Candidate, NCE, is Manager – Low Voltage Products at Control Concepts/ Liebert, a division of Emerson Electric focusing on the EDCO brand of surge protection . He has been involved with the R&D of one-port and two-port surge protection devices, electromagnetic interference (EMI) filters, and harmonic filters for fourteen years, and is a member of the IEEE’s Power Engineering, Engineering Management, EMC, and Product Safety Societies.
William Feldman writes often on electrical topics.
Control Concepts Corporation was started in 1971, when it introduced the patented Active Tracking® Filtering technology for low-energy transients and high-frequency noise . Since then, they have stayed on the cutting edge in developing some of the most advanced technology in filtering and surge suppression, including series filters, AC and low-voltage surge protective devices . In 1990, Control Concepts was acquired as a wholly owned subsidiary of the Liebert Corporation, a division of Emerson Electric Company . Today, Control Concepts is an integral part of the Emerson Network Power group, giving their customers not only the benefit of a single-source power quality manufacturer, but cross-company access to Emerson’s global infrastructure . For further information: 800-288-6169, 607-724-2484, www.control-concepts.com, Bryan.Cole@Edcosurge.com
