Ferraz Shawmut TPMOV Principles and Operation

Transcription

1 TPMOV Origin Due to recent changes in the Underwriters Laboratories Standard for Safety for Surge Protective Devices UL 1449 additional requirements have been instituted to maintain listings of Surge Protective Devices (SPDs) with which some SPD designs and components could not comply. A number of new designs for both SPDs and SPD components have been released to overcome the additional objectives presented by the updated UL 1449 standard ANSI/UL (commonly referred to as Third Edition ). One such SPD component is the TPMOV (thermally protected metal oxide varistor) provided by Ferraz Shawmut (Mersen). The early development of the TPMOV extends back to the 1990s. Forms of the basic theory and operation of the TPMOV have been used in IEC (mostly European) designs for a number of years. One of the differences between the TPMOV and the European designs is that the TPMOV design has received much focus as an offthe-shelf solution to the recently established UL 1449 requirements. TPMOV Components and Operation The figures below are of the external appearance of the TPMOV (middle), a view of the internal components separated (right), a TPMOV with the top of the enclosure removed (left top) and a TPMOV with the top of the enclosure removed and the thermal element operated (bottom left). Page 1 of 5

2 To aid in the description of the components and operation of the TPMOV refer to the figures herein and on Page 1. [Note: (#) references the numbered circles from Page 1.] The plastic, composite enclosure (1) of the TPMOV provides a degree of protection for the internal components of the device; however, its design and purpose are integral to the success of the overall device as well. Beyond providing a cover for the internal components, it also forms the cylindrical channels that house the compressed springs (3) and act as guides for positioning the insulating screen (5) between the shaped metallic contact (6) and the metal oxide varistor (MOV) body (2). The insulating screen (5) rests within the cylindrical channels above the compressed springs (3). The insulating screen (5) and compressed springs (3) are held in place by the solder joint between the MOV body (2) and the shaped metallic contact (6) that forms the thermal element (4). The figure below shows the channels that guide and house the insulating screen (5) and compressed springs (3). The figure below shows a top view of the contacts for the micro-switch (7) and the openings in the top of the enclosure for the visual indicators to protrude when activated (visual indicators shown in activated state below). Contacts for the micro-switch Visual indicators Note that the visual indicators are an integral part of the insulating screen (5) and must be pushed through the openings by the compressed springs (3) once the insulating screen (5) has been forced up the channels in the enclosure and in between the body of the MOV (2) and the shaped metallic contact (6). Note that the visual indicators may be excluded as an option. MOV Thermal Element and Metallic Contact Insulating Screen Note the shape of the cylindrical channels. They are generally round (to house the round compression spring) with a rectangular cutout that channels or guides the insulating screen (5). Openings for the visual indicators Micro-switch The opposing portion of the enclosure (1) shown above serves as a mechanical mount for the normally open microswitch (7) that is supplied with the TPMOV. Further, it provides the openings for the visual indicators that are integral to the insulating screen (5) to protrude through the top of the enclosure. The figure shows a view from the inside of the enclosure (1). The micro-switch (7) is visible along with the openings for the visual indicators. Page 2 of 5 Opening for MOV contact Shaped metallic contact Enclosure The base of the enclosure (1) is shown above. The MOV contact protrudes through the opening on the left. The shaped metallic contact (6) protrudes on the right. Although the opening around the shaped metallic contact (6) forms a close fit, the opening around the MOV contact does not leaving roughly 10 mm x 3.8 mm (0.4 x 0.15 ) opening. The openings in base of the enclosure (1) and the mechanical nature of the TPMOV prevent the component from being sealed. This limitation may create an opportunity for contaminants to enter the device. Further, it prevents the device from being encapsulated which may aid in the improvement of the dielectric strength and protection against the effects of shock and vibration. Along with protection against the effects of shock and vibration, the ability to encapsulate a circuit provides protection against pollution and improves the performance with regard to clearances and creepage distances. This is evident in the UL 1449 standard as it allows encapsulated

3 circuits to be considered to have an improved pollution degree rating. Although the openings in the top and bottom of the enclosure (1) of the TPMOV serve specific purposes, they may also create complications during the assembly process. Many manufacturing facilities use automated soldering processes that typically include a cleaning process to remove contaminants and flux from the circuits being assembled. The openings in the TPMOV may prevent the cleaning process from being used or if it is used, the openings may allow detergents, water or other contaminants to enter the TPMOV mechanism if the openings are not sealed in some fashion. These contaminants may be corrosive in nature if not rinsed properly and could potentially degrade the operation of the compressed springs (3). The metal oxide varistor or MOV (2) used within the TPMOV has a 34 mm (1.34 ) nominally square shape and the thickness (of the actual MOV body) varies based upon the voltage rating of the MOV/TPMOV. This size MOV is a fairly common component used in Type 1 and Type 2 SPDs. By inspection, it can be shown that the MOV is supplied by multiple approved (certified) suppliers. The compressed springs (3) are key elements in the operation of the TPMOV. Although, the composition of the springs is not published, their construction and supply is subject to the follow up services of Underwriters Laboratories by way of the UL certification of the device. The springs are housed within the channels of the enclosure (1) and are the source of the mechanical force to thrust the insulating screen (5) or arc shield into place between the MOV body (2) and the shaped metallic contact (6), then, up through the openings in the enclosure (1) providing visual indication and activating the micro-switch (7). See Endnote #1 for additional information regarding springs. Visual indicators Insulating screen Springs (notcompressed) The thermal element (4) is the solder connection between the shaped metallic contact (6) and the MOV body (2). The thermal element (4) is critical to the operation of the interrupting mechanism. The melting temperature of the solder must be precise to the point that the heat produced by the MOV body during the abnormal operation (overvoltage) is sufficient to cause the solder alloy to melt so that the shaped metallic contact (6) can separate from the MOV body (2). Further, the thermal transfer from the MOV body (2) to the shaped metallic contact (6) must be adequate to allow the heat transfer through the thermal element (4) to quickly melt and separate to allow the mechanism to safely operate. The interrupting mechanism of the TPMOV solely relies upon the operation of the thermal element (4). The interrupting mechanism of the TPMOV operates differently than that of systems that rely upon fuses, breakers or more conventional means of interruption. Although the TPMOV mechanism interrupts the flow of current through the MOV body (2) at very low levels, it does not react to the actual current flowing like a traditional fuse or breaker. The action of the TPMOV mechanism depends on the heat generated by the MOV body during an overvoltage and the conduction of current through the MOV that follows. The thermal element (4) is closely coupled to the MOV body (2). Over time, this allows heat transfer to occur and the shaped metallic contact (6) to release from the MOV body (2). See Endnote #2 for additional detail. The timing of these events is paramount to the operation of the TPMOV mechanism. This system operates properly when the TPMOV is exposed to overvoltages that are roughly twice the operating voltage of the MOV like the voltages used in the Current Tests (exposure to abnormal overvoltages) of ANSI/UL The application of this level of overvoltage allows for significant heat buildup in the MOV and time for the heat to transfer to the thermal element (4). In turn, the thermal element (4) opens and interrupts the flow of current through the MOV body (2). However, in circumstances where the TPMOV is exposed to higher voltages (for example, the misapplication of lower voltage device to higher voltage system), the heat buildup in the MOV could be very fast and the rate of heat transfer to the thermal element may be not be sufficient to allow for proper operation of the thermal element and may result in an undesired condition due to the excessive heating of the MOV. In this circumstance since the TPMOV does not incorporate any current limiting protection or backup fusing, the interruption of current flow will depend on an upstream fuse or breaker, if one exists (the TPMOV specification sheet states that no additional overcurrent protection device (fuses) required for available short-circuit currents up to the short-circuit current rating or 200,000 Amps of available fault current). As examples of an application that may use TPMOVs, Type 1 and Type 2 SPDs could be candidates for the use of the TPMOV. Type 1 SPDs are permitted to be installed on the line side of the service equipment; therefore, the upstream Page 3 of 5

4 fuse could be located at the serving transformer and could have a high current rating. Further, Type 1 or Type 2 SPDs can be installed on the load side of the service equipment which may make the serving fuse or breaker the main disconnect/fuse which, again, may have a high current rating. As with any electrical device, caution must be taken to be sure the SPD is intended for the system to which it is being installed. Without any overcurrent protection, this is particularly important with the TPMOV. Although not published, the insulating screen (5) appears to be comprised of a fiberglass resin composition similar to or, possibly, the same material used in fabricating most circuit boards (FR-4 fiberglass). The primary purpose of the insulating screen (5) is to provide a high dielectric insulator between the shaped metallic contact (6) and the MOV body (2) when the thermal element (4) is operated to interrupt and prevent the flow of current. Secondarily, the insulating screen (5) also activates the micro-switch (7) through physical contact (electrically isolated) and provides visual indication via the extended tabs that protrude through the top of the standard TPMOV enclosure (1). The dielectric strength of FR-4 fiberglass is roughly 400 to 800 Volts/mil (1 mil = inches). The insulating screen (5) of the TPMOV is approximately 30 mils thick. Thus, if the insulating screen of the TPMOV is made of FR-4 fiberglass, the dielectric strength of the insulating screen (5) is approximately between 12,000 and 24,000 volts. The micro-switch (7) provides indication of the operation of the TPMOV s interrupting mechanism by changing state from a normally open to a normally closed contact position when the TPMOV interruption mechanism operates. The micro-switch (7) may be utilized with low-voltage diagnostic circuits. The visual indicators integral to the insulating screen (5) also provide indication of the operation of the TPMOV s interrupting mechanism. TPMOV Ratings and Certifications The TPMOV is reported to be available in a range of 150 to 550 Vac. The TPMOV specification sheet indicates that the TPMOV has a 50 ka peak surge current rating for an 8/20 μs current impulse. The TPMOV has a short-circuit current rating (SCCR) of 200 ka. The operating and storage temperature range of the device is -25 to 60 C (-32 to 140 F). The TPMOV is a recognized component under ANSI/UL as a Type 4 SPD used in Type 2 SPD applications (surge protective devices employing integral thermal-links, for SPD Type 2 applications). The URL (accessed on February 16, 2011) was the source for the ratings and certifications reported above. Summary Considerations for the application of the TPMOV may be*: 1. The proper design, compression and released force of the two independent compressed springs 2. The retention of the initial designed force of the compression springs as the springs age and remain in a compressed state 3. The compressed springs not being subjected to temperatures or corrosion that may negatively impact their operation or spring rate 4. The coefficient of friction between the insulating screen and the channels of the enclosure and the shaped metallic contact 5. The proper alignment and motion (sliding) of the insulating screen (to prevent the insulating screen from jamming or locking against any of the sides of either channel or the shaped metallic contact) 6. The proper operation of the thermal element (releasing the shaped metallic contact from the body of the MOV at the appropriate temperature) 7. The force being applied to the insulating screen to push the shaped metallic contact away from the body of the MOV 8. The ability of insulating screen to interrupt the flow of current through the shaped metallic contact and the MOV body 9. The ability of insulating screen to interrupt any arc that may form as the shaped metallic contact separates from the MOV body 10. The ability of the insulating screen to prevent an arc from forming once it is positioned between the shaped metallic contact and the body of the MOV (act as an arc shield) 11. The ability of the MOV to produce sufficient heat at an appropriate rate to activate (release) the thermal element during an overvoltage event 12. Adequate rate of thermal transfer between body of the MOV and the shaped metallic contact via the thermal element to allow the interrupting mechanism to operate in a timely manner during an overvoltage event 13. The level of system overvoltage is limited to a magnitude that will not cause the MOV to overheat too quickly and, therefore, not allow the interrupting mechanism to operate properly 14. The temperature of the application environment is within the operating temperature range (-40 to 85 C or -40 to 185 F) *It should be noted that the safe operation of the TPMOV during the UL 1449 Overvoltage Tests is not dependent on the arc shield and springs of the TPMOV. The TPMOV safely disconnects without the operation of these components. Page 4 of 5

5 Endnotes #1 - Generally, compression springs are considered reliable and the performance of the materials used are consistent particularly when subject to inspection and qualification of a certifying body. However, many spring materials are susceptible to temperature and oxidation (corrosion) that can impact their consistent operation. In addition, oxidation or corrosion can impact the spring constant of a compression spring. The spring constant determines the amount of force the spring will apply to a load. Temperature and oxidation can negatively impact these properties, changing the performance of the spring. The combination of MOV and fuse allowed a peak value of 2,442 amps of current to flow before interrupting. These values are shown in the upper right of each oscillogram. These oscillograms are shown to demonstrate the difference in operation between a TPMOV and an MOV protected by a fuse. The TPMOV conducts at a consistent current level near the peak value of the current for a period of time until the MOV body transfers enough heat to the thermal element to interrupt the current flow. With the MOV and fuse combination, a current that is much smaller than the peak current is conducted for a short period of time. Then, the MOV ruptures or tunnels through causing the current to spike. The fuse operates which interrupts the current flow. #2 The interrupting mechanism of the TPMOV, when subjected to overvoltages within its ability to interrupt, operates at very low currents. The mechanism allows a relatively low current to develop before the thermal transfer between the MOV body and the shaped metallic contact releases the thermal element. This differs from other methods of interruption such as fuses or breakers. The large diameter MOV used in the construction of the TPMOV aids in this action. The large diameter MOV can conduct low level currents for (relatively) long periods of time before rupturing or tunneling through which forms a spike in the current due to the lower impedance of the rupture or tunnel. MOV and Fuse Oscillogram 240 Vac with 100,000 Amps available TPMOV Oscillogram 240 Vac with 100,000 Amps available As shown in the oscillogram for the TPMOV, the TPMOV tested in this example only allowed a peak value of 83.5 amps of current to flow into the MOV before interrupting. This particular test was performed on a 150 Vac TPMOV with 240 Vac applied and 100,000 Amps of available shortcircuit current. Further, the oscillogram from a 150 Vac MOV with a protecting fuse tested with 240 Vac applied and 100,000 Amps of available short-circuit current is shown. The time required for the TPMOV to interrupt the current flow during this level of overvoltage and available shortcircuit current is much longer than that of the MOV and fuse combination. In this example, the TPMOV conducted current for about 1.98 seconds while the MOV and fuse combination conducted current for about 0.2 seconds. In the TPMOV, during this time of current flow, the heat that is generated by the conducting MOV is transferred to the thermal element and shaped metallic contact causing the thermal element to operate. In this example, it is shown that the TPMOV interrupts only a relatively small amount of current while the fuse used in conjunction with the MOV interrupts a much larger peak value of current (2,442 amps versus 83.5 amps). Because the interrupting mechanism of the TPMOV is integral to its package, it does not offer overcurrent protection to the surrounding assembly (circuit board, wiring, diagnostics, etc.). Page 5 of 5