Thermally Protected MOVs Resist Overvoltage Failures

Metal-oxide varistors (MOVs) are the most widely used device for protecting sensitive electronics from damaging overvoltage transients. To date, most consider MOVs to be sacrificial devices, expected to disconnect (open) in the event of a failure. Unfortunately, this is not the case. MOVs used in surge protection devices (SPDs) can fail explosively when subjected to sustained steady-state power frequency overvoltages.

Traditional MOVs are highly susceptible to damage from sustained/temporary overvoltage conditions (Fig. 1). During these overvoltage conditions, MOVs enter a partial conductive state where they absorb the associated energy — eventually generating enough heat to force the component to rupture and initiate a short circuit condition.

For the above reasons, many design engineers have pursued alternatives to include an MOV-based suppressor in their design — adding series components, extra costs, and engineering time. However, the thermally protected MOV (TPMOV) addresses the inadequacies of the traditional MOV, making this technology a viable consideration.

Traditional MOV Deficiencies

An overvoltage may occur when misapplying the MOV and using it in a system whose voltage exceeds its highest rating. This can occur if the available voltage is greater than the maximum continuous operating voltage (MCOV) specified for the MOV. Typically, protection against these destructive consequences of an MOV failure is provided by:

• A current-limiting fuse that reduces the damage in the event of MOV failure (the fuse by itself only minimizes damage, but doesn't eliminate the condition).
• A thermal fuse that detects and disconnects the MOV if the disk temperature exceeds specified levels.
• Filter networks.
• Use of packed sand, epoxy, or other material around the MOV to minimize the MOV failure from propagating to surrounding equipment.

However, none of the above methods can adequately circumvent the explosive results of traditional MOV failure. The prominent failure mode for the MOV is sustained overvoltage. Under these conditions, the MOV begins to conduct power frequency (50 Hz to 60 Hz) current. The MOV ultimately reaches its maximum energy capacity and enters thermal runaway. For normal operating conditions, the MOV absorbs random short duration transient currents, transforming the energy into heat. But the MOV has a finite energy capacity. When this capacity is exceeded, the MOV can no longer effectively dissipate the heat — causing a milliohm short circuit.

These reasons instigated development of the thermally protected MOV (TPMOV) that provides “no damage” protection to sensitive downstream electrical components.

TPMOV

The TPMOV assists in minimizing the failure characteristics of traditional MOVs. It consists of a voltage clamping device, two forms of isolated indication, and a disconnecting apparatus that monitors the status of the metal-oxide disk. Fig. 2, on page 64, shows the TPMOV's construction.

If its metal-oxide disk is broken down or is approaching breakdown, the TPMOV disconnects from system power. It can withstand 40 kA of 8/20μsec surge current, and its available MCOV ratings range is 150Vac to 550Vac. It has two built-in, isolated indicating features. One is a visual indicator composed of two pins that proceed through the top of the unit if it has disconnected from system power. This allows operators to see which component is off-line without requiring them to disconnect system power from the array.

The second built-in, isolated indicating feature is a remote indicator composed of a normally open 12Vdc-status micro-switch. If the TPMOV was disconnected from system power, the status switch closes, which you can then use to indicate this condition to other circuits.

MOV Failure Modes

For sustained steady-state overvoltages, a typical test requirement for a traditional MOV withstands 150% of its rated voltage for a given duration. But MOVs may fail to a short-circuit condition when subjected to sustained steady-state overvoltages above this level. In service, abnormal system operating conditions may cause such overvoltages. However, a more common situation is the incorrect selection or wiring of the SPD, with failure occurring after initial switch-on — due to bulk overheating of the MOV. One approach to remedy this situation is to use MOVs with higher knee voltages; however, this provides poorer transient overvoltage clamping levels. A better approach is to use a thermal protector that responds to the MOV temperature and disconnects it from power, should the temperature exceed a certain level. The TPMOV accomplishes these tasks by withstanding 40 kA surge and offering a quick disconnect response in the event that the MOV disk has been degraded.

After extensive testing on a variety of traditional MOVs, significant damage was apparent, due to ineffective electrical protection. In this research, many MOV-based transient suppressors used short circuit and thermal fusing. But short circuit fusing alone does not provide complete protection — so many designs rely on thermal sensing to disconnect under limited current conditions (fewer than 10A) and rely on short circuit fusing to disconnect under high fault current conditions. Both of these conditions are a function of the energy capacity of the MOV-based suppressor. Under limited current conditions, the MOV displays few signs of physical damage but can generate enormous amounts of heat over time — eventually transferring the heat and melting the thermal element.

Because of the location of the thermal sensor, the majority of these thermal fuses fail to operate properly under high-fault current applications. When the thermal fuse (or thermal leg) is not located close enough to the MOV disk, it's unable to sense the actual temperature — negating its intended purpose. With high available fault currents, the traditional MOV cannot effectively transfer heat to the thermal sensor prior to thermal runaway.

The TPMOV features thermal sensing capabilities that react to the MOV disk's actual temperature. So, the TPMOV can disconnect from 6mA to 100 kA of available fault current. The available short circuit current does not depict the TPMOV's performance, because it disconnects before the available short circuit current can physically damage the MOV disk.

Complete electrical protection for SPDs must include both short circuit protection and thermal sensing. Beyond that, the thermal protector must have a thermal response that coordinates closely with the response of the MOV. Fig. 3, on page 66, shows the behavior of a 320V MOV when subjected to a sustained overvoltage of 150% (480V). In this case, the supply circuit had an available short-circuit current of 41.2 kA at a 15.2% power factor. Fig. 3(a) shows the current wave for an ineffectively thermally protected MOV. In this figure, the MOV conducts during the portion of the ac cycle where the supply voltage exceeds the knee voltage of the MOV, and the amplitude of the current pulses depends on the supply impedance and the MOV resistance. The temperature of the MOV increases, leading to MOV failure after 0.176 sec. Fig. 3(b) shows the corresponding MOV voltage.

In contrast, Fig. 3(c) illustrates the response of the TPMOV. Current pulses are higher than in Fig. 3(a) due to the normal variations in MOV characteristics. But in this case the TPMOV operates after 0.045 sec and isolates the MOV with zero arcing. If the available short circuit current reduces to a level where the on resistance limits MOV conduction, the amplitude of the current pulses fall and the operating times increase. Nevertheless, the TPMOV still operates prior to failure.

Surge Capacity

The TPMOV disconnects in the event of thermal degradation and withstands 40 kA of IEEE spec 8/20μsec surge current waveform (Fig. 4, on page 66). Designers created the TPMOV with this specific waveform capacity. To achieve effective thermal protection, the capacity of surge needed to be established with minimal I²t at 40 kA. The waveform used to meet the 40 kA peak has a 6μsec front time and a 17μsec tail time. The waveform tolerances are specified within IEEE C62 for surge protection products. If the TPMOV is subjected to surges in excess of the specified energy capacity, the thermal sensor disconnects. And as with all MOVs, if the incoming surges are greater than its ability to dissipate the heat energy, the MOV will be degraded.

But realistically, the TPMOV will seldom be subjected to surges greater than 10 kA. The design philosophy is to ensure effective protection from the predominant failure mode — thermal runaway. So, the TPMOV offers extremely fast response to thermal conditions and a high-energy capacity for dissipating random incoming transients.

The TPMOV provides two forms of built-in indication (Fig.5). Although it is critical to disconnect very quickly in the event the MOV disk has been degraded, it's also important that it quickly detects when the transient protector has been disconnected from sensitive electronics. The TPMOV accomplishes both tasks. It can disconnect as quickly as 20 ms (1.5 cycles), and its indication response time is not more than 2 ms from the time the thermal leg is removed from the MOV disk (Fig. 6).

The TPMOV offers excellent clamping ability to suppress incoming high overvoltage transients. Also, users can reduce the number of series components typically used to protect against thermal runaway. The TPMOV has built-in thermal protection with very little area between the MOV disk and mounting location. This approach reduces the effect of induced voltages added to the clamping ability during high energy/high frequency transients.

Fig. 7 represents the clamping ability of a high amplitude incoming transient. The red trace represents the shunted transient current, the blue trace represents a highly damaging transient voltage, and the green trace represents how the TPMOV suppresses the incoming overvoltage to levels that will not damage sensitive electronic devices or equipment.

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