Over-Current Issues For Transient Voltage Surge Suppressors

As of February 9, 2007, suppliers of UL Listed transient voltage surge suppressors (TVSSs) must comply with the latest revisions of UL 1449, the Standard for Transient Voltage Surge Suppressors, 2nd Edition, in order to maintain their UL Listing. Changes to the abnormal over-voltage tests are intended to ensure adequate protection against thermal runaway caused by over-currents from over-voltage conditions.

Since Article 285 of NEC 2005 requires that TVSS devices be listed and labeled with a short circuit current rating (SCCR), the loss of their UL listing means they can’t be used. Likewise, UL508A uses the SCCR of the TVSS to develop a SCCR for industrial control panels.

Failure Modes & Short Circuit Hazards
Under normal voltage conditions, a MOV (metal oxide varistor) is in a very high resistance (non-conductive) state. When a high-voltage transient is imposed on the power line, and the transient voltage exceeds the MCOV (maximum continuous operating voltage) of the MOV, the MOV changes to a low-resistance, highly conductive state. The majority of the energy of the transient is then shunted through the MOV and the voltage imposed on the protected equipment is limited to a safe level. After the transient voltage decreases below the MCOV, the MOV returns to its high-resistance state.

The MOV must then dissipate the heat generated during the conductive state. If subsequent conduction states occur too soon after the first transient, the MOV temperature can rise to damaging levels.

Exceeding Maximum Ratings
MOVs have a maximum single pulse peak current rating. Typically, this rating is determined with the 8/20S waveshape defined in IEEE C62.45-2002. This rating identifies the maximum transient energy that can be absorbed by the MOV without damage to the device. This type of failure is not common but can occur when a TVSS with a single pulse peak current rating is improperly selected for an application where it’s exposed to pulse energies beyond its rating.

Under sustained over-voltages an MOV can fail due to a phenomena referred to as thermal runaway. Thermal runaway can occur when the normal 60 Hz power system voltage becomes higher than the MCOV rating of the MOV and initiates a flow of continuous 60Hz current through the MOV.

When the voltage is above the MCOV, the MOV enters its conductive state and current flows through the device until the next zero crossing of the voltage. The magnitude of this current flow depends on the dynamic resistance of the MOV in its conductive state. The continuous heating generated by this current will exceed the energy capacity of the MOV and eventually cause the device to rupture.

Repetitive Pulses
MOVs also have multiple pulse ratings. Every time an MOV conducts transient energy, its life is slightly reduced. The pulse lifetime ratings are based on the definition of rated life, which is the point at which the nominal voltage of the MOV has decreased by 10 percent. One surge near its capacity of 10kA may degrade the MOV’s life as much as 20 surges at 0.2kA.

Surges can break down some of the junctions between the zinc oxide grains in the MOV. The greater the energy absorbed during a transient, the greater the number of junctions damaged. The result is a reduction of the MCOV and of the MOV energy absorbing abilities. This change in structure does not effect the ability of the MOV to clamp surges. Conversely, the voltage required for MOV conduction is reduced.

If the MCOV is reduced to the point that the MOV conducts during part of the normal 60 Hz voltage wave, the MOV will go into thermal runaway as previously described. Failure (case rupture) will occur when the heat generated by the 60 Hz current through the MOV exceeds the MOV’s energy capacity.

Standards & Requirements
Several changes have been made to North American codes and standards in recent years to address safety concerns about surge suppression products. Here are the ones you should know and be sure to properly address:
* National Electric Code:
• Article 285, Transient Voltage Surge Suppressors: TVSS, was added to the 2002 edition of the National Electric Code to cover the safe application of surge suppression products permanently installed on premise wiring systems.
• Section 285.5 requires that such products be listed devices.
• Section 285.6 requires that the TVSS be marked with short circuit current ratings (SCCR) to ensure that the TVSS is not installed in a location where the available fault current is greater than the capability of the surge suppressor’s protection system.

* UL 1449 Standard for Transient Voltage Surge Suppressors:
• TVSS devices listed to the UL 1449 standard are acceptable for application on wiring systems per NEC 285.5. They must successfully pass the tests without creating conditions that would increase the risk of fire or shock. Changes to this standard in recent years have had major implications in the selection of over-current protection schemes.
• The second edition of UL 1449, initially issued in 1996, added the Abnormal Overvoltage Tests to address field failures caused by temporary overvoltage (TOV) conditions with low follow currents. Overcurrent protective devices (OCPD) typically selected to protect MOVs from case rupture and sized to pass larger surge currents without opening, were not able to open for small follow currents caused by thermal runaway. To pass the Limited Current Abnormal Overvoltage Test, a TVSS needed to meet the standard’s requirements for test currents of 5, 2.5 0.5 and 0.125 amps.
• After further investigations, UL issued a revision to UL 1449 in February of 2005, adding new test current levels, again, in response to field failures.
• To address concerns of intermediate short circuit current levels, the Full Phase Voltage — Short Circuit Current Abnormal Overvoltage Test of Section 37.3 now requires tests to be performed with fault currents of 100, 500 and 1,000 amps and the selected SCCR of the device.
• The Limited Current Abnormal Overvoltage test of Section 37.4 requires tests at 10, 5, 2.5 and 0.5 amps. Compliance to the standard’s requirements at all test current levels is required to maintain the UL Listing of existing TVSS designs.

Over-Current Protection Options
Fuses used in series with MOVs are not used to protect MOVs from damage due to excessive heating from surge currents. Instead, they are used to clear high-level 60 Hz follow currents that could result from an MOV failure.

Fuses have been successfully used to protect equipment from the catastrophic failure of MOVs when they’re connected across the line terminals of equipment with large available fault currents and fail in a short circuit mode.

To achieve optimum transient protection, the fuse must be selected to allow the MOV’s rated surge current to pass without opening. In the case of MOV failure, the fuse must safely interrupt the potentially large follow current that will result from the low-resistance state of the MOV failure. To protect against case rupture from the heating of the follow current, the fuses used for these applications need to be very current limiting.
 
Applications
Current flow through an MOV due to 60 Hz overvoltages causes heating of the MOV. Thermal protectors are designed to sense the temperature rise of the MOV and disconnect it from the circuit before catastrophic failure occurs. Complete electrical protection typically includes short circuit protection and thermal sensing. The thermal p