Metal Oxide Varistors (MOVs) are commonly used for transient over-voltage suppression in many applications such as: Transient Voltage Surge Suppressors (TVSS), Uninterruptable Power Supplies (UPS), AC Power Taps, AC Power Meters or other products. Events such as lightning, inductive load switching, or capacitor bank switching, are often the sources of these transients. Additionally, in these applications, the possibility for a sustained abnormal over-voltage, with a limited current condition may exist which necessitates the need to protect the MOVs from an over-dissipation thermal condition.
The UL1449 standard clearly defines abnormal over-voltage, limited current test conditions. The intent of this paper is to outline these conditions, explore the use of MOVs in combination with a TCO (Thermal Cut-Off or Thermal Cut-Out) device, and compare performance with a thermally self-protected MOV technology. Graphs depicting the Epoxy temperature of MOVs without thermal protection will be shown along with a MOV/TCO combination and the internally protected MOV.
Under normal operating conditions, the AC line voltage applied to an MOV is not expected to exceed the MOV’s Maximum ACRMS Voltage Rating. Occasionally, over-voltage transients may occur that exceed these limits. These transients are clamped to a suitable voltage level by the MOV provided the transient energy does not exceed the MOV’s maximum rating. If, unlike a short duration transient, an MOV is subjected to a sustained abnormal over-voltage, limited current condition (as is required in UL1449), the MOV may go into thermal runaway resulting in overheating, smoke, and potentially fire. For end products to comply with UL1449, some level of protection must be afforded to the MOV to prevent this failure mode. That protection has traditionally been a thermal fuse or Thermal Cut-Off (TCO) device.
2. The UL1449 Abnormal Over-voltage Standard
In AC line applications, Neutral and Ground are typically near or at the same potential. In the event of a loss of a Neutral-Ground connection, there exists a risk that a sustained over-voltage may be applied to an MOV that is rated for a much lower continuous voltage. In an unlimited current condition, the MOV will first fail short, but due to the high amount of energy available, it most often ruptures instantaneously. If, however, there are loads tied to the AC line that limit current flow, the MOV can overheat and potentially cause the TVSS device to overheat resulting in smoke, out-gassing and eventually fire. This potential condition is specifically identified and addressed in the UL1449 TVSS Standard. See Table 1. In many cases, it requires that end-product manufacturers include a thermal protection element for an MOV.
|Device Rating||Phase||Test Voltage (a)||Voltage Rating of Conductor Pair that the test voltage is to be applied to|
High Leg Delta
High Leg Delta
|All110-120V120V All220-240V120V All
|(a) For device ratings not specified in this table, the test voltage shall be the maximum phase voltage (if available) or twice the conductor pair voltage ratings up to 600V max.|
Table 1: Test Voltage Selection Table
Table 1. defines the test voltage that should be applied to various TVSS devices depending on the designer’s desired device1 rating. Each test voltage is applied across each conductor pair with a short circuit current of 5A, 2.5A, 0.5A and 0.125A respectively across each of four TVSS devices. Since this test is destructive, four devices are needed to test for each of the four short circuit currents. The four devices must be energized for 7 hours, or until current or temperatures within the TVSS device attain equilibrium, or until the TVSS becomes disconnected from the AC Line.
For example, in a standard 120V AC Line application, the requirement is for a 240VACRMS test voltage to be applied across all conductor pairs. There are three pairs; Line–Neutral (L–N), Line–Ground (L–G), and Neutral–Ground (N–G). This test voltage is chosen because very commonly in the U.S., 120V AC power is commonly fed from a center–tapped 240V transformer. If a break occurs at X—X (see Figure 1.), then the load in the bottom phase acts as a current limiter and the line fuse may not clear. Thermally unprotected MOVs are typically rated from 130Vacrms to 150Vacrms and will heat up, out- gas and may catch fire in such circumstances.
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