IEEE P62.42.2-2020 pdf download

IEEE P62.42.2-2020 pdf download.Draft Guide for the Aplication of Surge-Protective Components in Surge Protective Devices and Equipmen tPorts – Part 2: Metal-Oxide Varistors(MOVs).
NOTE— – Reprinted with permission from Littelfuse Inc. (see Figure 2a in EC638 [B32j.)
Clamping voltages can range from 17 V to 4000 V depending on the varistor voltage rating and disc size.
The user should be cautioned against selecting a varistor with a clamping voltage so low that it would begin to conduct excessively during high ac or dc power line conditions. This might then interfere with normal operation of the equipment being protected, overheat the varistor and shorten its life due to extended operation above rating, see 7.3 for further discussion about selecting a varistor with adequate rating.
A factor that can affect the varistor’s measured peak voltage is lead and circuit inductance, due to L di/dt voltage, see 6.5.
Users should be aware that the clamping voltage can increase slightly, typically less than 0.01 %/°C, with increasing operating temperature. Also, clamping voltage can change after being exposed to a number of high current transients, and the amount of change could be significant depending on the application.
6.2 Leakage current (DC) (ILd)
The dc leakage current is the current conducted by a MOV from the power source. The important consideration is that the self-heating of the varistor should not be significant compared to its rated average power dissipation, PN’t (7.6). Standby current by definition is measured with rated dc voltage Vm(dc) applied. Values of VIfl(dC) can be obtained from manufacturer’s specifications, and should not be exceeded. Users should design with adequate derating margin to allow for power source voltage variations.
Users can estimate typical standby current. For example, consider a varistor with V-i characteristics as illustrated in Figure 5. If the nominal power source voltage were 350 V de, then the current drain would be about 10 nA. If the power source voltage increased to 400 V under high line conditions, then the typical standby current would increase to about 50 nA. The system power drains under these two conditions would be about 3.5 p.W and 20 j.iW respectively, small values compared to a P1 rating of about 1 W, and not enough to elevate component temperature. Typical values for DC leakage current ranges from 0.5 iA to <100 jiA for disc type varistors and 0.5 jiA to <200 pA for large block type varistors. Users can estimate the effects of temperature on standby current. Figure 10 illustrates the typical temperature coefficient of voltage for varistors shown in Figure 5. For the example above, the varistor voltage characteristic would be expected to decrease at a rate of about 0.15 %f°C with increasing temperature. If the ambient temperature were to increase to 60 °C above room value, the varistor voltage characteristic would shift downward by about 9.4%. Therefore, the characteristic point, which was 350 V at 25 °C, would become about 317 V at 85 °C, and typical standby current would increase from about 10 nA to 25 nA. NOTE— Reprinted with permission from Littelfuse Inc. (see Figure 14 in AN9767.l [B29].).) The leakage current of a varistor is subject to change over operating life. On typical specimens the measured value has been observed to be stable or even to decrease somewhat during the initial period of service. With an accumulation of stresses over time standby current may increase significantly, due to the V-I characteristic shifting in the leakage region similarly to the effects of increased ambient temperature. This could eventually lead to failure in the degradation mode, or to even failure by thermal runaway. However, there is no single value that can determine the standby current limit appropriate for all applications; but rather an evaluation process taking into consideration the desired component life, anticipated ambient temperature, incidence of high energy pulses, and selection of the end-of-life (failure) criteria. These considerations are within the realm of the manufacturer’s and user’s component engineering functions. 6.3 Nominal varistor voltage (VN) The nominal varistor voltage is a test characteristic that is commonly used to classify varistors by type. A test current of 1 mA is the value most frequently used, although values of 0.1 mA and 10 mA also are used in some manufacturers’ specifications for some types. These test currents are relatively low and are compatible with many models of commercially available automatic test equipment used by the electronics industry. Therefore, the nominal varistor voltage fulfils the role of a varistor characteristic convenient for both manufacturers and users to measure.