Fred Waechter said, >Hi all, ...I need some ideas/help regarding correct fuse selection. >1. When testing to IEC801-5 surge test, I want to be sure that the fuse will open but not physically disintegrate. There is a MOV directly after the fuse. >2. For fuse selection in an environment threatened by lightning, is IxIxT rating or interrupt rating more important? I assume IxIxT for normal operation and interrupt rating for surge operation. >3. IEC127 Fuse Standard defines interrupt rating between 35A - 1500A @ 250Vac. UL198.G/CSA 22.2 No. 59 defines interrupt rating of 10,000A @ 125 Vac. >4. IEC801-5 calls out waveform Voltage 1.2us/50us 6000V Current 8us/20us 3000A >5. Question - How can I be sure for European fuse application, if I apply 3000A current surge, that fuse will not disintegrate, as fuse standard calls out 1500A? I accept different manufacturers will probably have different ratings which will exceed 1500A spec., by different amounts.
As usual, Rich Nute has some good comments. Let me add some more: (Deap breath...) 1. MOVs tend to fail shorted. While shorted and still hot they exhibit a fairly low impedance, but still large in comparison with an intact fuse, so most of the fault energy will be dissipated outside the fuse. 2. I-squared-t (actually the integral of current-squared with respect to time) is called the "fusing integral" and represents what it takes to open the fuse. For current below the rating of the fuse it goes to infinity (the fuse doesn+t open). For currents just above the fuse clearing current (1.1 to 1.35 times rating for US fuses, 1.5 to 2.0 times rating for European fuses) it is a factor of how efficiently the structure removes heat from the fuse element, and will be higher for sand or powder-filled fuses. For higher currents, up to the break capacity of the fuse, it will tend to be constant, and is a good indicator of proper fuse operation. At higher currents it tends to become erratic, indicating impending fuse failure and/or disintegration. The interrupt rating is the critical factor in high-current performance. Designs with high break capacity tend to have powder or sand fill, often of a proprietary material, robust, pressure-tight enclosures, low fusing integral (in the constant range described above) which limits heating and ion production, larger sizes, higher price tags. 3. Interrupt ratings of 100,000A and higher are available, but seldom needed, and devilishly hard to verify. The interrupt rating is *NOT* the current through the fuse! Rather, it is the short-circuit current available at a test source. A warning: some fuse specifications add up to 4 feet of wire in series with the fuse, that is present when testing the fuse, but not present when measuring the source short-circuit current. This rather deceptive practice leads to listed fuses which will NOT safely break their rated break current. In addition, AC fuses specify specific current phase angles, and DC fuses specify certain source inductance, which may or may not represent your use situation. In real life, there is generally some minimal amount of circuit impedance which can limit short-circuit current. In your case, even a shorted, melted MOV has significant impedance. 4. The IEC 801-5 waveform represents the envelope of the transient caused by a nearby lightning strike. The peak open-circuit voltage and the peak short-circuit current are combined into a "source impedance" R(source)= E(peak) / I(peak). This isn+t really a source impedance since the source is not linear, but it+s a useful measure. The short-circuit current (and "source impedance") are defined based on how much current-limiting distribution circuitry lies between the lightning strike or other source, and the instrument being tested. In most cases you expect the product to absorb such a transient without damage and without blowing the fuse; the actual IxIxT is lower than the fusing integral, and the transient is gone before the fuse has time to open. However, if your circuit is overly susceptible a component may short, allowing "follow through" by the mains voltage. It+s this follow through that causes fires and for which the fuse must be designed. 5. If in doubt, test the fuses with the transient, in your circuit. Do some margin testing. A well-designed circuit should continue to operate. But an intact or just-melting fuse has relatively low impedance, and is unlikely to be destroyed by the transient: it+s over too quickly. Fuse break capacity can generally be specified for the worst-case short-circuit source current, based on source and circuit parameters, without regard to very short-duration transients.
