| Electronics > Beginners |
| Ceramic capacitor is of too low voltage?...som,etimes blows up at switch on. |
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| SeanB:
Well, getting the electric system at the old building checked, and the supply impedance at the furtherest plug outlet is 0.4R. Take that with the voltage being pretty constant at 238VAC ( 200kVA transformer supply feeding this panel via 20m of 16mm copper cable and a 200A distribution fuse and a 100A main breaker) and you get a prospective fault current of over 500A, and this is a best case current, the closer sockets are higher, and you need to have a prospective fault current over 400A to blow fuses and trip breakers in the instantaneous ( single cycle of 20ms) range. Now, if you are not feeding a lot of 2.5mm cable, but instead are using 16mm cable as typical in a street lighting circuit, where you have only 10m of 1.5mm cable at most providing supply impedance ( the supply will be as close to 0.1R as you can think, just from the cable size, irrespective of the length of the cable used and that it will be fused with a 100A fuse at the point of supply) your switch on surges can be pretty high, and without inrush limiting resistors ( could be a PTC only, but better to have a combination of PTC and resistor and a damped power line inductor to take the sharper spike edges off) you can get really high current inrush. |
| james_s:
--- Quote from: exe on July 08, 2018, 11:27:43 am --- --- Quote from: james_s on July 08, 2018, 03:46:19 am ---If we assume 170V peak and ignore the source impedance of the distribution feeding the panel, resistance of the breaker, receptacle, power cord, etc then I get 1,070A as the absolute peak you could get. Now that's still a whole lot of current, but it's not 2,000A. Apologies for a pedantic moment. --- End quote --- There are some counties with 230VAC RMS :P --- End quote --- Sure, but Tim is in the US so I was assuming he was referring to a standard 15 or 20A 120V socket. I don't know what wire sizes are typically used in other parts of the world so I did the calculation based on what I'm familiar with and didn't have to go look up. Not that it really matters, I was mostly being facetious. |
| T3sl4co1l:
--- Quote from: james_s on July 08, 2018, 04:27:00 pm ---Sure, but Tim is in the US so I was assuming he was referring to a standard 15 or 20A 120V socket. I don't know what wire sizes are typically used in other parts of the world so I did the calculation based on what I'm familiar with and didn't have to go look up. Not that it really matters, I was mostly being facetious. --- End quote --- I actually originally did the same calculation when I cooked off a FWB, on a 240V 50A circuit. Which had about the same length but obviously much lower wire gauge than your estimate. (If you're curious, only one diode failed in the FWB. The other one ((s) at least) started at the same initial temperature, yet survived the fault surge!) FWIW, inductance makes up about 5-10% of most circuits, IIRC. It's not all that much at 60Hz, but enough to make note of, and it's significant for elevated frequencies, like for surge protection. Tim |
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