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python01

Power supply protection

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I need to know how to determing fuse size for Power Supply. Lets say the power supply specs show max input current at 1A @ 200VAC If I conntect to 200VAC 2 phase should I install 2 x .5A fuses so each phase get .5A fuse which will make it 1A total or should 2 x 1A fuses be installed?

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I can't imagine where you managed to run into 2 phase power these days, much less a power supply. The voltage spec is even stranger. 200 VAC? Really? That's very unusual indeed in any country. 208, 220, and 230 VAC are the common ones, and in any case, if the phases are 180 degrees apart, that's not "two phase". It's simply single phase without a neutral. However, 1A is 1A, no matter how many phases are involved. You don't use the exact size since you need a little headroom. The power supply manual should give you some idea how much to oversize (usually about 25%). If not, you can often estimate it. You didn't mention the TYPE of fuse either, and that's important as well. I highly suggest you get a book on fuses to understand how to do this correctly. Cooper Bussman and Ferraz Shawmut both have excellent free textbooks explaining it. Fuse vendors are also usually more than willing to come out and help you out. I've never had any problems getting free technical support from them.

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I guess one fuse will protect it anyways. Got the bussman book, will look through it soon. Thanks for the info.

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Each line voltage leg should be fused. You can omit a fuse only if the line from which it is omitted is a grounded current carrying conductor (neutral). I don't know about CSA rules, but I doubt that they are much different on the matter.

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Yeah, as far as I know they actually hate when you fuse the grounded conductor (aka neutral). Because if that fuse blows, things can get really dangerous.

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What about fuse size? If it is 1A device should each fuse be slightly larger than maximum current usage so lets say 2 fuses 1.25A or 1.5A each? How would fuses in 3 phase system be calculated?

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For 3 phase I think you need to put a fuse on each line (each phase). Most 3 phase systems have a neutral don't put one there.

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Whoa, be careful with that assumption there. There's high leg deltas, ungrounded deltas, "solidly" grounded wye's, low and high resistance wye's, impedance grounded wye's, mid-span delta's, and probably another few that I've missed. All have a neutral but it is not accessible in all instances. The neutral in an "ungrounded" delta is a moving target for instance. Sometimes you fuse the neutral and sometimes you don't. In the most common case for low voltage systems (208Y/120 or 480Y/277 solidly grounded), the neutral is normally not fused since protection is assumed to be provided by either overcurrent devices or using core balanced CT's, and the neutral is assumed to be a non-current carrying conductor in a 3-phase balanced load. Even in a 480Y/277 system, if you are using unbalanced circuits there are situations where the neutral carries both current and voltage relative to ground and must be protected against a line-ground fault and overcurrent.

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Hi You want to fuse both legs to your power supply (when you using two legs out of 3Phase) with same fuse size or .25% larger what manufacturer specs and make sure it's right model of fuse (time curve) because of inrush current.

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current in circuit is the same so if the power suppply draws 1A, then it is 1A on each phase (or phase and neutral). only phases must be fused so if you are powering PSU from two phases then both need fuse. if supplying from phase and neutral, only phase is to be fused. Canadian Electrical Code (CEC) is explicit about overcurrent protection (get the book and become familiar with it). switching neutral is ok only if done by device that interrupts phases as well (circuit breaker, non-fused disconnect sw. etc.). I would not use 1A fuses though, as you would likely get nuisance trips. fuses are there to protect wiring between fuse and PSU. If the wires are #14 for example (2.5mm^2) you can even use 15A. CEC also helps selecting proper wire size. reading CEC on your own will be dounting task. If you have not become familiar with it I would recommend signing up to some eveninig course (Humber College etc.). they last for 2-3 months, 2 nights a week.

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