I need to size and build a bleeder resistor network or could be as simple as two bleed resistors….im not sure but I have an application where a we have a step up transformer from 120VAC single phase to circa 1200VAC on the O/P side. When we power up the line, and the line could be anywhere from 5 to 120km with cross sections ranging from 6mm to 25mm for each of the conductors, ie L(6mm) and N(6mm) for application 1 and L(25mm)/ N(25mm) for application 2 they obvious act like a capacitors ie when we switch the power supply off, there is still a residual charge in the line. We then have to "dump" that to earth before we can work on it or you get a shock. So, question is, where to put the bleed resistors in the circuit, on the 120VAC "input" side of the XMFR or do they have to be on the O/P side? If they have to be on the output side, how do i connect them so they are not in circuit when the transformer is on but when it is switched off they are activated? Finding relays / contactors etc for 1200VAC rating is difficult. Also, We would need to bleed the circuit within approx 5 to 10 mins, so how do we go about sizing the resistors? The 1200VAC power supply is approx 2.5kVA rated.
Electrical – Discharge / Bleeder resistors
dischargeresistors
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Best Answer
Because the residual charge would be DC, anything on the 120V AC mains side of the xfmr can't very well bleed it. However, if a slight loss of efficiency if acceptable, you could place a 1Mohm to 1.2Mohm resistor from L to G on your output. it would only bleed about 1-1.2mA of current (about 1.2-1.5W) during operation, but your 5-10 minute "powerdown time" gives 300-600 seconds for that "trickle" resistor to discharge the line significantly. Alternatively, if you can source/afford/build an inductor with a similar resistance at your AC frequency, your efficiency loss would be similarly minimal (attaching the inductor from L to N, same as resistor), but would appear as nearly a "dead short" to 5he stored, capacitive DC charge, and most likely discharge the capacitance below 1V in >0.25sec after mains input power is cut.