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Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
Maybe someone can explain some unexpected NST behavior. I am using an old-fashioned NST for a project. It is a Magnetek rated at 15 KV and 60 ma. If I connect the NST to 120 VAC, I can draw an arc between either HV terminal and an earth ground. The NST is not grounded, the case is floating. How can this be? There does not appear to be a path for a complete circuit. I know that the center tap of the secondary is connected to the core but the core shouldn't be connected to anything else other than the case, which is floating.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546
The 120 volt power line is connected to an earth ground. One side is directly connected, The other side is connected also to the earth ground through an extremely low impedance. Across this impedance is an AC voltage of 120.
The case of the NST has an electrostatic capacity to both line terminals.
The 7500 volts flows through this capacitance. Lets assume it is 1E-10 F. The current would be 0.3 mA.
That would pull an arc.
Also arcing the transformer like that is asking for trouble. You set up high voltages in unexpected places, like across the output insulators to case.
An energized NST needs a load.
If you have a DVM, with all the power off, use the capacity scales to measure case to output, output to line.
These days a 6000 count DVM from China costs about $30. and can measure capacity over a wide range.
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Unusual NST behavior
