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Background: I have a small soldered joint on a 500 VDC (0.1 A) lead that's exposed to air. The edges of the joint are fairly ridged and have a few significant edges. In the higher voltage range (40 kV) these edges can potentially cause a loss in current due to the ionization of air. Unfortunately, the joint is at a point that I cannot cover with liquid electrical tape or transformer oil.

Question: Will the current drop due to this exposed joint (the exposed lead and joint measure 2 cm) be significant? How much of a current drop should I expect due to the ionization of air, if any?

EDIT: Here are the picture of the exposed sections. Please note that only the white cable has the 500 VDC, others are just ground or 12 VDC inputs.

The two sections that are exposed to air are the connection to the board and the end section (the part covered in solder). I used a gold US dollar for reference.

This is the connection to the board with the dollar as reference

This is a slightly more zoomed in image of the connection to the board, notice the small wires

Soldered end of the wire, my apologies for the glare

  • Now you just need to send each of us one of those coins so we can see how big it really is. You should use a scale instead...you have an international audience. – Elliot Alderson Feb 17 '20 at 17:25
  • @ElliotAlderson Fortunately, the coin's dimensions are well documented. According to Google, it's 26.5 mm in diameter. –  Feb 17 '20 at 17:45
  • "you have an international audience" Indeed (UK here). No problem, I'll just Google the size ... ah, there are three different sizes. As this is a [Presidential dollar](https://en.wikipedia.org/wiki/Dollar_coin_(United_States)#Presidential_Dollar_Coins_(2007%E2%80%932016)) it has a 26.5 mm (1.043") diameter. – Graham Nye Feb 17 '20 at 18:02
  • About 10% bigger than a US Quarter. – Mattman944 Feb 17 '20 at 18:31
  • I would trim the stripped ends of the wires going into the terminal block so that the wire insulation can go right against the body of the terminal block (or at least much closer than shown). – Peter Bennett Feb 17 '20 at 19:14
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    @GrahamNye's advice re NOT soldering wires used in terminal blocks is VITALLY IMPORTANT. The wire not just CAN spread under pressure with time but WILL spread. The 400V wire will become a progressively looser fit. It MAY pull free or may just cause a high resistant joint, possibly causing arcing and/or fire. This is a genuine real-world hazard. – Russell McMahon Feb 17 '20 at 20:18
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    I live in the US. I have no idea what that coin is or how big it is without doing a lot of google searching first. Please be reasonable and not defensive when people make suggestions. We really are trying to help and there are a lot of things you can do to make it easier. – user57037 Feb 17 '20 at 22:32
  • I would be worried about clearance from the 500V net to all other nets primarily from a safety perspective. Is there any way a single fault like a small metal fragment or solder ball could bridge from 500V to other lower voltages? Is there any chance someone may be touching those lower voltages so that they are subject to shock hazard? With 500V if you have a small air gap you won't have much leakage current. But touch safety is also a big concern. – user57037 Feb 17 '20 at 22:36
  • @RussellMcMahon Thanks. Updated "can" to "will" in my answer accordingly. – Graham Nye Feb 17 '20 at 22:58

3 Answers3

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I wouldn't worry about air ionisation unless your 500 V circuit was exceptionally high impedance (many MΩ).

The wire to resistor joint would ideally have some heat-shrink insulation over it, more for strain relief and protection against electric shock than concern over ionisation.

The connection to the screw terminal has a large margin of uninsulated wire which looks tinned (though it might just be out of focus). If stranded wire is soldered together to go under a screw terminal the solder will spread under the pressure resulting in a loose connection developing over time. It's better to either leave the wire unsoldered (but with the strands twisted together) or crimp on an insulated ferrule. The wire insulation should continue up to the edge of the screw terminal (but not inside it) or finish inside the insulated boot of a ferrule.

Graham Nye
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You will not get corona discharge at 500VDC using any kind of sensible setup (like you have).

For higher voltages it's good to avoid sharp points which increase the electric field gradient.

That terminal strip (5mm pitch?) is probably not rated for 500VDC, but it's probably okay enough.

Spehro Pefhany
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Current leaks at 500VDC

Question: Will the current drop due to this exposed joint (the exposed lead and joint measure 2 cm) be significant? How much of a current drop should I expect due to the ionization of air, if any?

Like other answers show, you don't need to worry about corona discharge. If you worry about leakage currents, you'd better worry about leakage due to creepage, especially between the solder pads of the blue terminal strip.

Unless this 500VDC is a current source, a leakage current will not cause a drop in current. High voltage power supplies typically are (non-ideal) voltage sources which try to maintain a steady voltage and provide as much current as required. Leakage currents will therefore cause an increase in current, not a drop in current.

Huisman
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