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Old 25th Feb 2018, 3:25 pm   #1
eddie_ce
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Default DC clearance question

Hi,
I am in the process of constructing my HV power supply. I have aquired some NOS RS tagboards to mount the passives on. The minimum clearance distance between adjacent tags is 0.1". Will this be OK if the PD between tags is ca. 450V DC?

Thanks for your replies,
Regards
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Old 25th Feb 2018, 4:48 pm   #2
Richard
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Default Re: DC clearance question

Hi

I think initially it will be fine, but in a few years time, with some dust and muck build up it could arc across, I would have a rethink.

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Old 25th Feb 2018, 4:49 pm   #3
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Default Re: DC clearance question

I'd worry about that.

I'd worry even more if the 450VDC is going to be augmented by any amount of AF/RF signal-voltage!

My off-the-peg rule for linear-amp design was that in free-air you needed 10mm-per-DC-Kilovolt, doubling this if there was any degree of RF or AF signal present. A flashover can ruin your day if it takes-out $600-worth of Eimac's finest in the process.

Also, design for worst-case humidity. 'Damp-creep' can really mess-up the traditional calculations.
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Old 25th Feb 2018, 7:18 pm   #4
Al (astral highway)
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Default Re: DC clearance question

Quote:
Originally Posted by eddie_ce View Post
Will this be OK if the PD between tags is ca. 450V DC?
I'd also worry about that. Among the points mentioned already, surges (even lasting a few mS) could take things well over the PD that you plan, and it's a tiny clearance . Maybe skip /remove alternate tags? That's still pretty cosy.
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Old 25th Feb 2018, 8:15 pm   #5
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Default Re: DC clearance question

Insulation breakdown is usually by surface "creepage". I noted that the bases I got for my mains-rated octal based relays have ridges between the adjacent sockets, just as the relays themselves have barriers between adjacent pins, in both cases to increase the surface distance between adjacent pins to around three times the air clearance. .
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Old 25th Feb 2018, 8:51 pm   #6
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Default Re: DC clearance question

all is in IPC-2221
according IPC-2221 for 450V external conductors, uncoated minimum spacing have to be 0.1"
of course what mentioned before "but in a few years time, with some dust and muck build up it could arc across"

DESIGN is most important
I will not recommend to use such a small clerance between Ground and 450V but it depend on design where you place soldering tags for specified voltage
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Old 25th Feb 2018, 9:20 pm   #7
eddie_ce
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Default Re: DC clearance question

Thanks guys,
will give it a rethink and either

1. use alternate tags, removing the unused ones or
2. design so that the PD between adjacent tags is lower than the full DC voltage.

The tags are quite wide, hence the small clearance so removing alternate tags will give much more clearance.
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Last edited by eddie_ce; 25th Feb 2018 at 9:21 pm. Reason: additional information
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Old 25th Feb 2018, 10:35 pm   #8
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Default Re: DC clearance question

Hi Eddie, IEC recommendations are very useful, but also ensure that solder joints are well rounded and no spikey wire ends anywhere.

Ed
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Old 27th Feb 2018, 7:35 am   #9
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Default Re: DC clearance question

I've had some success with using epoxy to give extra insurance between and on HV terminals etc. I've also sprayed the underside of copper clad board PCB's with clear lacquer where there might be an aching problem, it's keeps contacts sealed, so no dirt can get in.

When mounting boards on a metal chassis I've used thick heavy duty gaffa tape under said boards.

Another technique used is to cut slots between pads or terminals of high PD, look at a good SMPSU and there are several methods used to isolate high PD and mains/DC out. Mostly it's good layout and thinking ahead.

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Old 27th Feb 2018, 7:14 pm   #10
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Default Re: DC clearance question

It's 2.54mm, which is not very much for 450V.

It'll be OK for a trial unit, but as others have said, long-term would be a worry. Especially on SRBP, moisture and dust can start the process of tracking, leading to carbonisation of the SRBP, leading to more leakage, leading to fizz-phut-phut-bang-smoke.

Even with double the creep age distance, cleaning the surfaces of flux residues is highly recommended, and maybe some anti-tracking varnish.

High voltage transistors in a TO-220 package are frequently found where the distance between the PCB pads is even less. But good design practice is to spread out the tracks immediately away from the pads.
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Old 28th Feb 2018, 1:14 am   #11
frankmcvey
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Default Re: DC clearance question

Hi, Eddie,

If you send me a drawing of your proposed tagboard, I could probably make one up for you in 2mm Tufnol.

Cheers,

Frank
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Old 28th Feb 2018, 5:18 pm   #12
eddie_ce
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Default Re: DC clearance question

Frank,

that is very kind of you. If I may, I'll get back to you over the weekend.

Many thanks to all who have so far replied.
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Old 1st Mar 2018, 10:35 pm   #13
Al (astral highway)
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Default Re: DC clearance question

Hey Eddie,

As luck would have it, I saw a clear case today which illustrates why a decent gap is important.

In the attached photo, the copper side of a buck converter is on display. It shows two soldered joints linking a surface mount resistor (R4, 125R) and another component.
The other component was a failed rectifier diode, part of a bridge.

As it broke down, its reverse characteristics changed such that an arc formed between one of its legs and R4, even though they were designed to be soldered together and shared the same pad.

The gap is 1.2 mm and the voltage is only mains.

The board was arcing away really fiercely, proving a surprising potential difference between the damaged component and its neighbour.

I replaced the diode and as it was now conducting properly, the problem stopped.

So, there we go, a nice example of the unexpected.
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