Spacing between HV conductors? - UK Vintage Radio Repair and Restoration Discussion Forum

Radio Tech · 3rd Jan 2016, 4:36 pm

Has anyone information on the safe flash over distance for 6kV between conductor and ground, by ground I mean the chassis. The reason I ask is that I am mounting some HV capacitors on a modified tag board to make it easier to mount the tag board as a small unit within an area where the original dual value capacitor came from. The distance I have is around 10mm between the conductor and the metal support. Conductors are mounted on a paxolin strip.

Regards

Ken

Humber888 · 3rd Jan 2016, 4:59 pm

This may help, but can't vouch for the accuracy of the statements therein: http://www.highvoltageconnection.com...agespacing.pdf.

Mike

Radio Wrangler · 3rd Jan 2016, 5:01 pm

The dielectric strength of air is around 3kV per mm. But that is the ultimate figure for clean, dry air measured with large-radius polished electrodes.

For rough edged tags etc this will come down somewhat, but for a straight air gap you should be OK. I suspect you'll have more trouble over creepage across the surface of insulation once dust gets on them.

We used to rout 5mm slots in PCBs in scope CRT EHT boards to substitute air gaps for creepage distance.

David

3rd Jan 2016, 5:22 pm

Can you slip in a bit of plastic? OHP (overhead projector) film is quite heat resistant too.

Refugee · 3rd Jan 2016, 5:38 pm

I have used the clear vinyl packaging that a curtain pole come in and it worked well and is flat too.
The other one I have used is squares cut from fizzy drink bottles but these need a bit of heat to flatten them out.
OHP film might be a bit thin.
I think the ozone smell from the tags is likely to be pretty strong.

Radio Tech · 3rd Jan 2016, 7:21 pm

Thanks everyone, the info is a great help, I will go ahead with my construction plan and just add a bit more insulation, might even encapsulate the whole unit some how.

Best wishes

Ken

joebog1 · 4th Jan 2016, 12:41 am

The other thing you can do, is to use brass bootlace ferrules, which you can rivet over using an ordinary centre punch. This removes the "sharp edges" of the tags. A bit of fibreglass circuit board is way more suitable than paxolin, You can also use nylon spacers and screws to increase the insulation resistance.
Simply drill some holes, the correct spacing for your caps, and of a diameter thats suits the bootlace ferrules. Custom made component mounting strips/boards!!.

Its very cheap too! The bootlace ferrules can be had from your local "chinese" shop for a couple of dollars for 4 or 5 sizes, and about 50 of each. For a bit more money ($8) you also get the punch and anvile for setting the rivets.
Joe

Herald1360 · 5th Jan 2016, 1:40 pm

The best place to find this sort of info is in safety standards for equipment. An out of date early version of something like IEC 60601-1 Medical Devices Standard for Safety should be findable for nowt somewhere. That provides comprehensive requirements for creepage and clearance distances which given the intended use of the equipment are nice and conservative.

Clearance required by that standard for 6kV peak voltage is 17.5mm for 1 or 2 MOOP (Means Of Operator Protection). Higher values are specified for MOPP (Means of Patient Protection).

John Caswell · 5th Jan 2016, 2:57 pm

"Blobby" solder on the joints will help as will sleeving any wiring in PTFE and/or heat shrink.

John

3rd Jan 2016, 4:36 pm	#1
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Spacing between HV conductors? Has anyone information on the safe flash over distance for 6kV between conductor and ground, by ground I mean the chassis. The reason I ask is that I am mounting some HV capacitors on a modified tag board to make it easier to mount the tag board as a small unit within an area where the original dual value capacitor came from. The distance I have is around 10mm between the conductor and the metal support. Conductors are mounted on a paxolin strip. Regards Ken

3rd Jan 2016, 4:59 pm	#2
Humber888 Heptode Join Date: Jul 2012 Location: Bromley, London, UK. Posts: 702	Re: spacing between HV conductor This may help, but can't vouch for the accuracy of the statements therein: http://www.highvoltageconnection.com...agespacing.pdf. Mike

3rd Jan 2016, 5:01 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: spacing between HV conductor The dielectric strength of air is around 3kV per mm. But that is the ultimate figure for clean, dry air measured with large-radius polished electrodes. For rough edged tags etc this will come down somewhat, but for a straight air gap you should be OK. I suspect you'll have more trouble over creepage across the surface of insulation once dust gets on them. We used to rout 5mm slots in PCBs in scope CRT EHT boards to substitute air gaps for creepage distance. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

3rd Jan 2016, 5:22 pm	#4
Guest Guest Posts: n/a	Re: spacing between HV conductor Can you slip in a bit of plastic? OHP (overhead projector) film is quite heat resistant too.

3rd Jan 2016, 5:38 pm	#5
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,553	Re: spacing between HV conductor I have used the clear vinyl packaging that a curtain pole come in and it worked well and is flat too. The other one I have used is squares cut from fizzy drink bottles but these need a bit of heat to flatten them out. OHP film might be a bit thin. I think the ozone smell from the tags is likely to be pretty strong.

3rd Jan 2016, 7:21 pm	#6
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: spacing between HV conductor Thanks everyone, the info is a great help, I will go ahead with my construction plan and just add a bit more insulation, might even encapsulate the whole unit some how. Best wishes Ken

4th Jan 2016, 12:41 am	#7
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: spacing between HV conductor The other thing you can do, is to use brass bootlace ferrules, which you can rivet over using an ordinary centre punch. This removes the "sharp edges" of the tags. A bit of fibreglass circuit board is way more suitable than paxolin, You can also use nylon spacers and screws to increase the insulation resistance. Simply drill some holes, the correct spacing for your caps, and of a diameter thats suits the bootlace ferrules. Custom made component mounting strips/boards!!. Its very cheap too! The bootlace ferrules can be had from your local "chinese" shop for a couple of dollars for 4 or 5 sizes, and about 50 of each. For a bit more money ($8) you also get the punch and anvile for setting the rivets. Joe

5th Jan 2016, 1:40 pm	#8
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: spacing between HV conductor The best place to find this sort of info is in safety standards for equipment. An out of date early version of something like IEC 60601-1 Medical Devices Standard for Safety should be findable for nowt somewhere. That provides comprehensive requirements for creepage and clearance distances which given the intended use of the equipment are nice and conservative. Clearance required by that standard for 6kV peak voltage is 17.5mm for 1 or 2 MOOP (Means Of Operator Protection). Higher values are specified for MOPP (Means of Patient Protection). __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

5th Jan 2016, 2:57 pm	#9
John Caswell Hexode Join Date: Jul 2009 Location: Wokingham, Berkshire, UK. Posts: 437	Re: spacing between HV conductor "Blobby" solder on the joints will help as will sleeving any wiring in PTFE and/or heat shrink. John