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Thread mating and wild electrons

If I tighten a male and female thread together so the bolt is under tension, presumably there comes a point where virtually all the load is on the flank on one side of the Vee with a good metal to metal contact giving good electrical contact. If now the thread is actually also a pipe fitting which needs to be both sealed and also a good electrical contact it rules out ptfe tapes etc. However if I were to use something like Loctite 542 hydraulic seal goo (which incidentally is fantastic stuff) would you expect there still to be good metal to metal contact on the side of the Vee as previously (assuming tightened so again under tension)? Or do these things creep under each other at the molecular level and sigificantly reduce the area of touching metal ? Using normal means I can detect no increase in resistance however in its final use this 3/4" BSP fitting may carry up to 1000 amps for short periods.

AWEM

Reply to
Andrew Mawson
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How much actual contact area do you need for you 1kA? How much is potentially available with your 3/4" BSP? Have you worked that out?

There must be quite a bit of localised deformation when the fitting is tightened which should ensure good contact. I'd be rather surprised if your Loctite could 'creep in' to upset that. My surprise is no barrier to it happening, though

Probably no help

Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

Reply to
Tim Leech

Just a thought. Is it possible to separate the two requirements (electrical contact & liquid-non-drippiness) by using a long thread with a backnut/sleeve and sealant? The 'ungoo-ed' female to male bit could provide the elctrical continuity. The 'goo-ed' backnut could provide the seal. Slightly difficult with taper threads though.

Reply to
John Montrose

Maybe you could try some busbar contact grease? It's not water soluble and is designed for high current applications. Although I've only seen it used for ~100A busbars, so I'm not sure about 1kA!!

It should be viscous enough to avoid being forced out if the threads are done up tightly and the internal pressure isn't too high.

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-Mark

Reply to
Mark Rae

Andrew, I think that as it is a seal it would also be a good insulator. Why cant you use a bonding strap.

Martin P

Reply to
campingstoveman

actually

incidentally

tightened

barrier

Hard one to call that Tim, as the fitting is cooled with chilled water. My current (sorry for the pun) solution being a belt and braces sort of chap is to have a big copper strap round the fitting, I suspect it to be redundant but don't want to take the risk!

AWEM

Reply to
Andrew Mawson

actually

incidentally

tightened

John, my current solution is a large copper strep round the fittings just to be on the safe side, but I suspect it's redundant - just I don't dare try without!

AWEM

Reply to
Andrew Mawson

actually

incidentally

tightened

high.

oooh - that's interesting. I've used the spray type for years which is an anti-oxidant and works wonders on switches. I have actually fitted copper straps round the fittings, the query is really a theoretical one.

AWEM

Reply to
Andrew Mawson

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under

actually

incidentally

tightened

however in

Martin, see my other responses - that is my present solution, the query is really at the theoretical level as I don't have the courage to remove the strap and try

AWEM

Reply to
Andrew Mawson

A lateral thought for you... If you have a current transformer that is happy with the frequencies you are using, measure the current in the strap to see what proportion of the total current that it is carrying. If it's not carrying most of the current, the pipe joint would probably be ok on its own.

This assumes that there is enough extra inductance in the strap that, with a perfect pipe joint, the current would prefer to go the straight route through the pipe joint.

Mark Rand RTFM

Reply to
Mark Rand

You could go the Hemp and paste route and electrically bond the pipe and fittings together.

Reply to
Neil Ellwood

Not my subject area but I would guess that copper to copper contact area would be stable or increase with time.

Not sure what what resistance measurement method you are using but 4 terminal methods can detect less than 100 micro Ohm change.

Pass 10 A DC through the joint and monitor the voltage drop with a separately connected 3 1/2 digit DVM on 200mV range. Repeat with reversed current connection to check for presence of any stray thermal EMFs.

Jim

Reply to
pentagrid

actually

incidentally

tightened

Jim,

Essentially that is the method I used except at about 100 amps, but that is still an order of magnitude below operational level !

AWEM

Reply to
Andrew Mawson

I guess a pretinent question is whether you ever need to disassemble the fitting. If not, why not just weld the joint & be done with it?

Regards, Tony

Reply to
Tony Jeffree

Use mercury as the pipe dope :-)

Mark Rand (running) RTFM

Reply to
Mark Rand

I'd put a copper strap round the fitting - 1000Amps - mad person!

Steve

Reply to
Steve W

How about using taper threads, and one of the copper or nickel-containing thread anti-sieze lubricants. "Copper-slip". The copper-coloured disk brake anti-squeal stuff is similar. Some of them contain 15% finely divided copper.

Reply to
Newshound

But look on the web first for sites offering instant coffin fitting.

Reply to
Neil Ellwood

I would say yes. Yes, it doesn't change the contact area. Where you do have metal/metal contact, the loctite is squeezed out. Without the loctite, you would simply have air gaps. But I might be completely wrong.

How about using some thin copper foil as a sealant and conductor?

Nick

Reply to
Nick Mueller

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