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Author Topic: conductive ballast - Aaaghhh  (Read 401 times)

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Offline Tom U

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conductive ballast - Aaaghhh
« on: May 23, 2020, 12:03:05 PM »
Well, here is something I never heard of before.  I thought |I was going nuts.
I have a fan of points with 5 roads going to a 4 bay shed and a turntable.
I ballasted it with a balast labelled as cinders.  Very dark grey with white flecks.
As the first two roads to the shed are quite long, I put in 2 switched isolation gaps so I could store several locos (DC power).  It is many months since I ballasted the area, and I have just got around to using it. 
The problem I noted was that the loco parked in an isolated spot would still move.  First I thought the gap had closed from expansion, so I re-cut the slots - same. 
I dismissed a switch problem because 4 switches don't fail all together.
I set the controller to 15V DC, and measured across each isolation gap on road 1.  There was still 11 volts after the first gap and 8 volts after the second.
So I set the meter to Ohms, and probed around on the ballast, sure enough I was getting continuity.  With the probes 1 cm apart on the ballast I was getting a reading of 0.8 (I think k Ohms) and 30 cm apart I was still getting continuity at 3.0 to 6 (I think k Ohms).
I checked all the normal grey ballast around the track, and nil reading.
Fortunately the cinder ballast came up quite easily by soaking it and vacuum cleaning, but fiddly in the sleepers. (I laid it the "standard" way with 50/50 PVA). I now have normal isolation switch operation.  I shall replace the ballast with a black ballast that I know is not conductive.

Unfortunatly, I discarded the bag and label of the cinders ballast, so I can't recall what vendor - but it was bought in a model rail hobby shop and sold as ballast, and it was from one of the normal vendors.  I would not have known that it was conductive had it not been for it partly shorting the isolation gaps.

Anyone encountered this before?

Tom.

Offline Newportnobby

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Re: conductive ballast - Aaaghhh
« Reply #1 on: May 23, 2020, 12:13:06 PM »

Anyone encountered this before?

Tom.

Not personally and it's one I've not heard of before either. How strange :uneasy:

Offline PLD

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Re: conductive ballast - Aaaghhh
« Reply #2 on: May 23, 2020, 12:30:18 PM »
Not come across ballast that was conductive, but once found a glue that was conductive. We'd borrowed it from another layout and it looked like PVA...  :-[

Offline Tom U

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Re: conductive ballast - Aaaghhh
« Reply #3 on: May 23, 2020, 12:34:43 PM »
Not come across ballast that was conductive, but once found a glue that was conductive. We'd borrowed it from another layout and it looked like PVA...  :-[

Logical theory, but I used the same PVA mix for the grey ballast and that is fine.
 ;)
Tom.

Offline Tfc49

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Re: conductive ballast - Aaaghhh
« Reply #4 on: May 23, 2020, 12:46:54 PM »
It could be that the glue is a red herring - the ballast might be the culprit, perhaps being white granules (e.g. aluminium oxide) with the predominant grey colouring being the white mixed with graphite powder, and once the glue had dried there was sufficient contact between graphite particles to give a high-resistance conductor.

Offline NinOz

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Re: conductive ballast - Aaaghhh
« Reply #5 on: May 23, 2020, 12:48:53 PM »
If it was real cinders and not just a name for the look and colour then yes it will be quite conductive when wetted.  All those lovely metal oxides will form all sorts of stuff.

I have had some weird experiences with PVA and various commercial sources of ballast:
1.  Attacked the nickel silver rail over a few months turning the ballast and glue a nice pale shade of blue.
2.  One lot would conduct sufficiently in humid weather to activate the current sensors causing occupation LEDs to light up and point CD units to fire.  Took a while to track down.
To be called pompous and arrogant - hell of a come down.
I tried so hard to be snobbish and haughty.

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Offline honk843

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Re: conductive ballast - Aaaghhh
« Reply #6 on: May 23, 2020, 01:17:25 PM »

anyone encountered this before?




Yep. It is the PVA -I got it shorting between rails on point frogs.
The answer is to be careful, making sure the ballast and glue don't touch the rail sides (good practice anyway). If it just the PVA it should peel away from the rails when dry and if Peco track is being used if it is not a problem if the ballast is kept below rail level. There is another thread on the forum regarding ballasting which might be helpful.

https://www.ngaugeforum.co.uk/SMFN/index.php?topic=48219.msg620392#msg620392

Offline honestjudge

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Re: conductive ballast - Aaaghhh
« Reply #7 on: May 23, 2020, 01:50:30 PM »
I've never encountered dried pva being conductive. None of my layouts would work if that was the case.
Do you think those cinders were the real deal? Being carbon they would conduct even if dry.

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Re: conductive ballast - Aaaghhh
« Reply #8 on: May 23, 2020, 02:35:11 PM »
I notice you are in Thailand, what is the humidity where your layout is.  It is possible that your PVA has not completely dried out.

NB if you pass power through PVA it can chemically change and form permanent conductive paths depending on the materials it is in contact with.

PVA is acidic by nature and will chemically modify any materials it comes in contact with and acid and power is the basis of most batteries.

Offline Tom U

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Re: conductive ballast - Aaaghhh
« Reply #9 on: May 23, 2020, 02:50:53 PM »
Thanks for some interesting comments.
 It is definately the cinders, not the PVA.
I used PVA from the same pot using the same methods for the main lines grey ballast, and that has no conductivity.
The ballast has been laid for many months, so is well dried out, and again the grey is in the same humidity conditions. (Actually, Chiang Mai being in the mountains is not so humid).

I just stuck the meter probes into the jar of leftover cinders, and even with the probes on opposite sides of the jar I got a reading.  In the jar of grey ballast, even with the probes within 2 mm, open circuit.

I suspect it is real cinders and as someone suggested, high in carbon.

Tom.

Offline NinOz

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Re: conductive ballast - Aaaghhh
« Reply #10 on: May 23, 2020, 11:13:45 PM »
I suspect it is real cinders and as someone suggested, high in carbon.
cinders = low in carbon, high in metals and very alkaline
charcoal = low in metals, high in carbon

PVA (poly vinyl acetate) is not acidic (fairly neutral actually) which it why it is used for wood and wood based products.  Can be cracked by alkali to form polyvinyl alcohol and acetic acid.  Also a poor conductor which is why it is used in manufacture and repair of high voltage electrical equipment.  Will become more conductive when ionic materials are mixed in or diffuse into the polymer (reason for my two problems with PVA/ballast).

Ballast is usually some type of mineral and its behaviour will depend on composition.
« Last Edit: May 23, 2020, 11:31:45 PM by NinOz »
To be called pompous and arrogant - hell of a come down.
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Re: conductive ballast - Aaaghhh
« Reply #11 on: Yesterday at 12:15:30 AM »
I must say I find this most interesting. I'm being genuine here, not sarcastic!  :thumbsup:

Thanks for posting!  :beers:

Just goes to show there's never a dull moment in the world of model railways!
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Re: conductive ballast - Aaaghhh
« Reply #12 on: Yesterday at 12:20:29 AM »
I suspect it is real cinders and as someone suggested, high in carbon.
cinders = low in carbon, high in metals and very alkaline
charcoal = low in metals, high in carbon

PVA (poly vinyl acetate) is not acidic (fairly neutral actually) which it why it is used for wood and wood based products.  Can be cracked by alkali to form polyvinyl alcohol and acetic acid.  Also a poor conductor which is why it is used in manufacture and repair of high voltage electrical equipment.  Will become more conductive when ionic materials are mixed in or diffuse into the polymer (reason for my two problems with PVA/ballast).

Ballast is usually some type of mineral and its behaviour will depend on composition.

The stuff I use shows red on litmus so it cannot be pure PVA.  Itís an outdoor waterproof variety and I have to watch the acidity as it affects the colour of printed inks, why I checked in the first place.

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Re: conductive ballast - Aaaghhh
« Reply #13 on: Yesterday at 12:39:45 AM »
I just looked up a chemical risk register and it gives the following.

Polyvinyl acetate

Risks
Combustible. pH of emulsions is usually acidic. PVAC resins release small amounts of acetic acid on curing and ageing.

So factory safety would require it to be treated as acidic.

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Re: conductive ballast - Aaaghhh
« Reply #14 on: Yesterday at 01:58:45 AM »
I must say I find this most interesting. I'm being genuine here, not sarcastic!  :thumbsup:

Thanks for posting!  :beers:

Just goes to show there's never a dull moment in the world of model railways!

Well there's a first!  :laughabovepost:

I agree though. I haven't done any ballasting yet and at the rate I work I won't be doing any until about the year 2035. I think I'll avoid cinders though no matter what she's doing at the time. I'll also test my ballast before laying it  :thumbsup:
John

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