Checking for shorts

[andruec]

Hi, had a spare hour this evening so decided to check my track for shorts/continuity. What I've noticed is that I don't always get '1' meaning infinite resistance through both rails on my multimeter. If I adjust the range to 20k or higher I start to get none-zero readings. Typically at 20k I get a reading of around 10 which I assume means a 10k resistance.

I do have some glue still drying in a few places but in testing that the resistance appears to be infinite. I'm going to change the batteries on the MM since they've likely been in there many years but in the meantime I'm curious as to whether I should be looking for 0 across the range or if I just need some minimum resistance.

Thanks.

[bnb]

When testing did you disconnect the power supply or unit feeding the track as this will become part of the circuit and give a resistance reading.

[andruec]

That's with no power to the track. I've just investigated further and the meter only reports a high resistance (as opposed to infinite) when the MM probes are put one way round. If they are swapped over it reports infinite. Looking online that implies there is power on the lines. But I've only just started laying this track and the only source of power is a 9v battery that is currently out of the way on a window sill :-/

For what it's worth it appears to relate to two points on the track, shown below.



This is where I've most recently been gluing and it's not all become transparent yet. Although those points are connected the track terminates shortly after both curves.

The non-infinite resistance is measured across the two highlight points or the rails attached to them and changing the turnouts can cause the non-infinite resistance to migrate to other bits of track (if that makes sense).

[Steve M]

In your first post you mentioned glue still drying. Is that on the ballast (if you have done that) or holding the track to the board?
Either way that could be the source of the problem. Let it dry thoroughly for a few days and try again.
Also, not sure how far you have got with laying track on your ambitious plan so post a few pics - someone may spot something that ain’t quite right.

[andruec]

There's no ballast yet, but the glue in a few places hasn't gone transparent (but appears dry).

Here's the work so far:

https://photos.app.goo.gl/nWZ4DeRKBRj2VXDX7

The short appears to be in the outermost section of track. Changing the turnout (the top highlighted one in the JPG) can remove the short from the short straight between the turnouts and make it appear on the rest of the track. That is the section which still has some white PVA visible (you can just about see it in the middle photo). Poking around it with the MM probes did occasionally and briefly show high resistance.

It seems odd that the resistance only shows with the probes in one direction but maybe that just proves that many years ago I bought a cheap MM

I'll see if things settle overnight. Given that it's high resistance rather than no resistance my own feeling is that it's moisture/contamination. Any kind of faulty turnout would be more obvious I think.

[teedoubleudee]

,..........It seems odd that the resistance only shows with the probes in one direction but maybe that just proves that many years ago I bought a cheap MM,.........

The resistance reading should be the same with probes connected either way round unless there is a power source or possibly diodes involved. Could be the internal resistance of the meter, try changing the battery inside the meter or a different meter.

[andruec]

Good news, folks! This morning that section of track reads infinite resistance across the rails. Meanwhile another section I had to lift to sort out a joint is reading high resistance when previously it was infinite. So it's the glue (or presumably the water in the glue).

Latest problem is that two of the turnouts are suddenly not routing power to one of the active lines. They just aren't quite making contact when switched, but I can force contact by pushing them. But they feel fine to my fingers and my test wagon(*) is happy so that problem will go away when I start installing droppers.

(*)A '21 ton' steel hopper. Its light weight means it's proven to be great at detecting issues. Give it a finger push to get it up to a ridiculous speed and it'll soon tell you if track is dodgy