Resistance between tracks.

[AndrueC]

Can I have a sanity check, please?  I've spent the last week painting track but this morning I fitted a new turnout controller. During the process I noticed that the resistance track to track is not 'infinite' with the DCC power off. I could have sworn it used to be. My breaker isn't tripping with power on though so it would appear that all is good for practical purposes.

 

My MM is now reporting '1.445...' in continuity mode and in resistance mode appears to be measuring around .5MΩ.

 

I was about to start worrying but then realised that I have 10 locos on the track along with their (unlit) rolling stock. So am I correct in thinking that I'm just measuring the locos' electronics and/or motors?

[Free At Last]

Measuring mine a while back I found I had 13kΩ between the track. Finding no obvious cause of shorts I have put it down to the graphite I use on the track, bridging over the insulated fishplates.
 

[WIMorrison]

1 hour ago, AndrueC said:

I was about to start worrying but then realised that I have 10 locos on the track along with their (unlit) rolling stock. So am I correct in thinking that I'm just measuring the locos' electronics and/or motors?

 

Yes, you are simply measuring the parallel resistance of all the items you have connected across the track bus. 

 

Assuming a track voltage of around 15v you are drawing around 0.03mA, which is absolutely nothing to worry about.

[kevinlms]

What CAN cause serious problems, is if you try ballasting with the traditional PVA and water sprayed on. Until it dries out throughly the insulation resistance can be rather low!

 

How did I find this out? Years ago on a DC layout with Twin T block detectors to detect occupancy and to turn signals red.

Couldn't figure out why all the signals were suddenly stuck on red. Took a while to remember that some patchy ballast had been redone last week. You could still run trains, so the resistance was high enough to not prevent that, but low enough to turn signals red. Just as well the signals weren't interlocked to track power. After another week, it had mostly dried out enough. This was in a clubroom, under several metres of concrete, so very even temperature.

 

Moral of the story DO NOT ballast the track, shortly before an exhibition, if you are using current detection for any purpose at all!

[RFS]

46 minutes ago, kevinlms said:

What CAN cause serious problems, is if you try ballasting with the traditional PVA and water sprayed on. Until it dries out throughly the insulation resistance can be rather low!

 

How did I find this out? Years ago on a DC layout with Twin T block detectors to detect occupancy and to turn signals red.

Couldn't figure out why all the signals were suddenly stuck on red. Took a while to remember that some patchy ballast had been redone last week. You could still run trains, so the resistance was high enough to not prevent that, but low enough to turn signals red. Just as well the signals weren't interlocked to track power. After another week, it had mostly dried out enough. This was in a clubroom, under several metres of concrete, so very even temperature.

 

Moral of the story DO NOT ballast the track, shortly before an exhibition, if you are using current detection for any purpose at all!

 

I can vouch for that too.  Having ballasted the track in that way, it was a couple of days before the occupancy detectors switched off again....

[GoingUnderground]

I'm not sure that I'd want to try measuring the resistance on a DCC layout even if the power is off if the controller is still connected to the layout. To measure the resistance you need a power source IIRC, which will be the battery inside the multimeter as resistance is calculated by dividing the voltage by the current flowing. AFAIK this still applies even to modern digitial multimeters. So I'd disconnect the DCC controller from the layout first to eliminate any risk of damaging it by exposing it to the power source in the multimeter. I know that on powering up most, if not all, DCC controllers isolate the output terminals from the internal electronics until the system has stabilised, but I still wouldn't want to risk it.

 

The resistance will only be infinite if there is absolutely nothing connected to the DCC bus or to the rails such as accessory decoders or most reversing loop modules, and nothing that takes power in anything sitting on the rails such as loco or interior lighting decoders or interior lighting taking its power straight from the rails or resistors between coach or wagon wheels to trigger occupancy detectors. Moisture from the glue in in newly laid ballast can also bridge the insulation between the rails as others have already said. 

[AndrueC]

I looked up the manual for the meter (Rapitest DM25) online and it turns out that what I'd thought of as 'continuity mode' doubles up as 'diode test' and the 1.44... reading is therefore what it sees as the 'forward voltage' although irrelevant here. Apparently it considers anything under 70Ω to be a short which is good to know.

Edited December 8, 2023 by AndrueC

[34theletterbetweenB&D]

1 hour ago, kevinlms said:

What CAN cause serious problems, is if you try ballasting with the traditional PVA and water sprayed on. Until it dries out throughly the insulation resistance can be rather low!

Shortly after adoption of DCC I ballasted five yards of twin track by this method. The DCC system operated normally on other sections of the layout where trains were running and it was only at the end of the evening's operation that I noticed the haze above the newly ballasted section, and condensation on nearby metal frame structure. (Much of that layout's multilevel frame was made from aluminium extrusion salvaged from secondary double glazing from our own and new neighbour's recently purchased homes.)

[Dagworth]

1 hour ago, kevinlms said:

What CAN cause serious problems, is if you try ballasting with the traditional PVA and water sprayed on. Until it dries out throughly the insulation resistance can be rather low!

 

How did I find this out? Years ago on a DC layout with Twin T block detectors to detect occupancy and to turn signals red.

Couldn't figure out why all the signals were suddenly stuck on red. Took a while to remember that some patchy ballast had been redone last week. You could still run trains, so the resistance was high enough to not prevent that, but low enough to turn signals red. Just as well the signals weren't interlocked to track power. After another week, it had mostly dried out enough. This was in a clubroom, under several metres of concrete, so very even temperature.

 

Moral of the story DO NOT ballast the track, shortly before an exhibition, if you are using current detection for any purpose at all!

Yep, I know that one very well, I ballasted a joint that had never been completed about a week before Dagworth’s first show. As per all my large layouts the signalling is fully interlocked with track circuits. When we got to the show the constant occupancy of that circuit meant I was unable to change points at one end of the station. I can’t remember exactly how I solved it but it involved bypassing that particular track circuit until it dried fully. 
 

Andi