My Insulated Fishplate melted

[DCB]

Checking for a partial short which slowed my trains to a crawl, it was a live frog polarity reed switch which was in opposition to the point blades, I found a melted plastic fishplate.   This is in a garden near Swindon not near the equator.   Any ideas.  

[AndyID]

That's a new one 😀

 

If there was an arc forming across the gap that would do it, but you'd have to be doing something very naughty (high voltage) for that to happen. I'm guessing that due to the short there was enough current flowing through the frog to make it hot enough to melt the plastic fishplate.

[RAF96]

You say out in the garden. Could it have simply gotten wet and shorted.

[Michael Hodgson]

Even if it had melted away completely, the rails either side of it ought not to be in contact with each other, so you should still have effective insulation in the form of an air gap.  So if there is (or has been) a short across it, the rails must have moved closer together.  Swindon may not be on the equator, but rails will still expand a bit when the winter frosts go away.  Garden railways can be more susceptible to problems of rail expansion on warm days, as are some loft layouts.  Rails may move within their plastic chairs or buckle if the temperature changes enough.  I've never felt that nylon fishplates are as strong as I would like but there's not a lot of other choices available. 

[kevinlms]

As soon as I read the problem, of things melting, I knew it had to be David!

[DCB]

6 hours ago, RAF96 said:

You say out in the garden. Could it have simply gotten wet and shorted.

Could be.  The branch is DC but wired for DCC. The switch on the frog to the right was not switching so there was opposite polarity each side of the joiner, The branch is sometimes powered up when trains aren't running but the main layout in the shed is in use, so it could just be the wet.   Thanks.  

 

 

[AndyID]

Rainwater isn't a very good conductor. If it was conductive enough to get that hot it would evaporate quite quickly and if there was more rain falling on it that would cool it down 🙂.  I doubt if those insulated joiners would melt even in boiling water but you could try an experiment to find out.

 

BTW, how many amps can your controllers put into your layout and if there was a dead-short created by the stuck switch, why didn't the controller's overload protection work?

[DCB]

9 hours ago, AndyID said:

BTW, how many amps can your controllers put into your layout and if there was a dead-short created by the stuck switch, why didn't the controller's overload protection work?

Normally its on a Morley Vector which has its slave at the terminus and main unit in the shed, but it can be on an OnTrack I guess both are 12VA, and variable voltage, I guess the cut out is a 1 amp polyfuse.  Only one controller at a time can be connected on this layout. The OnTrack has a Relco     This is not the layout where I run long trains with 4 or 5 Triang Transcontinental locos and power them with 2 or more controllers, That is a floor layout, Triang Super 4 track. Great fun.  Must dig it out before the summer ends.

[AndyID]

12 minutes ago, DCB said:

Normally its on a Morley Vector which has its slave at the terminus and main unit in the shed, but it can be on an OnTrack I guess both are 12VA, and variable voltage, I guess the cut out is a 1 amp polyfuse.  Only one controller at a time can be connected on this layout. The OnTrack has a Relco     This is not the layout where I run long trains with 4 or 5 Triang Transcontinental locos and power them with 2 or more controllers, That is a floor layout, Triang Super 4 track. Great fun.  Must dig it out before the summer ends.

 

Thanks. Looked it up and they say 1.1 amps.

 

We know you had a short caused by the stuck reed switch so it's almost certain that's what caused the joiner to melt. It would be a remarkable coincidence if something else caused it. We also know the short was affecting the frog where the joiner melted.

 

Here's what I think happened: There is some corrosion at the metal joiner on the other leg of the frog. Enough corrosion to produce a several ohms of resistance between the frog and the rail. The problem with the reed was causing all the current to flow through that joiner which was really acting like a resistor because of the corrosion.

 

The Morley can happily pump out 1 amp without tripping and if the resistance at that joiner was say 8 ohms then 8 watts would be dissipated at that joiner (I squared R). Over time that's quite enough power to heat up the frog and melt the plastic joiner on the other leg.

 

At least that's what I think did it 😀

 

 

[Titanius Anglesmith]

13 hours ago, DCB said:

The switch on the frog to the right was not switching so there was opposite polarity each side of the joiner

It’s an insulated joiner. Its only purpose in life (apart from mechanically joining the rails, obviously) is to separate opposite polarities. If it never had opposite polarities across it, there would be no need for it to be insulated. 
 

>>edit- unless it’s the non-insulated joiner on the other rail that you’re talking about? <<
 

I think @Michael Hodgson has it right, a short with a small cross section developed at the insulated joint (perhaps due to rail expansion), and the heat from that short melted the joiner. The Reed switch, being the only current path to the frog, would have to carry that fault current and that’s probably what ruined the switch, not the other way round. 

Edited April 19, 2022 by Titanius Anglesmith

[AndyID]

1 hour ago, Titanius Anglesmith said:

I think @Michael Hodgson has it right, a short with a small cross section developed at the insulated joint (perhaps due to rail expansion), and the heat from that short melted the joiner. The Reed switch, being the only current path to the frog, would have to carry that fault current and that’s probably what ruined the switch, not the other way round. 

 

The reed switch is not the only current path to the frog. Normal Peco electrofrog wiring supplies the frog from the point blades. I believe David added the reed switch to improve reliability on his outdoor layout.

 

The gap at the insulated joiner must be about two millimeters. It's highly unlikely something metallic made contact across that gap but if it did where did it go?

[Titanius Anglesmith]

1 hour ago, AndyID said:

 

The gap at the insulated joiner must be about two millimeters. It's highly unlikely something metallic made contact across that gap but if it did where did it go?

I’ve been on track circuit failures on the real thing where metallic dust has bridged across 12mm of insulated T-piece. It doesn’t take much. If the rails have ever been cleaned with a track rubber or other abrasive, that can drop  dust / filings into the joint. 
 

I took it as a given that if a reed switch was being used then the links on the bottom of the points would have been cut, and the switch rails bonded directly to the stock rails. You’re right of course, they may not have been. 

[DCB]

No the links haven't been cut, the reed was feeding the frog from one rail and the point blade feeding the frog from the other rail when set straight through. The reed is fine, that's the other weird thing, it has worked reliably for ages and suddenly the magnet needs to be 15mm away so as not to trip the reed not the 10mm clearance I allowed it which it always used to need.
 

[AndyID]

6 hours ago, DCB said:

No the links haven't been cut, the reed was feeding the frog from one rail and the point blade feeding the frog from the other rail when set straight through. The reed is fine, that's the other weird thing, it has worked reliably for ages and suddenly the magnet needs to be 15mm away so as not to trip the reed not the 10mm clearance I allowed it which it always used to need.
 

 

That would mean that the current path was from one rail to the point blade then through the frog and the reed switch then back to the other rail. If there is very little resistance through that path the controller would have tripped but if the path has a few ohms resistance the controller will not trip and the place with the highest resistance in the path will heat up. That might be where the blade is hinged and that would still transfer heat into the frog.

 

You might try to reproduce the problem by deliberately setting the reed switch the wrong way and measure all the voltage drops round the path. It should also heat-up at the high resistance "weak spot".

 

BTW, I realize it's a bit late now, but if you are going to polarize the frog with an external switch it's a good idea to isolate the frog completely from the blades and bond them to the stock rails. The reason for that is using the Peco method there are two changeover switches in parallel. That works OK as long as they are properly synchronized but if they are not they can put a dead short across the track. That might be very short duration but it does not do the switch and blade contacts much good. It's not impossible that had something to do with the initial problem with the reed switch.