Electrical-leak testing could reduce Metro meltdowns

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Electrical-leak testing could reduce Metro meltdowns

Postby davinp » Mon May 08, 2017 8:30 am

Metro has 40 year old rail fastener and copper cables that are worn and need to be replaced. This causes the electrical arching smoking problems.

A preventive maintenance program starting in July that will allow workers to begin testing for electrical leaks along the system’s power cables and tracks. But there’s one problem: The testing process is slow, cumbersome and time-intensive — and it will probably be a while before riders start noticing results.

https://www.washingtonpost.com/local/tr ... 64fce147ef
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby JackRussell » Mon May 08, 2017 9:51 am

davinp wrote:Metro has 40 year old rail fastener and copper cables that are worn and need to be replaced. This causes the electrical arching smoking problems.

A preventive maintenance program starting in July that will allow workers to begin testing for electrical leaks along the system’s power cables and tracks. But there’s one problem: The testing process is slow, cumbersome and time-intensive — and it will probably be a while before riders start noticing results.


I saw that, but it immediately raised the question as to when it simply makes more sense to not waste time testing, and simply replace all of the insulating gaskets along a section of rail.

And if the gaskets are truly rubber (you always have to wonder whether the media dumbed it down by using the word "rubber"), ought they not consider a gasket made out of a different material that has a longer lifespan?

It seems that there were a number of PM programs that were discontinued some years back - I am wondering how many others there are that we don't know about yet.
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby Sand Box John » Mon May 08, 2017 10:02 pm

The term "gaskets" is not an apt description.

The anatomy of that track fastener is two steel plates sandwiching a block of neoprene. The bottom plate is secured with at cap stud epoxied into a hole drilled into the grout pads (contemporary track fastener design) or a threaded stud epoxied into a hole drilled into the grout pads with a nuts and lock washers securing the bottom plate to the grout pad (1970 vintage track fastener design), The rail is secured the the top plate with pair of pandrol clips (contemporary) or a pair clips secured to T studs with a nuts and lock washers (1970 vintage).

In WMATA's application, the track fastener preforms three functions:

  • Holds the rail in gauge.
  • Absorbed vibration.
  • And electrically isolates the negative traction current return from going to ground through the concrete track bed.

The stray currents are not the result of a defective track fastener, they are cause by the accumulation of conductive material on the track fastener creating a path for current to flow between the top plate and the bottom plate and into the concrete track bed.
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby MCL1981 » Mon May 08, 2017 11:11 pm

What problems are caused by the running rails grounding electrically to earth?
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby Sand Box John » Tue May 09, 2017 6:40 am

"MCL1981"
What problems are caused by the running rails grounding electrically to earth?


The question should be; What problems are caused by the running rails grounding electrically to earth along the incorrect path?

Accelerated corrosion of near by ferrous and non ferrous metals, rebar reinforcement in concrete, segmented steel tunnel linings, steel shoring in rock and soil bored tunnels. . .
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby farecard » Thu May 11, 2017 9:02 am

Sand Box John wrote:"MCL1981"
What problems are caused by the running rails grounding electrically to earth?


The question should be; What problems are caused by the running rails grounding electrically to earth along the incorrect path?

Accelerated corrosion of near by ferrous and non ferrous metals, rebar reinforcement in concrete, segmented steel tunnel linings, steel shoring in rock and soil bored tunnels. . .


And I suspect, more noise afflicting the signaling systems.

I think WMATA needs a battalion of workers with pressure washers; cleaning the insulators, the cable, the above joints, everything.....
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby Sand Box John » Thu May 11, 2017 1:08 pm

"farecard"
And I suspect, more noise afflicting the signaling systems.


The new hardware is significantly less fragile then the legacy hardware it replaced.

I think WMATA needs a battalion of workers with pressure washers; cleaning the insulators, the cable, the above joints, everything.....

Everything in the tunnels including the tunnel need to be cleaned because what was not cleaned will eventually soil what was cleaned.
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby dcmike » Fri May 12, 2017 6:02 pm

To add to what SBJ already said, when return current travels through fasteners and other unintended paths it significantly accelerates corrosion.

And while contemporary signalling hardware is decidedly less fragile, it's also far more sensitive. If there are return current leakage paths along the running rail, the AF cab signals are going to find their through in as well. This is a particularly thorny issue to troubleshoot because in the absence of a train the track circuit may appear to operating normally. But with a train in the area dumping return current through the rails the electrical characteristics become less predictable.
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby Sand Box John » Sat May 13, 2017 10:00 am

"dcmike"
And while contemporary signalling hardware is decidedly less fragile, it's also far more sensitive. If there are return current leakage paths along the running rail, the AF cab signals are going to find their through in as well. This is a particularly thorny issue to troubleshoot because in the absence of a train the track circuit may appear to operating normally. But with a train in the area dumping return current through the rails the electrical characteristics become less predictable.


That's an angle that would have never occurred to me.
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Re: Electrical-leak testing could reduce Metro meltdowns

Postby farecard » Sat May 13, 2017 12:00 pm

It's rather hard to hunt for a few millivolts of intelligence under 750v/thousands of amps of brute force.

Corrugated rails are another factor adding to the noise; ditto third rail shoes.

When you see sparking, think listening to a whisper while standing on the aircraft carrier deck during a catapult launch....
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