Overhead Lines Which Fall

[Lukespenceruk1]

A random question and sorry if it sounds Silly.

But I was at Leyton Midland Road station Yesterday which has just had overhead lines installed along the Barking - Gospel Oak route ready for when the Class 710's are introduced.

I was wondering is there a sort of safety feature built into them should the overhead cables fall down from damaged, Bad weather, Vandalism etc..

Is there a safety feature that cuts out the power supply so if this happened at a station, Passengers wouldn't be in danger from electrocution.?

 

[TheEdge]

Circuit breaker, as soon as they short, break etc in theory there are circuit breakers that trip the sections out

 

[TwistedMentat]

Circuit breaker, as soon as they short, break etc in theory there are circuit breakers that trip the sections out

That ^

I would imagine they share a lot of the tech used in overhead power cables for that sort of thing. Detect short and either fuses or breakers will go.

Though I believe with overhead systems you can do setups that will send a test pulse after a period of time to see if it was a false trip. Not enough to be dangerous but enough to do detection. A second trip then kepping it off until someone manually turns it back on.

 

[Highlandspring]

For OLE at least (don’t know anything about third rail stuff) the ECRO will try to reset a tripped breaker up to three times with a minute in between each. If it won’t go live after three attempts then it stays off and staff will be called out to investigate. There is a maximum amount of time allowed for the protection relays and breakers to operate on a short circuit or overcurrent, in milliseconds, and they’re tested regularly to prove they will work when required. The ECR uses a tool called Optimho which measures the distance along a circuit to a trip (I think based on the time it takes for the breakers at either end to come out) which gives a rough idea of where the problem is located.

 

[swt_passenger]

They do attempt to re-close the supply breaker in response to a trip on OHLE. It happened at St Pancras while a broken cable was hanging down onto the platform, there’s a RAIB report about it.
https://www.gov.uk/raib-reports/overhead-line-failure-at-st-pancras-international

On 23 September 2009, at approximately 18:35 hrs, shortly after the arrival of the 17:13 hrs train from Paris Nord at platform 9 of St Pancras International station, London, the overhead wire supplying electric power to the train, which was live at 25kV, parted and fell to the ground. This occurred during the preparation of the train for its next journey, while many passengers who had left the train were walking along the platform.

The immediate cause was the parting of the overhead wire because of local overheating caused by the high electric current flowing from the wire to one of the train’s pantographs.

Correct me if I’m unaware of some specific monitoring device, but how do they detect an open circuit that doesn’t cause a short to earth?

 

[Highlandspring]

The protection relays will detect an excess current and bring the breakers out. This can happen when there are an unusually high number of trains attempting to draw power in the same section at once, for example on weekday early mornings on the West Coast Main Line in the section between Law Jn and Carstairs there are two liners booked to pass each other, both worked by pairs of class 90s that are renowned for bringing the whole of Gowkthrapple feeder station out, grid feeder and all!

 

[philthetube]

I know there is an issue with residual current and that wires should not be touched even if the juice is known to be off so a wire should never be assumed to be safe, perhaps someone could explain further?

 

[snowball]

I know there is an issue with residual current and that wires should not be touched even if the juice is known to be off so a wire should never be assumed to be safe, perhaps someone could explain further?

Current in one wire can induce a current in another wire, especially if the two are parallel over a long distance.

 

[The Lad]

Capacitive charge?

 

[edwin_m]

More of an inductive effect, a bit like a very large and very inefficient single-coil transformer.

 

[Elecman]

They do attempt to re-close the supply breaker in response to a trip on OHLE. It happened at St Pancras while a broken cable was hanging down onto the platform, there’s a RAIB report about it.

Correct me if I’m unaware of some specific monitoring device, but how do they detect an open circuit that doesn’t cause a short to earth?

They can’t, that was the cause of the Virgin driver getting the severe electric shock at Weaver th3 Protection only operates on Short Circuit to earth or overload

 

[apk55]

When working on overhead line (railways or otherwise such as power lines) it is normal safety procedure to short the line to earth via a visible link. Not only does this provide protection against re-application of power but discharges the line from any induced voltages either from adjacent lines or even atmospheric current flow (which could be significant in some cases) or lightening strike.

 

[Lukespenceruk1]

So I take it there has not been incidents of wires falling onto station platforms and putting passengers in danger?

 

[yorkie]

So I take it there has not been incidents of wires falling onto station platforms and putting passengers in danger?

See post above:

They do attempt to re-close the supply breaker in response to a trip on OHLE. It happened at St Pancras while a broken cable was hanging down onto the platform, there’s a RAIB report about it.

 

[Elecman]

Yes there has as linked above At St Pancras