Third Rail Section Gaps

[Urban-Savage]

Apols if this has been asked.
travelling a lot on Merseyrail I see a fair few signs that say ‘section gap’
I assume they simply mean a gap in the third rail supply and therefore a different source of power for the units
my question is…
are there any issues with the trains shoes very briefly drawing power from two arguably separate supplies and how easy / common is it for trains to be ‘gapped’
cheers

 

[Gloster]

Apols if this has been asked.
travelling a lot on Merseyrail I see a fair few signs that say ‘section gap’
I assume they simply mean a gap in the third rail supply and therefore a different source of power for the units
my question is…
are there any issues with the trains shoes very briefly drawing power from two arguably separate supplies and how easy / common is it for trains to be ‘gapped’
cheers

As far as the second point is concerned, when track layouts are altered they should be designed so that no train will be gapped. This will be taken into account when planning and if it is found that there is a location where it would be possible for a train to be gapped, they will just have to adjust the layout. Planning for the third rail is an integral part of the process. If you get new trains, they will have the same shoe layout or you will need to come up with a very good explanation.

 

[Snow1964]

Historically trains in tube style tunnels didn't have their pick up shoes buslined (all linked together by power cable on the train). This dates back to higher fire risk on the cables from when insulation materials were different. The same is true of tube stock in London

Because they were not linked it is possible for shoes in that car to be gapped, so motors on that car won't work as only gets power from shoes on that vehicle.

Not really a problem these days when trains are 3 or 6 cars, as bound to be some part of it on the power rail. But in years gone by, there were some short formations, eg motor and driving trailer used off peak, and a single motor car could get stranded.

The other reason for marking them, is similar to the neutral sections on overhead. If a train is accelerating hard and passes through it on full power, tend to get jerky (snatching) ride. So better to ease off through the sections. I am guessing a train on heavy regenerative braking would also get a snatch effect passing a dead section.

 

[Railsigns]

The other reason for marking them, is similar to the neutral sections on overhead. If a train is accelerating hard and passes through it on full power, tend to get jerky (snatching) ride. So better to ease off through the sections.

Unlike the neutral section signs on AC overhead electrified lines, the section gap signs on DC third rail electrified lines (known as "isolation demarcation boards") don't have any relevance to train drivers.