Signal alignment, gantry access & OHLE

[eMeS]

Last week whilst filming, I noted that a signal gantry on the WCML (near Old Linslade church) had protected areas for workers' access, and this got me wondering how the signal beams were aligned along the track, and whether the signal lamp itself could be raised for repair and adjustment, or whether adjustment etc. was done at OHLE level. And whether the 25KV was ON or OFF?

 

[John Webb]

In the older filament bulb signal heads, the bulbs are carefully made to place the filament at the same place, so that lampholders can be aligned in the factory and sealed into a set position, eliminating the need for on-site adjustments of the individual beams in a signal head. The heads also incorporate cast-in sighting holes to allow alignment of the whole head on site.
So there should be no need for a head to be moved once erected other than for very minor adjustments, which are built into the head and its mounting foot. Only if the whole head is being replaced will it need removal - that would require a possession and the OHLE would, of course, be turned off.
Bulbs were replaced at regular intervals to minimise the potential for failure, even though the bulbs were usually made with two filaments, main and standby. The bulbs would be changed by the S&T people using the protected access on the signal gantry and accessing the head through the doors on the rear of the signal. I believe the lens fronts would be given a clean at the same time. The OHLE would remain on and trains run normally during such work.

Modern LED heads are also set up in the factory as part of their manufacture, and again, once on site, should need no adjustment once the initial set-up has been done. There are no bulbs to replace regularly either. Some LED signals are now mounted on folding posts so they can be cleaned and otherwise maintained at ground level, eliminating the need for ladders, protective cages and possibly gantries.

 

[snowball]

The steel mesh cages make it perfectly safe to work on the signals with the traction current on. They are earthed and work as Faraday cages - as well as preventing anyone from accidentally reaching out towards the power cables.

A more extreme example - not sure how often it's done in this country but it certainly happens a good deal in the USA - is that people can work close to live power transmission cables, at higher voltages than are used on the railways, wearing wire mesh suits. As I understand it, such suits are not earthed and can come into contact with the live wire, and ensure that the potential difference across the wearer's body is close to zero.

 

[eMeS]

Thank you for your replies.
Some years ago I had to advise on HV electrical safety in industry, and I wondered whether the same guidance applied "out in the open" where weather etc. could have a serious effect.

 

[Railsigns]

The steel mesh cages make it perfectly safe to work on the signals with the traction current on. They are earthed and work as Faraday cages - as well as preventing anyone from accidentally reaching out towards the power cables.

I very much doubt whether the mesh screens on signals act as a Faraday cage. For one thing, it doesn't enclose anything so, by definition, it's not a cage. As far as I can see, it's no more than a physical barrier.

 

[SpacePhoenix]

A more extreme example - not sure how often it's done in this country but it certainly happens a good deal in the USA - is that people can work close to live power transmission cables, at higher voltages than are used on the railways, wearing wire mesh suits. As I understand it, such suits are not earthed and can come into contact with the live wire, and ensure that the potential difference across the wearer's body is close to zero.

Saw that on tv once (on Nat Geo I think), they use a probe type thing (long pole) to make contact with the live cable so that there's no PD between the helicopter and the cable. There's probably a clip on youtube somewhere

 

[The Snap]

The steel mesh cages make it perfectly safe to work on the signals with the traction current on. They are earthed and work as Faraday cages - as well as preventing anyone from accidentally reaching out towards the power cables.

I very much doubt whether the mesh screens on signals act as a Faraday cage. For one thing, it doesn't enclose anything so, by definition, it's not a cage. As far as I can see, it's no more than a physical barrier.

Touch-potential fence. These days, quite often they are made from GRP to prevent them requiring earthing/bonding, and of course becoming live in the case of arcing or OLE failure.

As for maintenance, as someone has commented, new lightweight folding signal posts are starting to be used, particularly for banner repeaters. For the eagle-eyed among you, we've just installed two on the Down Slow on the WCML between Stafford and Crewe...

 

[edwin_m]

Don't the signal gantries installed for the Trent Valley widening allow the entire signal head to be winched up to the working platform for maintenance access? I presume anything of this type (including hinging down a hinged signal or simply changing a bulb) would require as a minimum trains to be stopped on the line the signal applies to.

I've seen a photo somewhere (probably on one of the recent modular signaling routes) of a gantry just consisting of a single girder frame with no ladder or platform arrangements whatever. I'm not sure if this hinges down somehow (which would block all the track underneath it) or a RRV platform vehicle is needed to access the signals - but either way clearly based on the assumption that access will be very infrequent. However there must be quite a bit of saving of metal, because the access measures increase the weight of the structure a bit and the wind loading quite a lot, requiring more structure to bear the load which increases the weight of the structure a lot more.

 

[mr_towers]

Stirling's new gantry at the south end sounds like this. They're all lightweight LED signals so some sort of external access equipment will be required for cleaning. Either that or a big stick!

 

[Tio Terry]

The sooner the Industry moves to ERTMS/ETCS the better. No more lights on sticks and all the problems that go with them!

 

[Railsigns]

Stirling's new gantry at the south end sounds like this. They're all lightweight LED signals so some sort of external access equipment will be required for cleaning.

It's a lightweight structure, although the signals themselves are not of the lightweight type.

 

[The Snap]

Don't the signal gantries installed for the Trent Valley widening allow the entire signal head to be winched up to the working platform for maintenance access? I presume anything of this type (including hinging down a hinged signal or simply changing a bulb) would require as a minimum trains to be stopped on the line the signal applies to.

Maintenance on the folding lightweight signal posts would most likely be done under possession or line blockage as a minimum I'd imagine. The ones we've installed are repeaters, so by their very nature if you drop one down, the following signal is not protected by an operational repeater and therefore not safe for the passage of trains. Don't hold me to that though, as I'm civils not signalling.

I've seen a photo somewhere (probably on one of the recent modular signaling routes) of a gantry just consisting of a single girder frame with no ladder or platform arrangements whatever. I'm not sure if this hinges down somehow (which would block all the track underneath it) or a RRV platform vehicle is needed to access the signals - but either way clearly based on the assumption that access will be very infrequent. However there must be quite a bit of saving of metal, because the access measures increase the weight of the structure a bit and the wind loading quite a lot, requiring more structure to bear the load which increases the weight of the structure a lot more.

As far as I'm aware, the key selling point for the folding lightweight structures is to remove the need for working at height, a high-risk activity. These are, however only single sign posts, not gantries.
The new gantry we installed on the WCML has full ladder and platform access; again, this removes the need for working at height from a MEWP.