650v trackside power supplies

[HSTEd]

Rather esoteric question for the S&T types amongst you.

On various cabinets on the trackside, seen from trains and from platforms, you see a lot of references to 400v power supplies (as one might expect) but also 650v power supplies.

I have seen them outside third rail areas so I have to assume that this is a signalling related thing.
What are the characteristics of such a supply? Why are they used? Are they simply produced from whatever power supplies might be convenient for the railway (230v/400v supplies at stations or other convenient points for example) or is there some greater distribution system?

Anyway, thanks for any replies on this rather obscure topics.

 

[swt_passenger]

It's basically done to deal with volt drop at longer distances from the supply. There was a short thread about it a couple of years back:

http://www.railforums.co.uk/showthread.php?p=1362119

One of the posters therein reckoned 650 Volts was historically some sort of maximum value before additional safety rules applied.

 

[HSTEd]

Wow - I am actually one of the posters at the end of that thread.
That is slightly worrying.

1000v is the very top of the definition of low voltage I believe, but I do believe this has changed recently.

EDIT:
Found the regulations - it is 1000v between conductors, and 600v between conductor and earth.
So I assume that these 650v supplies are considered isolated to step around that restriction, or that they are operated as +/- 325V or that the railway has some sort of grandfather permission.

 

[lincolnshire]

Wow - I am actually one of the posters at the end of that thread.
That is slightly worrying.

1000v is the very top of the definition of low voltage I believe, but I do believe this has changed

EDIT:
Found the regulations - it is 1000v between conductors, and 600v between conductor and earth.
So I assume that these 650v supplies are considered isolated to step around that restriction, or that they are operated as +/- 325V or that the railway has some sort of grandfather permission.

Thinking about it, the upper limit for voltage at one time if I remember was 700v above that it became High Voltage which required permit to work system under the Electric Supply Regulations which has now become 1000 volts above that Permit to Work Area. Some of the 650v supplies go for very long distances local to me one is about 10 miles one direction from the REC supply point.
When the signalling was installed it needed balancing up as the supply voltage had to be above the 650 volts so that the far end was within the limit required, so the en-route step up transformers needed adjustment to up the voltage for the far end and to be able to alter the transformer settings at the source back down to 650v so the system was within the voltage limits and didn,t become High Voltage 700v.
To get that distance you need transformers to step the voltage back up again en-route and the transformers in the line side locations set to give the correct secondary voltage required 110v for the local equipment.

The usual set up is a 400v x 2 phase primary input and 650v single phase secondary output. Also the usual set up is a 650v single phase standby generator to back up the REC supply in case of power failure.

If it was High Voltage then you would require staff trained for H.V. and all the permit to work/ sanction to test etc paperwork and the physical separation of equipment and voltages within location cabinets and relay rooms so that only staff passed out and certified can work on the equipment etc.

 

[MarkyT]

So I assume that these 650v supplies are considered isolated to step around that restriction, or that they are operated as +/- 325V or that the railway has some sort of grandfather permission.

Signalling has a number of grandfather permissions and similar clauses. For instance, it is explicitly excluded from the scope of the IET wiring regulations along with traction and rolling stock equipment. Some modern schemes have a more standard industrial style 3-phase distribution system running at 415V, but the traditional signalling supply was always 650V single phase with both poles floating. Groups of trackside equipment cabinets derive local 110VAC supplies from step-down transformers. Signalling supply points usually have two separate utility supply sources and either a generator back up in non-electrified areas, or an alternative backup feed derived from traction power supplies where provided. UPS systems are now common for processor-based systems, where re-boot times can be significant and there are risks of not being able to restart at all. Relay-based systems are more tolerant and restart quickly and reliably after a short blackout whilst the generator starts.

 

[infobleep]

Fascinating replies. The area around Wimbledon on the South West Mainline is 650v. Is that the most common voltage for signalling on the railways, excluding the trackside equipment which is stepped down?

 

[Joseph_Locke]

Fascinating replies. The area around Wimbledon on the South West Mainline is 650v. Is that the most common voltage for signalling on the railways, excluding the trackside equipment which is stepped down?

Yes it is

 

[MarkyT]

. . . 650v. Is that the most common voltage for signalling on the railways

Yes it is

In the beginning, there was no power in signalling. Block telegraphs were the first application, for which primary cells were used. When the first shaky utility supplies appeared, they were used to charge low voltage secondary batteries for backup, powering the rapidly increasing number of electrical devices being used: track circuits, signal indicators, electric locks, electric signals, point machines, etc. Once better utility supplies were available with reliable diesel generators for backup, signals gradually migrated to 110VAC and a general conversion to point machines became more practical. In both cases with the higher voltage the items could be further away from their power and control cabinets and the signal box. The era of "power signalling" and panel signal boxes during the 1960s and 70s dramatically increased the demand load over even wider extents and the 650VAC single phase system quickly became the standard trackside distribution solution, a practical compromise between safety standards, cable costs and resistance losses.
--- old post above --- --- new post below ---
Yes batteries, batteries everywhere. Store rooms and lockers, vans full of batteries, old batteries lying by the track, slung down embankments, under bushes with remnants of porn mags. The rural mechanically signalled railway, mainline and branch, was powered by batteries, dry or wet primary cells of various types for particular applications that had to be changed regularly. That was certainly the case for GW lines in Devon until the mid-1980s when Exeter resignalling consigned the last of the old boxes to history. With the power supply so limited, to be practical in the field it was important for early electrical signalling devices to be extremely efficient and use as little current as possible. When bigger, more reliable supplies became available the efficiency imperative lost its primacy through the powerbox era. The latest modular signalling techniques revert to that lean, low voltage, distributed trackside architecture to an extent, with efficient LED signals and axle counters instead of track circuits, although control clearly is very much more centralised today.

 

[HSTEd]

Would the return to efficiency not be driven by the desire to reduce the amount of copper [I imagine its mostly aluminium for power distribution now?] on trackside and thus reduce the impetus for theft, rather than to conserve energy?
After all, electricity is still rather cheap in terms of the costs of operating the railway.

 

[MarkyT]

Would the return to efficiency not be driven by the desire to reduce the amount of copper [I imagine its mostly aluminium for power distribution now?] on trackside and thus reduce the impetus for theft, rather than to conserve energy?
After all, electricity is still rather cheap in terms of the costs of operating the railway.

Yes. Supply cable is usually aluminium today and even with a dedicated feeder along the trackside, going lean with the equipment saves the size and cost of its conductors. Modern signals and axle counters use next to nothing and clusters of point machines can use a 'point battery' to avoid excessive transient voltage drop on motoring, a very traditional solution frowned on a little in the macho power signalling days, although to be fair a big lead acid bank did require a lot more maintenance in the 1960s than a modern battery.