650V POWER

[Archie]

I have been installing a lot of power cables into LOCs of late, 70mm, 50mm, 25mm, can some please explain the relevance for the different sizes.

Has it got anything to do with distance from the feed itself related to resistance?

I am not sure so any help would be greatly appreciated.

Many thanks

[PJW]

Yes very likely.

Large cable diameter is likely to be primarily for low resistance to minimise volt drop from end to end, rather than for current carrying capacity. Rarely are the currents which flow in signalling circuits close to the nominal rating of the cable. Even things like point machines which can draw say 15A are doing so or so short a time that the unwanted heating effect within the cabling is not a concern; however the fact that there may only be 100V appearing across the point machine terminals from the 120V battery terminal voltage would be- they would be slow to operate and may not have enough power to drive points aver when they are heavy, slide chairs not well lubricated etc and hece would be prone to unreliability.

Note that for power distribution, which I suspect some at least of those quoted are, then the need for low resistance is often set not by consideration of the normal expected current but by the current that flows in faut condition. Basically if there is an interconnection or earth fault from one leg to earth at the far end of a long feeder, then it is essential that the feed fuse blows within a short time; this limits the risk due to hazardous voltage for staff. Therefore the feeder needs to have a low enough resistance so that in fault conitions it pulls enough current to blow the feed fuse.

Suppose the feeder normally can be expected to supply 7A, the rating of the feed fuse would need to be set to be say 10A- there needs to be a margin so that it isn't constantly running hot and being close to blowing or thermally aging. Note that the fuse would not immediately blow if 11A is drawn though it- indeed it probably wouldn't blow at all or only after a considerable time. If 15A is drawn then it would blow much more quickly but it may take some time, whereas 20A would ensure that it didn't last long at all and 25A would blow it even quicker. Although there is a little more to fuse design than that, simplistically a fuse consists of a piece of wire which is intended to melt at the right time but not otherwise; hence it is "I squared t" that is relevant- the heating effect multiplied by the duration. There are tables that show how quickly a fuse will blow for a given current and these are consulted to determine the maximum resistance of the feeder can be. Then knowing its length that then gives a figure for the maximum toleraable "ohms per kilometer" and from this the minimum permissible cross sectional area that th cable can be whilst still achieiving the objective of a fuse that does not fail in normal use yet blows quickly enough to ensure that staff are nor expoed to hazardous voltages in fault conditions for any longer than is strictly necessary.
In the real world though copper (or even Aluminium) costs a lot of money and the greater the cable size the more expensive (and indeed the greater probability that some thieves will steal itand cause the railway chaos as a result). Typically, except when feeders are re-configurable to be fed from either end, then a power feeder will tend to be of greatest cable size close to the supply and then step down in size section after section as the end of the feeder is reached. This is because the normal current being supplied by the feeder diminishes as the distance increases; this reflects the distributed nature of the load as only a small percentage of the total load is at the very far end and thus this portion carries little current, whereas the piece closest to the supply point carries the whole total. Therefore for minimising the voltdrop, the best return on investment is to have the biggest cable closest to the supply (as the volt drop is I x R).

So sizing of cable has to consider various factors but fundamentally it is about that trade off between cost and resistance whilst ensuring that adequately low resistance is achieved for those portions where such is needed.

Probably more than you wanted to know, but hopefully useful for someone!

PJW

I have been installing a lot of power cables into LOCs of late, 70mm, 50mm, 25mm, can some please explain the relevance for the different sizes.

Has it got anything to do with distance from the feed itself related to resistance?

I am not sure so any help would be greatly appreciated.

Many thanks

[Peter]

I have been installing a lot of power cables into LOCs of late, 70mm, 50mm, 25mm, can some please explain the relevance for the different sizes.

Has it got anything to do with distance from the feed itself related to resistance?

I am not sure so any help would be greatly appreciated.

Many thanks

PJW's notes cover the main issue which is the fault. Fault conditions do not tend to be well understood by signal desingers. Although about a specific technique for detecting faults, this lecture will also give some useful background information.