What actually is a 'Signal Failure'?

On the operating side, there are basically 7 reasons for a signal "remaining at danger":

For signals that work automatically (ie, no intervention from a signaller):

1 - train in section
2 - metal obstruction on the track
3 - broken rail
4 - equipment failure
5 - flooding

For signals that work semi-automatically (ie, usually operated by a signaller):

Same as 1- 5 above, plus:
6 - lever in the normal position (signal correctly showing a red aspect)
7 - lever in the reverse position (signal had been cleared to green and has since correctly returned to danger (red) following the passage of a train - the lever has to be replaced to normal then reversed again to clear the signal)

As you can probably tell, reasons 2 - 5 are the ones that will be treated as a failure. Most are self explanatory, but "equipment failure" is the one that can cover pretty much any single element of the whole signalling system. It could be a thin piece of metal joining two track circuits together (bits of plastic are used in track joints to separate the signalling track circuits), a blown fuse, a blown bulb in the signal head itself, a train stop failure, a part of the electronic or mechanical interlocking failed, a cable fault, computer (or other type of automatic routing equipment) fault, power failure - quite literally anything associated with the signalling system.

The one thing you never hear of is a points failure. The simple reason for that is that a points failure will inevitably mean that a signal cannot 'read over' the problem points and thus as a consequence cannot be cleared; so ultimately the problem becomes a signal failure and is thus covered by equipment failure.

We have a number of rules & procedures that have to be followed to the letter when passing a signal remaining at danger. It all depends on the location, time (peak or off peak), day of the week, signal type and exact reason for failure as to whether it can be easily managed with a fairly decent frequency of service or whether it'll crucify the service or even cause a full suspension. It really does depend on a number of factors unique to each failure scenario.

We've had this come up before, and I'm sure it's been said quite recently, though putting links in this instance isn't necessary...

The only time the phrase "signalling problem" is used is where there has a been a suspected SPAD, or other suspected human error. In all other cases, including any type of equipment failure, the correct phrase is "signal failure". It's a subtle difference to those not in the know.

I can perhaps understand why the likes of those of you who maintain the signalling system get the hump when, in your view, you see the signalling system being blamed (ie, "signalling problem" is being put out) - but the bottom line is that a delay is occurring and it is ultimately because of the signalling system not allowing trains to proceed normally.

I thought it was generally regarded that signal failures have reduced over time.

So when the Victoria / Jubilee line are having their signals replaced does this actually mean that antiquated old copper wires are being replaced by sophisticated, elegant, Ursula Andress in Dr. No cables which will never short circuit or blow?