From the new dumbed-down Torygraph.
"Although the optical fibres that transport data are extremely thin, the cables have to be reinforced with layers of tubing, steel wires and plastic to prevent damage. The cables have become a target for shark attacks, with sharks possibly drawn to the electromagnetic signals running through them"
Um...
Are you absolutely sure there aren't optical repeaters, in which case currents might flow in the sheath?
I would still expect copper cables within the steel reinforcing to supply the power.
There most definitely are repeaters, cos I worked on the design of one, and yes, sharks do get attracted to the currents
What do you think is incorrect?
Power for the repeaters?
En el artículo , Andy Burns escribió:
A few amps. And the cable's shielded by armour. Several times.
He doesn't know. He doesn't realise just how thick he is.
No wonder he wanted to talk about angle grinders (not that he's ever used one). [1]
Cheers, T i m
[1] Then starts a thread about something else non d-i-y.
Maybe the armour doubles as a conductor?
With the sea as a return? Possibly. It'd save the cost running separate power wires alongside the fibre. That, or run one core and use the armour as the return.
AKAIC the outer armour is 'earth' and an inner copper tube that carries the fibres, or a discrete wire, powers the repeaters in series down the cable.
Sharks in common with many fish have very sensitive electric field sensors. So sensitive they can pick up the electrical activity generated by nerve signals of their prey (eg buried in mud) So, it's very possible.
I assumed the repeaters were powered in series.
I've heard that sharks can detect an AA battery carried by divers, so any stray voltage might well 'interest' them ...
En el artículo , Fredxxx escribió:
So when one goes bang it's time to play the traditional Christmas game of "find the blown bulb?"
:)
Very much so.
Then out go the grapplers to try and snag the cable
You have to remember that sharks can smell your thoughts. :-)
"Sharks are the most electrically sensitive animals known, responding to DC fields as low as 5 nV/cm."
Could you two love birds go have a tiff elsewhere?
If they were in parallel are you suggesting that finding the offending failed repeater would be simpler?
Are you advocating there is trail of lights on the sea bed or something?
This thread and your responses aren't doing you any favours.
;-)
Unfortunately, to have 'a tiff' you don't normally have one person willing to discuss reasonably and the other running away and hiding?
That's just called 'someone throwing their toys out the pram'.
OOI, I didn't notice if you *also* stepped in when Tomlinson gratuitously insults me from behind his cowardly killfile (I apologise if you did).
Cheers, T i m
Using the sea water as a 'Ground return' is a non-starter in this case. ISTR seeing a reference to the power requirements for the TAT undersea cables requiring 7Kv feeds from each end (positive at one end, negative at the other - a grand total of 14KV in all to power the repeaters which AFAICR were spaced at 2Km intervals, extracting some 20 to 30 volts off the line to power the repeaters which had been designed at the GPO (now BT)'s research establishment (Dollis Hill, Martlesham) using the special long life transistors that had also been developed there).
Unless I'm many miles out, I figured a total route length approximating to some 4,800Km, implying a total of 2,400 undersea repeaters. Even assuming 20v per repeater, a simple series arrangement would have demanded a total supply voltage of 48Kv (24KV at each end, way higher than the 7KV figure I recalled). Possibly the 20 or 30 volt drops were shared between 3 repeaters at a time with the section of centre conductor between each trio acting as a common bus bar supply.
In either case, such very long undersea cable routes would require power feeding of the amplifiers (old analogue system) or regenerator/laser amplifiers (today's modern optical fibre systems) where several kilovolts are applied at each end of the power conductors (centre conductor of each co-axial tube in the old analogue FDM days with the outer tube shield/ return acting as the ground return for the DC supply which neatly nulled out any magnetic leakage which might attract the attentions of deep sea sharks) so that each amplifier/regenerator could tap voltage off the constant current supplied from each shore station.
Effectively, each repeater (or group of repeaters)'s power connections are in series with each other rather like the heater strings in the early valved (tubed) TV sets to minimise inefficiency in powering 6 to 12v (or even higher?) 300mA valve heaters off a 120 or 240 volt mains supply in the absence of an expensively screened and weighty mains transformer.
I should imagine that a modern undersea optical fibre cable would use an outer screening conductor as the ground return for the DC power to effect cancellation of magnetic field leakage. Whether this would rely on the outer stainless steel armouring to supplement a copper tube screen ground or simply as a ground return conductor in its own right, I couldn't say but it's odds on that such a co-axial conductor power feeding arrangement would be use whatever the technology of the repeaters simply to eliminate any emanation of magnetic flux from the cable.
Although the use of good quality stainless steel in the armouring of such under sea cables should eliminate distortions in the local magnetic field, it's possible that the shark's electro senses may still be detecting the cable's presence, especially in sections of the route where the cable crosses trenches or depressions in the ocean floor's topology where it will form a catenary suspension above the ocean floor to occupy the space where sharks would be freely swimming and more likely to detect the slightest of such anomalies. Sections of undersea cable lying on the sea floor, perhaps sunk into mud or soft sand, will be largely screened from such electro detection by sharks.
In view of the above, it's perhaps not surprising that the reporter glossed over the complexities of modern undersea cables when describing why sharks might have been attracted to the cable(s) and taking exploratory nibbles out of this potential food source.
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