On 28/07/2014 19:18, The Natural Philosopher wrote:
On 28/07/14 17:11, John Williamson wrote:
On 28/07/2014 14:48, wrote:
Nightjar wrote:
It's losses in the dielectric, so it applies to underground cables
too.
Like the man says, although the problem is significantly greater under
water. 30km is about the limit for AC transmission under water, (...)

Sorry, I still don't understand this. We're talking cables not
capacitors.
How does a 16mm2 PVC insulated cable have a higher resistance if it's
immersed in water than if it's in a vacuum?

The *resistance* doesn't change. When transmitting AC, however, the
*impedance* does change, both due to the capacitance to earth,


so far so good


and, if
the frequency is high enough, "skin effect", which is where the current
flow is restricted to the areas near the surface of the strands also
starts to come into play.


Yep a couple of hundred megahertz is what we transmit power at these days!

It can seriously affect stuff as low as 455kHz, if you remember the Litz
wire we had to use for IF transformers in AM radios back in the day.

The thicker the conductor, the stronger the effect, and it is even
noticeable at 20kHZ, according to this site:-

http://www.belden.com/blog/broadcast...-Frequency.cfm

Not noticeable in the special case we're discussing here, but I did say
"if the frequency is high enough" in the general case.
--
Tciao for Now!

John.