Snaking Direct Buried Cables 1

[gordonl]

What is the typical measure for quantifying the amount of snaking in direct buried cable, and what are some of the values typically used.

Thank You,
Gord

 

[ItAintMe]

we've laid thousands of km of direct buried cables. There's no need for snaking as they are rigidly fixed by the soil. You only need snaking where the cables are not securely held - eg in cable trays or tunnels where thermal effects can cause uneven expansion and damage joints if they are held more rigidly than the cables. We don't snake in ductlines either (the ducts aren't bent!) but we do account for cable movement when locating the joints in jointing pits.

 

[jbartos]

Suggestion: It appears to be prudent to leave some slacks (~5% to 10% depending on the run length) for potential splices for the case of cable damages by material defects, rodents, digging, etc.

 

[SphincterBoy]

To jbartos: Right on! Allowing excess slack in the cable
is wise, especially when the elbows fail, as has been
my experience.

 

[gordonl]

From the point of view of slack, snacking seems useless to me, you would need to dig up a fair bit of trench to recover the slack. Anybody else have experience with this issue?

The reasoning we did so in the past was to allow slack for the expansion and contraction of the cable due to load cycling.

All input greatly appreciated.

 

[ausphil]

Depends if you want to snake to allow greater flexibility of the cable laid, or to eliminate mechanical problems associated with laying cables in roadways. We had a situation where transmission cables layed in the roadway actually pulled out of the joints enough to cause arcing and eventual failure in the joint. The original design (some 30 years ago) was for a certain traffic flow, and as the traffic increases over time, the cable movement increases. The phenomena is known as "waveriding" and occurs with high traffic flow, and usually soft ground conditions. The snaking of the cable is believed to increase the coefficient of friction between the cable and the ground, hence reducing the movement. Studies have been done as to the method of movement with different backfills, different road surfaces etc. You'll find some info in the report as to failure of cables in New Zealand in the PDF file at

http://www.med.govt.nz/inquiry/failtech.pdf

. This report makes good reading in general as to how a number of causes (design, maintenance, downright poor engineering) can lead to a fairly spectactular failure (customer outage wise). They were backfeeding parts of the CBD from ships generators in the harbour for some weeks before supply could be fully restored.

 

[busbar]

On the Aukland power report,

http://www.med.govt.nz/inquiry/

seems to work better.