Thermo-mechanical Design, Snaking and Fault Forces

A cable that heats up expands, and if it cannot expand it pushes. On a large conductor the numbers are not small: CIGRE TB 669 puts the restrained thrust at roughly 60 to 80 kN per phase for a 400 kV 2500 mm² copper cable. In a rigid system that force is independent of route length, so a short run is no safer than a long one. The job is to decide whether the system is rigid, flexible or semi-flexible, and then to design for whichever it is.

When You Need It

  • · A large conductor is being installed in ducts or cleated on a rack, and nobody has calculated what the expansion does
  • · There is a transition from duct to a sealing end, a GIS pit or a joint bay, which is where the thrust arrives
  • · Snaking has been proposed, or assumed, and the amplitude and pitch have not been checked against the sheath strain limit
  • · Cleat spacing has been proposed and nobody has calculated the fault force it has to survive
  • · A support or structure design needs the cable loads to be given to it

Standards Applied

CIGRE TB 669CIGRE TB 889IEC 61914CIGRE TB 283IEC 60287

The report states the clause and the edition it works to, so the calculation can be reproduced by whoever reviews it.

What You Get

System classification: rigid, flexible or semi-flexible, because that fork decides everything after it
Restrained thrust for the temperature range the circuit will see, and the movement that has to go somewhere if it is not restrained
Snaking design where the system is flexible: amplitude and pitch, checked against minimum bending radius, cyclic sheath strain, accessory force and core movement within the sheath
Transition detailing where a duct meets a sealing end, a GIS pit or a joint bay
Peak electromagnetic force between conductors at the stated peak fault current and spacing
Maximum cleat spacing, and the load each cleat and support has to carry
Where the assessment finds the proposed arrangement inadequate, what to change

Frequently Asked Questions

Is a longer route worse for thermo-mechanical force?

Not in a rigid system. The restrained thrust is a function of the conductor, its temperature rise and its restraint, not of how far the cable runs, so a short rigid section carries the same force as a long one. What length changes is how much movement has to be accommodated once the system is not rigid.

What actually is snaking?

Laying the cable as an approximate sinusoid rather than straight, and cleating it at the inflection points, so thermal expansion is taken up as a change in amplitude instead of as thrust. It is not a refinement on a straight lay. In CIGRE TB 669 it is what a flexible system is, and it comes in two planes: vertical, which is sagging, and horizontal, which is waving.

What limits the snake?

Four things, and the binding one varies. Minimum bending radius, cyclic sheath strain, the force the accessories will accept, and movement of the core within the sheath. The strain limit is usually the one people have not checked: roughly 0.35 per cent for aluminium sheath and 0.12 per cent for lead alloy.

Do you need the peak fault current or the RMS?

The peak make value, which is not the RMS symmetrical figure. If you have the RMS and the X/R ratio we will derive the peak and show it.

Can this be done from a cable schedule alone?

No. It needs the physical arrangement: spacing, support type, span and fixing, because the force is a function of geometry.

The Rest of the Cable Chain

Review, calculation, drawings, testing and fault location, from one place. See the whole picture.

Cable Design Review

Cable Sizing and Current Rating

Cyclic and Emergency Ratings

Short Circuit and Screen Sizing

Induced Voltage and Bonding

EMF Assessment

Cable Pulling Calculations

Route Design and Drawings

Testing and Fault Location

Request a Cable Sizing Study

A free calculator answers one question at standard conditions. A study takes the actual route, burial depth, soil resistivity, grouping and load profile, and gives you a signed report you can issue to the DNO or the client. We take single calculations as readily as full cable system designs.

Or call 07951 651 013 or email enquiries@stardeltapower.co.uk