The Whole Chain, Not One Slice of It
Most firms of our size do one part of this. The reason to bring the cable system to us is that the calculations, the drawings and the site work end up in the same hands, so the trench section on the drawing matches the rating that was claimed for it.
Review
Read somebody else’s design before you sign it off.
Calculate
Rating, withstand, bonding, forces and field, to the standard.
Draw
Trench sections, joint bays, bonding diagrams, schedules.
Test
Prove it on site, and find the fault when there is one.
What We Deliver
Each of these is a separately purchasable piece of work. You do not have to buy the set.
Cable Design Review
Cable Sizing and Current Rating
Cyclic and Emergency Ratings
Short Circuit and Screen Sizing
Induced Voltage and Bonding
Thermo-mechanical and Fault Forces
EMF Assessment
Cable Pulling Calculations
Route Design and Drawings
Testing and Fault Location
Grouping and Mutual Heating
Circuits sharing a corridor heat each other. Once they do, it is the group that sets the conductor temperature, not the circuit on its own, and a single-circuit rating understates it. That temperature is then the input to everything downstream, including whether the route needs a snake at all.
One circuit, on its own
The same circuit, ten in the corridor
Grouping factor, 15 K spread centre to edge
Ten circuits of three single-core cables in individual ducts, solved by image-source superposition to IEC 60287-2-1 §4.2.3, converged in six iterations.
Circuit count does not force you to a numerical model. Soil uniformity does. The closed-form method carries as many circuits as you like; there is no count at which it gives up. What it does assume is uniform soil across the whole envelope, and across a wide corridor that assumption is the one that fails first. That is the point at which a finite-element treatment earns its place, and it is worth establishing which side of the line a route sits on before anyone commissions anything.
Thermo-mechanical Design and Snaking
A heated cable expands, and if it cannot expand it pushes. On a large conductor that is not a small number: 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 does not depend on route length, so a short run is no safer than a long one.
Force assessment
Restrained thrust across the temperature range the circuit will actually see, and the movement that has to go somewhere if the system is not rigid.
Rigid, flexible or semi-flexible
The fork that decides everything after it. Semi-flexible is the duct case, and it is the one most often assumed rather than established.
Snaking design
Amplitude and pitch for vertical snaking (sagging) or horizontal (waving), checked against four limits: bending radius, cyclic sheath strain, accessory force and core movement within the sheath.
Transitions
Duct to sealing end, duct to GIS pit, and the rigid-to-flexible interface. The transition is where the thrust arrives, so it is where the design usually fails.
Cleat and support loads
Maximum cleat spacing and the load each cleat and support has to carry, given as numbers an installer can build to.
Short circuit forces
The electromagnetic force between conductors at peak make current, at the same locations, because the cleats have to hold for both.
Where the Expansion Goes
Held rigid, a large cable warmed by 70 K pushes tens of kilonewtons into its termination. Laid in a locking wave, friction at each bend bleeds that thrust away before it arrives.
The CIGRE TB 669 Annex 5 worked example: 60 kN rigid, 5.5 kN at the flexible loop, and the checks that prove the wave is long enough.
Clamp Spacing Is Set by the Fault
On a fault the phases repel with a force that rises with the square of the peak current. Clamps too far apart are overloaded, let go, and the cable whips.
An illustrative 132 kV example at 100 kA peak, with the clamp rated on its manufacturer's published test.
Worked to CIGRE TB 669 and CIGRE TB 889. More on thermo-mechanical design →
No minimum package
One calculation is a job. If all you need is a screen size checked against the fault level, ask for that and nothing else.
Fast, because the engine is ours
The rating calculations run on software we wrote and maintain rather than bought in, so expedited turnaround is available where the programme needs it.
Reproducible reports
Every input is stated, every clause is named. A competent engineer can rebuild the calculation from the report, which is the only real test of one.
Frequently Asked Questions
Will you take a single calculation, or only a full package?
A single calculation is normal work for us and there is no minimum package. One circuit, one screen size, one bonding check. If the answer turns out to need more than that we will say so before starting rather than after.
How quickly can a cable sizing study be turned round?
Faster than the industry expects, because the calculation engine is ours and the report is templated. Expedited turnaround is available where a programme demands it, and we have delivered multi-case 132 kV verifications that way. Committed dates are agreed against a complete information set rather than against optimism, so ask and we will give you a date we can hold.
Do you cover the whole chain or only the calculations?
The whole chain. Independent review of somebody else’s design, the calculations, the drawings to build from, and testing and fault location on site. Most firms our size do one slice of that.
Which standards do you work to?
IEC 60287 for steady state rating, IEC 60853 for cyclic and emergency, IEC 60949 for short circuit withstand, CIGRE TB 283 and TB 347 for bonding, CIGRE TB 889 for pulling, and ENA ER C55 for the GB standing voltage limit. The report names the clause so a reviewer can follow it.
Can you review a design produced by somebody else?
Yes, and it is one of the strongest things we do. A manufacturer’s submittal package gets read line by line and comes back with a comment, a status and a required action against every item.