A cable entry can be correctly installed and later become strained when the transformer foundation and trench settle differently. The relevant quantity is their relative movement. A settlement measurement on the trench alone cannot determine the load reaching the cable termination.
Map the supports on both sides of the interface. Identify fixed clamps, bends, free lengths, joints, seals and the points where the cable can move. This is a service-movement problem, distinct from the pulling forces during initial installation.
Define the displacement between supports
Let the two support displacement vectors be u1 and u2, in millimetres along a defined coordinate system. Relative displacement is Δu = u2 − u1. The vertical and horizontal translation components depend on the defined coordinates. Record support rotations separately: the vector difference alone does not contain them. Equal downward motion produces a different cable demand from one support moving while the other remains fixed.
| Interface element | Movement evidence | Mechanical question |
|---|---|---|
| Transformer-side support | Foundation and bracket displacement | How is motion imposed on the cable? |
| Trench-side support | Trench settlement and rotation | What relative motion develops? |
| Bend/free length | As-installed geometry | Can deformation occur without exceeding limits? |
| Entry seal | Construction and movement capability | Does it restrain or tolerate the motion? |
| Joint/termination | Position and support arrangement | What load reaches the accessory? |
| Protective edge | Clearance through the movement range | Can cable contact create damage? |
Follow the load path
A flexible-looking cable route may still be tightly restrained by a seal or a hidden clamp. If movement cannot distribute through the intended free length, force can concentrate at a bend or accessory. Conversely, a long free length may change support and vibration behavior. The mechanical assessment should represent the real restraint arrangement.
Use verified cable and accessory limits for bend geometry, strain and permitted loading. Do not infer service movement capability from a maximum installation pulling force. Those quantities describe different conditions and may involve different vulnerable components. Southwire’s installation guide distinguishes final cable training from pulling under tension; neither gives a universal service settlement allowance. [1]
Inspect change against a baseline
Retain baseline photographs and measured support positions with stable datums. Later observations should identify displaced supports, changed bend shape, abrasion, seal movement and accessory alignment. An intact outer jacket is useful evidence but does not establish that internal strain remained acceptable.
Settlement can also change drainage or local thermal surroundings at the entry. Record these as separate effects rather than trying to cover them with a mechanical inspection alone.
Keep survey uncertainty separate from the permitted movement envelope. Stable datums and registration corrections are needed before a change in position can be compared with its engineering margin. [2]
Resolve support movement by component
Assumptions: transformer support translation is (0, −5) mm and trench support translation is (8, −20) mm in horizontal/vertical coordinates. Relative motion is (8, −15) mm, with magnitude 17 mm. The approved service envelope is assumed to allow ±10 mm horizontally and ±12 mm vertically, with no support rotation. Horizontal demand has 2 mm margin, but vertical demand exceeds its limit by 3 mm. Reject the arrangement and revise the free length or restraint before accepting the settlement. A scalar check of 17 mm against an assumed 20 mm radial allowance would miss this directional failure; use the actual component envelope and independently check rotations.
References
- Southwire Company, Power Cable Installation Guide, 2005, Training Radius and general field practices.
- B. N. Taylor and C. E. Kuyatt, NIST Technical Note 1297, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, 1994, sections 5 and 6.