The engineer sees armour heating, burned screen bonds, termination damage, or a measurable voltage at the isolated end of a single-core AC cable. Follow the electromagnetic path: conductor current produces magnetic flux, the flux links the metallic armour or screen, and the bonding arrangement either interrupts the resulting current or closes a loop through earth and bonding conductors. Trefoil formation reduces the external field of the three-phase group, but it does not by itself prove that the voltage induced along each individual metallic layer is zero.
What conductive path does the cable construction provide?
Layer one first. Identify every metallic layer before choosing a bonding scheme. Armour and screen are not interchangeable names: armour primarily provides mechanical protection, while a metallic screen controls the electric field and carries charging or fault current. A cable may contain both layers, and a termination may bond them together even when drawings describe only “armour grounding.”
| Reading or inspection | Outcome | Next check |
|---|---|---|
| Manufacturer cable construction and termination drawing | Separate armour and screen connections | Trace each layer independently through every termination and joint. |
| Armour and screen joined at a termination | The combined metallic path controls induced current and touch voltage | Record where the combined path is grounded, isolated, or sectionalized. |
| Continuity between both cable ends before external bonds are installed | A metallic layer or unintended parallel path is continuous | Locate joints, glands, supports, earth conductors, and equipment enclosures that complete it. |
| No continuity end to end | The layer is sectionalized, isolated, or open | Compare the result with the approved bonding design; an accidental open bond is not a bonding method. |
Confirm the armour material from the cable data rather than from appearance. Magnetic armour around one AC conductor can add magnetic and eddy-current loss. Specifying a nonmagnetic armour avoids that particular magnetic-material problem, but conductivity remains: a conductive armour bonded at both ends can still form an induced-current loop. “Nonmagnetic” therefore does not automatically authorize solid bonding at both ends.
Where does the induced voltage originate?
AC load current in the core creates a changing magnetic field. Some of that field links the cable’s metallic screen and armour, producing a longitudinal induced electromotive force. Its magnitude depends on conductor current, cable length, phase spacing, physical arrangement, metallic-layer geometry, and the relative positions of the phase conductors.
For a metallic section, the governing relationships are:
E_induced = integral of induced electric field along the metallic section
I_metal = E_induced / Z_loop
P_loss = |I_metal|^2 × R_metalWith one end intentionally isolated, the loop is open and sustained circulating current is suppressed. The induced electromotive force then appears mainly as voltage between the isolated metallic layer and local earth. With both ends bonded, earth conductors, equipment frames, and the metallic layer can close the circuit. Current then flows according to total loop impedance and produces heat in the metallic layer, bonds, glands, and terminations.
Bonding both ends does not create cable capacitance. Capacitance exists between the conductor, insulation, metallic screen, armour, and earth regardless of the bonding method. The problematic continuous current associated with double-ended bonding is principally the current driven around the closed metallic loop by magnetic induction. Cable charging current is a separate dielectric-capacitance effect and must not be used as an explanation for armour-loop heating without measurements that separate the two paths.
Does trefoil formation cancel the armour voltage?
Trefoil placement brings the three phase conductors close together, so their vector-summed magnetic field falls more rapidly outside the group than it would in a widely spaced flat formation. That reduces external magnetic field and can reduce induced effects. It does not guarantee cancellation of flux linking each individual cable screen or armour.
| Physical condition | Expected effect | Decision |
|---|---|---|
| Symmetrical trefoil, equal phase currents, consistent cable positions | Improved field cancellation outside the three-cable group | Still calculate or measure metallic-layer voltage and current. |
| Unbalanced phase currents | Residual magnetic field increases | Use the highest credible operating imbalance in the bonding assessment. |
| Trefoil orientation changes, phases separate, or spacing varies | Flux linkage changes along the route | Inspect the entire route, including bends, entries, joints, and terminations. |
| Armours or screens interconnected phase-to-phase or through common hardware | Additional loop paths may exist | Trace continuity through clamps, glands, enclosures, and earth bars. |
Cancellation requires geometry and current symmetry, not merely a drawing note that says “trefoil.” Phase transposition or cross-bonding can make induced voltages from successive cable sections oppose one another, but the section lengths and phase sequence must form a designed system. Randomly exchanging bonds or joining screens at intermediate points can create new circulating-current loops.
Which symptom identifies the active branch?
| Symptom or reading | Probable mechanism | Next action |
|---|---|---|
| Substantial AC current in a bond during steady load | Closed armour or screen loop, or an unintended parallel earth path | Trace both ends and all intermediate metallic connections. |
| Bond current rises with conductor load | Magnetically induced circulating current | Compare all phases and inspect geometry where readings diverge. |
| High voltage from isolated metallic layer to local earth | Single-point bonding has interrupted current but allowed induced voltage to accumulate | Assess touch voltage and the need for sectionalizing, cross-bonding, or a sheath voltage limiter. |
| Burned armour, screen bonds, glands, or termination connections | Excess loop loss, inadequate bond connection, fault-current duty, or a combination | Inspect damage before energization and measure both load-related current and connection resistance. |
| One phase differs materially from the other two | Asymmetric routing, wrong phase position, damaged layer, or unintended bond | Compare phase arrangement and continuity section by section. |
| Heating with little measurable bond current | Magnetic armour loss, core-conductor heating, poor power connection, or measurement at the wrong branch | Measure temperature and current at every available bond and verify the armour material. |
Field failures associated with incorrect bonding include burned armour and screen bonds and damaged cable systems. A reported 33 kV installation experienced four failures within six months after a termination arrangement grounded one end while bonding armour and screen at the other without grounding that end. That arrangement cannot be judged from the verbal description alone: the deciding test is whether the armour-screen connection and external hardware completed a loop or left an excessive standing voltage. Monthly failures were also reported on 33 kV and 11 kV single-core systems, reinforcing the need to test the installed path rather than accept termination labels.
Does high-resistance system grounding change the bonding choice?
The 6 kV and 400 V systems are described as high-resistance grounded. That neutral-grounding method controls the power-system response to a phase-to-ground fault; it does not remove the magnetic field created by normal phase current. It therefore does not eliminate load-induced voltage in a single-core cable’s armour or screen.
Treat two current paths separately. First, evaluate continuous induced current during normal loading. Second, verify the metallic layer, bonds, grounding conductors, and any limiter against the ground-fault current and clearing behavior of the actual protection system. Read the neutral-grounding equipment rating, relay settings, cable manufacturer bonding instructions, and project fault study. A normal-load clamp reading cannot prove adequate fault-current capability, while a fault-duty calculation cannot prove acceptable continuous sheath loss.
At 6 kV, a metallic screen is commonly part of the cable’s electric-field control system, so its continuity and termination affect more than mechanical armour. At 400 V, construction may differ. Inspect the actual cable schedules and termination kits for each voltage class instead of applying one generic armour detail to both.
When is single-point bonding insufficient?
Single-point bonding trades circulating current for standing voltage. As the uninterrupted cable length and conductor current increase, voltage at the isolated end can rise to a hazardous or insulation-stressing level. Several thousand volts have been encountered on long single-core circuits under some system conditions, but no generic length threshold follows from that observation. Calculate the actual route or measure it during controlled operation.
| Bonding arrangement | Continuous-current behavior | Voltage behavior | Engineering response |
|---|---|---|---|
| Single-point bonded | Interrupts the normal circulating-current loop | Standing voltage develops at the isolated end | Control access, insulate the termination, and verify maximum induced voltage. |
| Solidly bonded at both ends | Allows induced loop current | Holds both ends near their local earth potentials | Calculate metallic-layer loss and apply cable ampacity correction or a larger cable if required. |
| Sectionalized or cross-bonded | Opposing section voltages can limit loop current | Section voltages remain design-dependent | Use engineered section lengths, phase transposition, insulated joints, and verified bonding links. |
| Single-point bonded with a sheath voltage limiter | Limiter remains nonconducting during ordinary induced voltage when correctly selected | Transient voltage is constrained by limiter operation | Select from the cable insulation and system transient study; inspect its earth connection and status. |
If both-end grounding is mandatory, include calculated armour and screen losses in ampacity. Extra metallic-layer heat can require derating or a larger conductor. If single-point bonding produces unacceptable voltage, use a designed cross-bonding, sectionalizing, or voltage-limiting arrangement rather than installing an undocumented second earth bond.
How should the resolving branch be implemented?
- Obtain the cable construction, route, phase arrangement, joint schedule, termination-kit instructions, maximum operating current, ground-fault study, and approved bonding design for both the 6 kV and 400 V circuits.
- Identify armour and screen separately. Mark every earth point, insulated gland, link box, joint, sheath voltage limiter, equipment enclosure, clamp, support, and parallel grounding conductor.
- With the circuit isolated and proved safe under the site procedure, test continuity from end to end and between each metallic layer, local earth, equipment enclosure, and the other phases. Reconcile every conductive path with the drawing.
- Inspect trefoil spacing and phase order along the whole route. Record deviations at bends, entries, joints, and locations where the cables separate.
- Choose the design branch. Use single-point bonding where calculated isolated-end voltage and transient stress remain acceptable. Use engineered sectionalizing, cross-bonding, or a sheath voltage limiter where standing voltage is excessive. Use both-end bonding only after calculating continuous metallic-layer current, losses, temperature effect, and fault duty.
- Install insulated terminations, bonding conductors, links, and limiters exactly as shown on the approved scheme. Prevent glands, enclosures, or temporary earths from bypassing intentional isolation.
- Before load, repeat continuity and insulation checks. Confirm that the nominally isolated end has no accidental earth path and that the grounded end has a sound, low-resistance connection appropriate to its duty.
- Energize under controlled conditions. Record core current, armour current, screen current where separately accessible, isolated-end voltage to local earth, bond temperatures, and phase-to-phase differences at increasing stable loads.
- Accept the installation only when measured paths match the drawing, bond currents and standing voltages remain within the project’s calculated limits, no connection shows abnormal temperature rise, and protection and limiter indications remain normal.
FAQ
Can I ground single-core AC cable armour at both ends?
Yes, only when the calculated induced loop current, armour or screen loss, cable ampacity, bond heating, and fault duty are acceptable. Measure bond current under load because nonmagnetic armour and trefoil formation do not prove that the closed-loop current is negligible.
Does trefoil formation eliminate induced screen voltage?
No. Trefoil reduces the external magnetic field when geometry and phase currents are symmetrical, but each cable’s metallic layer can retain longitudinal induced voltage. Verify it by measuring the isolated metallic layer to local earth at the maximum controlled load available.
Can I leave the remote armour end ungrounded?
Only as part of a designed single-point bonding scheme with an insulated, inaccessible or guarded termination and acceptable normal and transient voltage. The final verification is to record isolated-end voltage, bond current, and bond temperature under stable load and compare all three phases with the approved calculated limits.