For the 11 kV cable sequence described, restore any circuit main earth (CME) removed for signal injection, disconnect the identification equipment, and apply the required CMEs before remotely spiking the cable. The described working practice uses earths at both cable ends during spiking and identifies no benefit in leaving them off. A network-specific instruction that says to replace a CME after spiking creates a procedural conflict; the controlling distribution network operator (DNO) safety rules, switching schedule, and issued safety document must resolve it before work continues.
Earth-State Decision
The visible problem is a conflict between two operations. Signal-injection identification may require one CME to be removed so the test signal can circulate through two selected cores. Cable spiking is a destructive safety operation performed only after identification is complete. The earth state used for identification therefore must not be carried forward automatically into the spiking state.
The number that matters is the current that could flow if the selected cable is energized, back-fed, induced, or simply the wrong cable. At 11 kV, a spike penetrating the cable cross-section can initiate a high-energy fault. Applied, adequately rated earths hold the isolated circuit conductors near earth potential and provide defined discharge and fault-current paths, subject to the approved earthing arrangement.
| Observed condition | Likely meaning | Required decision |
|---|---|---|
| One CME is off and signal-injection equipment is connected | Cable identification is still in progress | Complete identification, remove the test equipment, and restore the CME before authorizing the spike. |
| Both-end CMEs are applied and identification equipment is removed | The described circuit is in its intended pre-spike earth state | Confirm the switching schedule, earth schedule, tool readiness, exclusion zone, and remote operating position. |
| A local Q&A says “replace the CME after spiking” | Its sequence conflicts with the both-earths-on practice | Stop and obtain an authoritative interpretation of that DNO rule. Do not combine fragments from the two procedures. |
| The spiking tool remains attached after operation | The tool is connected mechanically and potentially electrically to primary conductors | Remove it only under the safety document and earth state specified by the controlling rules. |
| The far end is inaccessible or terminates at a bottle end | Normal two-ended signal-injection identification may be unavailable | Use the approved alternative identification method; the described alternative is insulation-resistance testing before and after spiking. |
Fault Current, Heat, and Operating Time
This is fault energy, not logic. The cartridge provides mechanical penetration; it does not limit electrical fault energy. If the cable is live, the spike can bridge conductors, metallic screens, armour, and earth, with current limited by the network impedance and cleared by upstream protection.
Thermal energy in the fault path rises with the time integral of current squared: thermal duty proportional to ∫I²dt. Neither prospective fault current nor clearing time is supplied for this installation, so no incident-energy boundary, CME rating, or safe distance can be calculated from the nominal voltage alone. Read those quantities from the network fault study, protection settings, approved tool data, and earthing-equipment markings.
| Quantity or limit | Why it controls the operation | Where to read it |
|---|---|---|
| 11 kV nominal system voltage | Defines the voltage class stated for the cable, not the available fault energy | Network diagram, cable records, and switching schedule |
| Prospective fault current | Sets the maximum current duty if the wrong or energized cable is penetrated | Current network fault-level study |
| Protection clearing time | Controls the duration component of I²t
|
Protection settings and relay coordination study |
| CME short-time rating | Must cover the duty assigned by the approved earthing arrangement | Earth-set markings, inspection record, and DNO equipment schedule |
| Spiking-tool voltage and application rating | Confirms that the complete tool is approved for the cable and network class | Tool identification plate and manufacturer instructions |
| Remote operating distance and exclusion zone | Separates personnel from arc, fragments, and mechanical movement | DNO procedure and approved tool instructions |
Safety-Document Boundaries
A sanction for test (SFT) controls testing that may require a temporary departure from the normal work earth state. A permit to work (PTW) controls the subsequent work on conductors placed in the defined safe condition. The boundary matters because the spiking tool remains attached to primary conductors after it fires.
One described sequence cancels the SFT after spiking, issues the PTW, and removes the tool under the PTW. Another possible sequence removes the tool under the SFT after the document records that earths have been replaced. Either can be valid only when explicitly authorized by the applicable DNO rules. The person controlling the operation must not improvise a hybrid sequence based on the names of the documents.
The safety document must identify the circuit, work location, isolation points, applied earths, test exceptions, and transfer point between testing and work. Its wording must also cover attachment, remote firing, post-spike inspection, and removal of the spiking tool. If any of those actions falls outside the document, pause and have the document corrected or reissued.
Cable Identification Before Spiking
Physical route, colour, position in a trench, and similarity to drawings are supporting observations, not positive identification. The described method connects a signal generator to two cores at one end with one earth removed, then detects that signal at the intended work point. Identification and earthing changes remain controlled operations because adjacent cables may still be live.
- Match the circuit designation, cable route, termination records, and work location to the switching schedule.
- Establish the isolation and initial earthing condition required by the DNO rules.
- Under the test authority, remove only the CME specified for signal injection.
- Connect the approved signal generator to the specified two cores and identify the cable at the work point using the approved receiver and method.
- Mark and retain control of the identified cable so the tool cannot migrate to an adjacent cable.
- Disconnect the signal generator and all identification leads.
- Restore the removed CME and independently confirm the final pre-spike earth state.
Where a bottle end prevents the signal-injection arrangement, the described alternative is an insulation-resistance test before and after spiking. Record the core-to-core and core-to-earth test configuration and results according to the approved test procedure. The post-spike result must show the expected electrical change on the same circuit; use the DNO acceptance criteria rather than inventing a resistance threshold.
Controlled Spiking Procedure
Only authorized personnel using an approved, inspected remote cable-spiking tool may execute the operation. The following sequence describes the decision points; the DNO procedure and tool instructions supply the equipment-specific loading, mounting, firing, and exclusion requirements.
- Confirm the cable identity, exact spike location, isolation points, safety document, and switching schedule.
- Verify that identification equipment has been removed and every temporarily removed CME has been restored.
- Confirm whether the approved earth schedule requires CMEs at both ends. For the sequence described here, both-end earths are on before spiking.
- Inspect the spiking tool, remote operating system, cartridge, restraints, and inspection status against the approved instructions.
- Fit the tool to the positively identified cable at or close to the intended work point. Spiking damages the cable, so the location normally becomes part of the jointing or repair work.
- Clear the exclusion zone, account for personnel, and operate the tool remotely from the prescribed position.
- Wait for the operating state required by the tool procedure, then verify complete penetration without approaching prematurely.
- Apply the prescribed post-spike test or inspection. For the inaccessible-end case, repeat the recorded insulation-resistance test and compare the result with the approved acceptance criteria.
- Transfer from the SFT to the PTW, or remove the tool under the SFT with earths replaced, exactly as directed by the controlling safety rules.
- Issue or accept the PTW before cable cutting, jointing, or other work begins.
Post-Spike Verification
A fired cartridge alone does not prove that the correct cable was penetrated. Verify that the spike passed through the required cable cross-section, that the tool operated fully, and that the electrical result matches the identified circuit. A partial mechanical stroke, a misplaced tool, or attachment to an adjacent cable invalidates the proof.
Maintain the CMEs while inspecting and removing the tool unless the approved procedure specifies a documented test exception. Treat the tool and spike as connected to primary conductors until the applicable safety document releases the task. Record the cable identity, spike location, earth state, tool result, test results, and document transition.
For a cable identified by the insulation-resistance method, compare the before-and-after records using the same test points and configuration. A result that fails to show the required change means the cable has not been positively confirmed. Keep the work stopped while the circuit identity, spike penetration, test setup, and records are re-examined.
Recurring Procedural Pitfalls
- Confusing identification state with spiking state: an earth removed for signal injection is a temporary test condition, not permission to spike with that earth absent.
- Treating “replace earths” as informal wording: it is a switching action that must be recorded, checked, and matched to the earth schedule.
- Assuming two earths are universally mandatory: both-end CMEs are used in the described practice, but the network topology and DNO rules determine the formal requirement for each circuit.
- Using a successful firing sound as proof: verify tool travel and the electrical effect on the identified circuit.
- Removing the tool between documents: tool removal must remain inside either the SFT or PTW scope; there must be no undocumented interval.
- Relying on nominal voltage for risk: fault current, clearing time, CME rating, and tool rating control the electrical duty.
- Starting work immediately after spiking: complete verification, document transfer, and PTW issue before cutting or jointing.
Frequently Asked Questions
Why does a CME come off during cable identification?
Signal injection may require one CME to be removed so the generator can drive a detectable signal through two cable cores. Disconnect the generator and restore that CME before spiking under the described sequence.
Why does the spiking gun need a PTW or SFT for removal?
After firing, the tool is attached to primary conductors and may form part of the earthing or fault path. Remove it only within the explicit scope and earth state of the controlling SFT or PTW.
Why does the procedure say to replace the CME after spiking?
That wording conflicts with the described practice of restoring all earths before the spike and may represent a different DNO-approved sequence. Stop when the safety documents, earth schedule, or tool procedure do not give one unambiguous order. Escalate the conflict through the DNO’s official operational or engineering support channel, and resume only after the authorized sequence is documented.