Author: Leon Zhang — Power Monitoring and Testing Equipment Team, Baoding Houfeng Trading Co., Ltd., sourcing cable test instruments and online monitoring systems for utilities, contractors and industrial plants
Technical review: reviewed by the in-house high voltage test engineering team against IEC 60270:2025, IEC 60885-3:2015 and IEEE 400.2-2024 practice
Last updated: 18 October 2026
Cable testing and monitoring covers three jobs that usually get discussed as one. Finding a fault that has already happened is the first. Diagnosing insulation that has not failed yet is the second. Working out which cable is which, before anyone cuts it, is the third. Each uses different instruments and answers a different question. Confusing them is the fastest way to buy equipment that does not solve the problem.
Most cable testing disappointments are not instrument failures. They are mismatches between the question being asked and the physics the instrument can measure. A time-domain reflectometer is sent to find a high-resistance fault that barely reflects. A loss-angle set is asked to prove a joint is sound, when a discharge measurement would have answered sooner. A locator is used to open a trench, with no pinpointer to mark the spot. This guide covers the whole range and points to the instrument that performs each job.
Key takeaways
- Cable testing and monitoring is three separate jobs — fault location, insulation diagnostics and identification — and each one has its own physics and its own instrument.
- Fault location is always two stages: pre-location gives a distance, pinpointing puts a mark on the ground. A locator alone cannot tell you where to dig.
- High-resistance faults have to be converted before they can be located; burn-down reduces the fault resistance until reflection methods can see it.
- Dissipation factor measured on cable insulation and the same quantity measured on transformer oil are different tests on different instruments, and mixing them up is a common procurement error.
- Online monitoring answers a different question from offline testing: it covers the hours the test set is not connected, which is where intermittent faults live.
- The most common cause of a cable incident is a crew working on the wrong cable, so identification equipment is a safety item, not an accessory.
Why cables fail: the case for cable testing and monitoring
Almost all cable failures in service come back to one of three places, and the three need different instruments, which is the practical reason a complete programme costs more than a single box. The insulation body fails from a defect that grows under voltage. A joint or termination fails from workmanship, moisture or thermal cycling. The metallic sheath fails from mechanical damage, corrosion or a bonding conductor that has been stolen or never installed. What the three have in common is that no single purchase covers them.
That asymmetry is the argument for a programme rather than a purchase. A fault locator only helps after the first failure mode has already happened. It says nothing useful about a sheath defect that has been collecting moisture for two years. It says nothing about a joint discharging quietly under load either.
Finding a fault: pre-location, then pinpointing
Fault location is a two-stage process, and the two stages use different instruments: pre-location gives you a distance from the test end, while pinpointing turns that distance into a mark on the ground. Buying one box and expecting it to do both is the most common specification error in this category, and it usually surfaces on the first real outage rather than in the quotation.
Low-voltage pulse methods and pulse-current methods both do this. The HF-CH-204 cable fault locator runs in low-voltage pulse mode on its own, and switches to pulse-current mode when paired with a high-voltage source. Waveform recognition and resolution decide whether the answer is a usable distance or a guess.
Pinpointing takes that distance and puts a mark on the ground. The HF-CH-205 cable fault pinpointer uses acoustic-magnetic synchronisation, with optional step-voltage modes, to confirm the position from more than one physical effect. That is how an operator avoids marking a point that merely looks plausible. It needs an impulse source. The HF-CH-218 surge generator is an integrated high-voltage source for pre-location and pinpointing. For test work rather than location, the HF-CH-217 high-frequency HV power supply is the alternative source.
Two situations break the simple version of that workflow.
The first is a high-resistance fault, which barely reflects a pulse and cannot be located directly. The answer is to convert it, not to buy a more sensitive locator. The HF-CH-212 cable fault burn-down system combines a burn-down function with an intelligent bridge, reaching up to 60 kV and roughly 600 mA. That is enough to reduce a high-resistance fault to a low-resistance one that ordinary location methods can see.
A quick way to tell which case you have before sending anyone to site:
| Symptom at the test end | What it usually means | First instrument | Why not the obvious one |
|---|---|---|---|
| Sharp reflection, readable distance | Low-resistance fault | Pulse TDR | Already works — go straight to the pinpointer |
| Very small reflection, high background | High-resistance fault | Burn-down unit, then TDR | A more sensitive TDR still cannot see below the noise floor |
| Trace looks clean, fault known to exist | Localised weak spot or moisture | Broadband impedance spectrum | TDR blurs the defect into the trace baseline |
| Intermittent, nothing during the test | Load- or weather-dependent fault | Travelling-wave monitoring | An offline test cannot see what is not happening |
The second is a faint, localised defect that conventional time-domain reflectometry cannot resolve. The HF-CH-213 cable impedance spectrum analyzer takes the broadband impedance spectrum (BIS) approach instead of TDR, as a non-destructive test for weak localised defects that reflection methods tend to blur into the noise floor.
Sheath and route: the failures outside the insulation
On a high-voltage cable the outer layers form a system of their own, and faults there show up through voltage gradient and sheath current rather than through a pulse reflection. That is why a fault locator never sees a sheath defect at all. A sheath test needs its own HV source, grounding reference and procedure. It is normally scheduled as a separate job, not bolted onto an insulation test.
The HF-CH-208 cable sheath fault locator targets breakdown faults on the outer protective sheath of buried cables with three positioning modes: step voltage, fault current and high-voltage probe DC voltage detection — three ways of pinning the same fault, for site conditions where any single method is unreliable. For EHV cable outer sheaths, the HF-CH-216 signal generator supplies the high-voltage signal with stepless voltage and current adjustment and continuous short-circuit capability, which is what a sheath test involves.
Where the question is which cable or which route, rather than where the fault is, route tracing and utility location come in. In this kind of programme it is the step that most often gets skipped. The HF-CH-223 smart utility locator traces buried cables and metallic pipelines with real-time depth measurement. The HF-CH-207 underground utility locator combines routing, cable identification and step-voltage fault detection in one unit.
Insulation diagnostics: tan delta and damped AC partial discharge
Finding faults is reactive work, and a complete cable testing and monitoring programme also carries methods that catch insulation on the way down rather than waiting for it to fail under load. Two of these, tan delta and damped AC partial discharge, share a voltage source and a single site visit yet answer genuinely different questions. Most programmes end up buying both. Dissipation factor trends dielectric loss across the whole insulation. Damped AC with partial discharge finds the discharge sites that a loss measurement cannot see. Neither is destructive.
Tan delta — the dissipation factor — measures the dielectric loss of capacitive insulation. On HV cable, motors and bushings, the HF-CH-209 high voltage cable tan delta tester works with an external high-voltage excitation source, either a test transformer or a series resonance set, for factory inspection and on-site insulation testing. Rising loss at a known voltage is the trend that matters. A single loss number in isolation rarely is, because the absolute value depends on the insulation type, the temperature and the length of cable under test.
It is worth keeping two different tests apart. The oil test measures the loss of an oil sample in a test cell, and belongs with oil quality testing, not with cable insulation. That is a separate instrument and a separate procedure, and Houfeng supplies it as the HF-TC-629 tan delta test of transformer oil set. The cable measurement is taken on the cable itself, at voltage, and the two results are not comparable.
The table below sets the four insulation and location methods side by side on the questions that decide which one to schedule. It is not a ranking. Each row answers a question the others cannot. A programme normally needs two of them, not one.
| Diagnostic method for cable testing and monitoring | What it measures | Destructive? | What it localises | Typical duration per circuit | What a pass does not prove |
|---|---|---|---|---|---|
| Dielectric loss | Loss tangent of the whole insulation, at voltage | Non-destructive | Nothing — trend only | 30–90 min, plus setup | That no local defect exists |
| Damped AC with PD | Discharge magnitude in pC, per site | Non-destructive | PD site in body or joint | 1–3 h incl. calibration | That the cable withstands a fault |
| VLF withstand (0.1 Hz) | Withstand capability under AC stress | Can fail a weak section | Nothing | 15–60 min per phase | That insulation is undamaged |
| Broadband impedance spectrum | Impedance deviation along the run | Non-destructive | Weak localised defect | 10–30 min | That PD is absent |
How to read the four rows against each other. Loss measurement (row one) is a whole-cable average, so it moves slowly and cannot localise anything. PD (row two) localises but only sees discharges inside its detection band, which is why a VLF withstand test can pass a joint that is already discharging above 10 pC. VLF (row three) is the only row that can actually fail a weak section, which makes it a proof test rather than a diagnostic one — the damage it can cause is the point, not a side effect. BIS (row four) fills the gap that TDR leaves at high fault resistance.
Damped AC with an overvoltage and discharge channel is the method for extruded MV cable, where PD activity in joints and terminations is the earliest available warning. The HF-CH-203 cable damped oscillating wave test system performs AC withstand voltage testing and discharge detection on medium-voltage cable. It also locates PD in cable bodies and joints, and it is built to the IEC 60270:2025 measurement standard. The HF-CH-202 OWTS system applies the same principle specifically to 35 kV MV cable, as an alternative to frequency-tuned series resonance withstand testing for handover and preventive testing programmes.
Identification and safe working
A large share of cable incidents are not electrical failures at all: they are a crew working on the wrong cable, and on HV systems identifying the right one is not a simple clamp-on job. Single-core cables are often grounded at one end, bundles run in the same trench, and several circuits can share a route for kilometres.
- Energised single-core cable at 35–500 kV. Most single-core cables are grounded at one end, which defeats conventional identifiers. The HF-CH-225 live cable identification tester uses satellite time synchronisation to identify energised single-core cables under those conditions.
- De-energised cable in a bundle. The HF-CH-201 power cable identifier picks the target cable out of a bundle using closed-loop detection with qualitative and quantitative parameter judgement.
- Confirming the cable is dead before cutting. The HF-CH-220 cable voltage detector judges energised status with a handheld receiver and stethoscope sensor. Where a positive destructive confirmation is required, the HF-CH-219 high voltage cable spiking tool performs a remote or automatic puncture: an energised cable arcs, a de-energised one does not. It is a deliberately destructive safety method. It is used where the consequence of cutting a live cable justifies a second, independent check.
- Phase and sequence. The HF-CH-221 phase sequence tester works by clamp-type non-contact inductive measurement, without disconnecting terminals or touching live conductors. The HF-CH-211 wireless high voltage phase detector covers contact and non-contact phase verification from 10 V to 550 kV.
Supporting equipment completes the set. The HF-CH-215 cable high voltage signal generator provides a 0–32 kV adjustable discharge, with a 2 μF energy storage capacitor for triggering and testing. The HF-CH-214 cable fault simulation system lets a training centre or a calibration lab set up fault types, properties and distances on 10 kV three-core cable. Crews and instruments can then be checked before they are needed. The crews who locate fastest are the ones who have practised on a simulated fault, not the ones with the newest locator. An operator’s first encounter with a reflection trace should not be an outage.
Three further units cover the awkward cases. The HF-CH-224 mining cable marking machine handles cable management in mining and drilling applications. The HF-CH-222 mining cable fault detector addresses the harsher case of mining cable damage, where conventional impulse methods damage the insulation further. The HF-CH-206 overhead line fault locator covers 6–35 kV overhead lines, including grounding, short-circuit and arc-grounding defects. For line model parameters, the HF-CH-210 transmission line parameter tester measures sequence impedance and capacitance.
Monitoring instead of testing: watching a cable that stays in service
Offline testing answers the question at the moment of the test, and online monitoring answers it continuously across the hours in between, which is the interval that matters most for cables that cannot be taken out of service. Failure mechanisms that only appear under load, or only in wet weather, live in exactly that gap. A scheduled test misses exactly those mechanisms. The test set is simply not connected when the conditions arrive.
- Partial discharge. The HF-C-201 cable partial discharge monitoring system combines HFCT, UHF and ultrasonic detection for continuous PD capture on medium and high voltage cable, with sheath circulating current and insulation variation as supporting channels and solar off-grid supply for sites without power.
- Sheath grounding current. The HF-C-202 cable sheath grounding current monitoring system watches the metallic sheath, which is where multi-point grounding, stolen grounding conductors and failed cross-bonding links show up — the conditions that quietly shorten cable life and reduce supply reliability.
- Temperature. The HF-C-203 cable temperature online monitoring system covers joints, trenches and branch boxes, where overheating is local and manual patrols miss it.
- Travelling-wave fault location. The HF-C-204 cable fault location monitoring system captures transient fault signals and latent insulation defects in service, so a fault is located from the record rather than by testing after the outage.
| Situation on the circuit | What answers it | Why the other half does not |
|---|---|---|
| Handover of a new feeder | Offline withstand + diagnostics | Nothing to monitor yet — no baseline exists |
| Joint PD suspected from loss trending | Offline damped AC PD | Monitoring confirms but does not localise the joint |
| Circuit must stay energised | Online PD and sheath monitoring | An outage for testing is the cost being avoided |
| Repeated sheath current alarms | Offline sheath fault location | Monitoring found it; locating it takes gradient methods |
| Theft or third-party damage risk | Cutting detection and route patrol | Neither test nor monitoring prevents physical access |
The choice between testing and monitoring is not either/or. A mature cable testing and monitoring programme uses offline testing at handover and for periodic verification. It uses online monitoring on the circuits where an outage is expensive. Cable theft and accidental third-party damage sit outside both: that is a security problem, and it is covered separately in transformer cable cutting detection.
The monitoring channels that pay back fastest are almost never the ones in the catalogue headline. On a paper-insulated circuit with a history of sheath damage, sheath grounding current earns its cost in the first year, because a stolen bonding conductor shows up as a step change in a channel that needs no PD calibration. On an extruded MV feeder with joint failures, PD wins. The choice should follow the circuit’s own failure record, not the sensor list.Choosing between the two. Test when you need a number you can sign. Monitor when you need to know what happened at 02:40 on a wet Tuesday. The first answers a commissioning question; the second answers an operational one. Circuits that matter enough to do both usually have an outage cost that already pays for the monitoring hardware.
The standards that apply
- IEC 60270:2025 defines discharge measurement itself — quantities, calibration and test circuits — and it is the reference behind PD testing equipment. The fourth edition replaced the 2000 edition and its 2015 amendment, renamed the document to Charge-based measurement of partial discharges and extended its scope to DC testing (IEC 60270:2025). A procedure written against the 2000 text should be checked for clause references rather than assumed current.
- IEC 60885-3:2015 gives the PD test methods for lengths of extruded power cable (IEC 60885-3:2015). Its boundary matters more than its content in day-to-day work. The standard explicitly does not cover measurements on installed cable systems. On-site PD work therefore refers back to IEC 60270:2025 and to the manufacturer’s procedure instead.
- IEC 60502-1:2021 covers construction, dimensions and test requirements for extruded-insulation cable at 1 kV and 3 kV (IEC 60502-1:2021); IEC 60502-2:2014, with Amendment 1:2024, covers 6 kV to 30 kV (IEC 60502-2:2014).
Using the numbers above on a live job. The edition year belongs in the work method statement, not just the quotation. A test procedure that cites IEC 60270:2000 without an amendment year was written before DC testing entered the scope, and a clause reference carried over from the 2000 text can point at text that no longer exists. Checking clause references against the 2025 edition takes minutes and avoids an argument on site.
- IEEE 400.2-2024 is the guide for field testing of shielded power cable systems using very low frequency (VLF, below 1 Hz), and it is the reference to follow when a VLF withstand or diagnostic test is specified (IEEE 400.2-2024).
Limitations and boundaries
A test set answers only the question its physics allows. These boundaries are the ones that most often get lost, between the test report and what someone takes the report to prove.
Pre-location accuracy is a percentage, and the percentage is applied to length.A locator quoted at one percent on a four-kilometre run gives a forty-metre window. That is why pinpointing is a separate instrument rather than a refinement of the first. It is also why planning to excavate from a pre-location distance alone is planning to dig a trench, not a hole.
Burn-down damages the fault, and the damage is permanent.Reducing a high-resistance fault to a locatable one degrades the insulation at that point in a controlled way. A cable that has been burnt down has a known defective section. The repair belongs in the same work order as the location, not in the next budget cycle.
Offline tests sample the calendar, not the load.A test at handover or on a fixed cycle says nothing about the hours the set was not connected, and nothing about failure modes that only appear under full load or in wet weather. Two years of records can be clean and still miss the mechanism that fails the circuit.
Online monitoring measures only what its sensors can see.A sheath current sensor reports sheath current. It does not report PD in a joint two kilometres away, and it will not detect a defect under a road crossing that no channel covers. Coverage is the sum of installed sensor types. Compare it against the failure modes actually recorded on that circuit, not against a general list.
A passing test is a statement about the span that was tested.Extruded insulation degrades locally, so a segment that passes at one joint says nothing about the segment beside it. Spatial coverage belongs in the report, and a report that does not state which spans were tested cannot be used to argue that the circuit is sound.
VLF and damped AC are not interchangeable for every insulation type.IEEE 400.2-2024 is written for shielded systems rated 5 kV and above, with extruded and laminated insulation. Applying VLF practice to paper-insulated or mass-impregnated cable without checking the manufacturer’s guidance is outside that guide’s scope rather than beyond it. The difference matters when a consultant specifies a test by standard number alone.
Identification equipment reduces one specific risk, and no amount of it replaces a permit.A live cable identifier confirms which cable is which; it does not confirm that the switching has been done or that the earths are applied. It closes one failure mode in a sequence, not the sequence itself.One line before the questions. Everything above reduces to one habit: write down which of the three jobs you are buying for, and refuse to let a single instrument quote answer all three. The FAQs below answer the questions that come up most often when a purchase order is being written.
Need a quotation for a cable test set?
Tell us the equipment you need to test, the voltage class and the quantity. Our engineers reply with a technical proposal and a quotation within 12 hours — English manual, remote video guidance and a one-year warranty on exported units.
Payment terms agreed per order · Lead time confirmed per order
Frequently asked questions
What is the difference between pre-location and pinpointing a cable fault?
In cable testing and monitoring, pre-location measures the distance from the test end to the fault, typically from pulse or pulse-current reflections, and gives you a number. Pinpointing uses that number with an impulse source and an acoustic-magnetic or step-voltage detector to mark the exact position on the ground. A locator alone cannot tell you where to dig. The distance it produces carries a percentage error that becomes tens of metres on a long run, which is why a serious programme budgets for both instruments.
Why can a high-resistance cable fault not be located directly?
Because the fault barely reflects the test pulse, so there is almost nothing for the instrument to measure above the noise floor. The standard approach is to convert it first. A burn-down unit applies high voltage and current to reduce the fault resistance, until ordinary location methods can see it clearly. The trade-off is deliberate damage at the fault point, which is why the repair is planned with the location work rather than after it.
Is the cable dissipation factor the same as the transformer oil test?
No. The cable test measures the dielectric loss of the insulation itself, at voltage, using an external high-voltage excitation source. The oil test measures the loss of an oil sample in a test cell and belongs with oil quality testing. They use different instruments, different procedures and different acceptance limits, and a result from one cannot be read as a result from the other.
When should a cable be monitored online instead of tested offline?
A circuit is a candidate for online cable monitoring when it cannot be taken out of service, when the failure mechanism is intermittent and only appears under load, or when the cost of an unplanned outage is high. Online PD and sheath-current monitoring capture conditions that a scheduled offline test will simply miss, because the test set is not connected during the conditions that cause the failure. Offline testing still has the advantage of full control over the applied voltage, which is why most programmes run both.
Why spike a cable instead of just testing it for voltage?
Because a spiking tool gives the crew a positive, physical confirmation that the cable is de-energised: an energised cable arcs during the puncture. It is a remote or automatic operation precisely because it is destructive at the point of contact, and it is used where the consequence of cutting a live cable makes a second, independent check worthwhile. Voltage detection remains the first check; spiking is the confirmation, not the substitute.
What changed in IEC 60270:2025?
The fourth edition, published in June 2025, replaced the 2000 edition and its 2015 amendment. It renamed the document to Charge-based measurement of partial discharges, extended the scope to DC testing and to AC up to 500 Hz, and focused the text on charge-based measurement. It also streamlined the performance checks for measurement system components and revised the normative annex on calibrator performance tests. Procedures written against earlier editions should have their clause references checked rather than carried over.
Conclusion
Cable testing and monitoring is bought one job at a time, and the specification should be written the same way. Which of the three jobs you actually have, what the physics of that job requires, and what the deliverable has to prove. A locator without a pinpointer leaves you with a distance and a trench. A loss-angle set without a PD channel leaves you with a trend and no location. A monitoring system installed without a named owner for its alarms is a data logger.
If you are not sure which of these jobs you actually have, describe the symptom rather than the specification — what happened, whether the cable is in service, what the last test result was, and what insulation type and voltage level you are working on. The cable test instrument range covers fault location, diagnostics and identification, and the online cable monitoring range covers circuits that stay energised.
Request a quotation for cable test instruments and monitoring systems matched to your voltage level, insulation type and failure history, and we will confirm the instrument combination your programme actually needs.
Technical references
- IEC, IEC 60270:2025: High-voltage test techniques — Charge-based measurement of partial discharges, 4th edition — webstore.iec.ch.
- IEC, IEC 60885-3:2015: Electrical test methods for electric cables — Part 3: Test methods for partial discharge measurements on lengths of extruded power cables, 2nd edition — webstore.iec.ch.
- IEC, IEC 60502-1:2021: Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV — Part 1: Cables for rated voltages of 1 kV (Um = 1,2 kV) and 3 kV (Um = 3,6 kV), 3rd edition — webstore.iec.ch.
- IEC, IEC 60502-2:2014: Power cables with extruded insulation and their accessories for rated voltages from 1 kV (Um = 1,2 kV) up to 30 kV — Part 2: Cables for rated voltages from 6 kV (Um = 7,2 kV) up to 30 kV (Um = 36 kV), 3rd edition, with Amendment 1:2024 — webstore.iec.ch.
- IEEE, IEEE 400.2-2024: IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF) (less than 1 Hz) — standards.ieee.org.