Electrical Tracking and Climatic Testing

Materials either track or erode — and the difference decides the test.

Everything up to this point has dealt with failures that announce themselves in seconds or microseconds: disruptive discharge under impulse, collapse of dielectric strength, the power arc. This module inverts the time scale. Electrical tracking is the failure that is born slowly — hours, months, years of milliampere surface currents that, drop by drop of contamination and moisture, write a conductive carbon track across the polymer until the final short circuit. It is the dominant failure mode of compact overhead networks with covered conductors, of polymeric insulators, and of every organic insulation exposed to weather.

Here the laboratory does not deliver a blow. It accelerates ageing. This module joins the standardised tracking test — the inclined plane of IEC 60587 — to the climatic tests that reproduce the environment in a chamber: salt spray, UV weathering, and fog or rain under voltage. By the end you should be able to conduct the test, read its three regimes, and above all write climatic requirements without ambiguity, knowing when each chamber makes sense and when it is simply wasted laboratory hours.

What you will be able to do

• Explain the physical mechanism of tracking and erosion in polymeric insulation — from loss of hydrophobicity to the dry band, from dry band arcing to carbonisation — and distinguish materials that track from materials that erode.

• Prepare specimens to repeatability criteria (dimensions, cleaning, conditioning, wettability) and run the inclined plane test per IEC 60587:2022 in both methods and both criteria.

• Interpret the three regimes of leakage current (initiation, acceleration, failure), recognise parasitic arcing and spurious contamination, and correlate laboratory results with field conditions while knowing the limits of that correlation.

• Specify climatic tests — salt spray per ISO 9227, UV weathering per ISO 4892-2 and -3, and fog or rain — choosing the test by the dominant degradation agent and writing acceptance requirements without ambiguity.

• Position the complete-component test (IEC 62217 with 1 000 h salt fog, and the low voltage comparative tracking index of IEC 60112) relative to the material test, understanding what each proves and what it does not.

The chain of slow failure

A compact protected overhead network lives, by design, with electric field on the surface of the covered conductor. Where there is tangential field, contamination and moisture, there is surface current. The same holds for polymeric line and substation insulators, cable terminations, bushings with polymeric sheds, and silicone-housed surge arresters. In all of these, ceramic gave way to polymer for excellent reasons — reduced mass, resistance to vandalism, better performance under pollution thanks to hydrophobicity — but at the price of a new vulnerability: the polymer is organic, and organic material degrades chemically under surface discharge, UV and weather.

The physics follows a well-known chain. First, the surface loses hydrophobicity, through contamination, UV ageing or prolonged exposure to moisture, and water stops forming discrete droplets and starts forming a continuous film. Second, that film, made conductive by deposited salts, carries a leakage current of milliamperes that heats it by Joule effect. Third, the heating evaporates water non-uniformly and opens dry bands: narrow stretches of dry surface that now hold practically the whole voltage. Fourth, the local field in those bands exceeds the strength of air and dry band arcs ignite — small arcs at temperatures in the thousands of kelvin, anchored on the polymer surface. Fifth, under those arcs, polymers with aromatic rings or free carbon in the chain pyrolyse into conductive carbon. The track is born, and it lengthens with every wetting cycle until it bridges the electrodes.

The inclined plane test in detail

The world reference test for tracking and erosion resistance of materials is the inclined plane of IEC 60587:2022, which cancelled and replaced the 2007 edition with an improved experimental description, a stricter electrode specification, and removal of Criterion B from Method 2 where it does not apply. The principle is to replicate, in six hours, the dry band to arc to carbonisation chain under rigidly controlled conditions.

The specimen is mounted with the test face downward, inclined at 45 ± 2°, with stainless steel electrodes of type 302 (18% Cr, 8% Ni), 0.5 mm thick, spaced 50 ± 0.5 mm apart, new for every test, with deburred edges. Between the upper electrode and the specimen sits a block of eight layers of filter paper 0.2 ± 0.02 mm thick that acts as a reservoir: the contaminant is pumped into the paper and runs as a film down the lower face to the bottom electrode, which drains it. The power circuit requires an AC source of 45 to 65 Hz, sinusoidal with total harmonic distortion of 5% or less and a crest factor of √2 ± 5%, variable to about 6 kV with a rated current of at least 0.1 A per specimen and voltage stabilised within ±5% under load. In series with each specimen sits a 200 W resistor whose value depends on the test voltage, and a time-delayed overcurrent relay that disconnects the specimen when 60 ± 6 mA persists for 3 ± 1 s — the physical embodiment of Criterion A. Multi-specimen tests require individual sectionalising so that one failure does not interrupt the others.

Methods, criteria and the honest reading of a statistical test

Method 1 at constant voltage is the preferred and most used approach, because it requires no continuous inspection: a fixed voltage — typically 2.5, 3.5 or 4.5 kV — is applied for 6 h, generally on five specimens simultaneously. The material is classified by the highest voltage at which all specimens survive the period, and the designation records method, criterion and voltage — class 1A4.5 means Method 1, Criterion A, 4.5 kV. Method 2 with stepped voltage starts at an initial level and rises 250 V every hour until failure, classifying the material by the last step survived. Method 2 is faster for comparative screening of formulations, but Method 1 is what appears in product specifications.

The inclined plane is a statistical test by nature. The position of dry band arcs is random, and scatter between nominally identical specimens is large, which is why the minimum set is five specimens and why all of them must survive for the class to be assigned. The classic sources of spurious variability are: contaminant outside specification with unverified resistivity; irregular flow or unevenly saturated filter paper; reused electrodes with damaged edges; heterogeneous wettability across specimens; and parasitic arcing at edges or mounting holes, which carbonises outside the useful region and can trip the relay without representing the material at all. A mature reading of the test is therefore never one specimen that failed, but the pattern: clustered times to failure, consistent erosion, and tracks starting from the lower electrode in the central region. A discrepant result from an isolated specimen calls for inspection of the mounting before condemning the material.

Specifying climatic tests without wasting laboratory hours

Climatic tests are not a menu from which you order one of each. Each chamber replicates a specific degradation agent, and specifying the wrong chamber costs weeks of testing and yields no information. The design question is always the same: which agent dominates degradation at the installation site? Salt atmosphere corrodes fittings and couplings. UV photodegrades the polymer. Moisture combined with pollution on an energised surface produces dry bands and tracking.

Three patterns of waste appear frequently in specifications. Demanding thousands of hours of salt spray for a component whose field failure mode is tracking, where the right chamber would be tracking and erosion or fog under voltage. Demanding both xenon and fluorescent UV simultaneously with no failure hypothesis that distinguishes them, which duplicates cost with no new information. And specifying chamber hours as if they were years of life, a conversion with no physical basis, because acceleration depends on the material, the mechanism and the site. The correct wording of a climatic requirement always has four elements: standard and method with edition; duration and cycle; the property measured before and after — retained elongation, colour change, erosion depth, residual tracking voltage; and a numerical acceptance criterion. A requirement without a measured property is ritual, not engineering.

Syllabus

• The chain of slow failure: hydrophobicity loss, conductive film, dry band, arcing, pyrolysis, track

• Tracking against erosion, and why the chemistry of the polymer decides which one happens

• Chemical defences: alumina trihydrate and the low-molecular-weight migration of silicone

• Hydrophobicity classes HC1 to HC7

• IEC 60587:2022 in detail: specimen, preparation, wettability, and what surface abrasion means

• Arrangement, electrodes, filter paper reservoir, series resistor and the overcurrent relay

• The contaminant: composition, wetting agent and controlled resistivity

• Coupled parameters: test voltage, series resistor and contaminant flow rate

• Method 1 at constant voltage and Method 2 in steps, and the resulting material classification

• End-of-test criteria A and B, and erosion depth measurement after the test

• Interpretation and the statistical nature of the test: sources of spurious variability

• From material to component: IEC 62217 and the 1 000 h salt fog test

• The low voltage relatives: comparative and proof tracking indices per IEC 60112

• Salt spray per ISO 9227: NSS, AASS and CASS variants

• UV and artificial weathering per ISO 4892-2 and -3: xenon arc against fluorescent UV

• Fog, rain and artificial pollution under voltage

• When a climatic test is wasted, and the four elements of a correct climatic requirement

• Laboratory-to-field correlation and its honest limits

Laboratory work

Attend a tracking test in progress and analyse the specimens afterwards, in three blocks. Before, verifying the setup: check the contaminant resistivity with a calibrated conductivity meter against the 3.95 ± 0.05 Ω·m target; check the flow rate with a timed graduated cylinder against the parameter table; inspect the electrodes for newness and deburring and the filter paper block; verify wettability on all five specimens and record any abrasion; check the overcurrent relay setting at 60 mA and 3 s and the individual sectionalising per specimen. During, reading the regimes: watch the leakage current of each specimen and classify, at each interval, the regime as initiation, acceleration or failure; visually locate the dry band arcs and distinguish them from parasitic edge arcs; record the instant of the transition from initiation to acceleration for every specimen. After, post-test analysis: photograph and map tracks and erosion zones; measure the maximum erosion depth of each specimen with a depth gauge; compare the failure pattern between specimens, clustered or discrepant; and write the classification, for example 1A3.5, plus the report paragraph with the surface caveats. Complementary directed study: given a hypothetical material that failed at 4.5 kV by erosion with no track, write the complete climatic specification clause — standard, method, duration, measured property and criterion — for that material applied in a coastal compact network.

Key takeaway

Specify the chamber that matches the dominant degradation agent, and write the requirement with a measured property and a numerical criterion or do not write it at all. Accelerated tests reject the inadequate; only field service approves anything for thirty years.

Module 22 of the Atlas Energy Academy · Diagnostic Methods · about 6 hours · Video series (6 × 8–12 min) plus a test setup visit · Prerequisites: Part IV completed