A quick answer is No, a PAT test cannot detect every electrical fault, and no competent tester will claim it can. A PAT test confirms that an appliance was safe at the moment it was tested — under test conditions, on that date, in one person’s hands.
Some faults sit outside the method’s reach entirely, others develop the day after the label goes on, and a badly performed test can pass a dangerous appliance. This page sets out what the test catches, what it structurally cannot, and which measures cover the rest.
What a PAT Test Actually Checks
A PAT test combines two stages: a visual inspection and a set of electrical measurements. The two stages find different faults, and neither works properly without the other.
The visual inspection comes first, and it does most of the work. Around 90% of faults surface at this stage — damaged plugs, frayed or repaired cables, incorrect fuses, burn marks around the pins, and wiring errors inside rewirable plugs. A tester who rushes this stage misses the majority of what the whole process exists to find. The full checklist sits in our guide to visual inspection in PAT testing.
The electrical tests then find what the eye cannot. Earth continuity confirms that the protective conductor still runs unbroken from the plug to the appliance’s exposed metalwork. Insulation resistance confirms that the barrier between live conductors and touchable parts has not broken down. Leakage current measures how much current escapes where it should not. A cable can look perfect and hide a snapped earth conductor inside intact sheathing — only the meter finds that. Both stages together make up portable appliance testing as the IET Code of Practice, 5th Edition defines it.
What faults can a PAT test detect?
A PAT test detects visible damage — broken plugs, frayed cables, incorrect fuses, poor repairs — plus hidden electrical faults: broken earth conductors, degraded insulation between live parts and touchable surfaces, and excessive leakage current.
The Faults a PAT Test Cannot Detect
The Health and Safety Executive settles the question of scope in its own guidance: most defects can be found by visual examination, some only by testing, and some types of electrical safety defect cannot be detected by testing alone. The limits below come from the regulator and from the physics of the method, not from hedging on our part.
| Fault | Why the PAT test misses it | What catches it instead |
|---|---|---|
| Intermittent faults | A loose connection that only fails when the cable flexes at one angle, or a motor fault that appears once the appliance heats up under load, passes a bench test at room temperature | User checks during use; investigation of reported symptoms |
| Damage after the test date | The appliance was sound on Tuesday; a trolley crushed the cable on Wednesday. The label records Tuesday | User checks before each use |
| Internal wear approaching failure | Insulation that reads above the 1 MΩ threshold today but is degrading still passes. A single pass/fail result hides the trend | Records of measured values compared over time |
| Fixed wiring and installation faults | PAT stops at the plug. A damaged socket, worn ring circuit or faulty RCD sits behind the point the tester can reach | An EICR on the fixed installation |
| Misuse and environment | Daisy-chained extension leads, cables run coiled under load, and appliances used in conditions they were never rated for all sit outside what a bench test examines | Risk assessment and user training |
Two of these cause the most surprise. An intermittent fault is invisible to a snapshot measurement by definition: the test applies a known voltage for a few seconds to a cold, stationary appliance, and a fault that needs heat, vibration or a specific cable position never appears in that window. A drill that trips its supply after twenty minutes of load will pass every bench test its owner pays for.
Degradation works the same way, because it is a trend rather than an event. An insulation reading of 1.2 MΩ passes this year, 1.05 MΩ passes next year, and the pattern only becomes visible to someone who records the numbers instead of the verdict.
Can an appliance pass a PAT test and still be dangerous?
es. An appliance can pass while carrying an intermittent fault, suffer damage after the test date, or hide degrading insulation that still reads above the pass threshold. A pass records one moment, not the future.
What faults can a PAT test not detect?
A PAT test cannot detect intermittent faults, damage that occurs after the test, gradual internal degradation still above pass thresholds, faults in the fixed wiring behind the socket, or hazards created by misuse and environment.
Why a Pass Label Is Not a Safety Guarantee
A pass label records one fact: the appliance met the pass criteria at the moment of test, under test conditions, measured by one person. It is not a warranty, and the method was never designed to produce one. The green label is a dated record, not a promise about the year ahead.
This reasoning drove the shift in the IET Code of Practice, 5th Edition, away from fixed retest intervals and towards risk assessment. The useful question is not “when was this last tested” but “how quickly could this appliance become dangerous in this environment”. A drill on a building site and a monitor on an office desk age at completely different speeds, and a single calendar rule serves neither. Retest intervals should follow the risk — the reasoning is set out in how often PAT testing should be done.
Does a PAT test guarantee an appliance is safe?
No. A pass confirms the appliance met the test criteria at the moment of testing. It carries no guarantee about future condition, which depends on how the appliance is used, handled and stored afterwards.
How long is a PAT test valid for?
A PAT test has no fixed validity period. The IET Code of Practice, 5th Edition recommends retest intervals based on risk assessment — from months for construction site equipment to several years for low-risk office items.
False Passes: When the Test Itself Goes Wrong
Even the faults the method is designed to catch can slip through when the test is performed badly. Four errors account for most false passes.
An appliance left switched off during the insulation or leakage test is the most common. The test voltage stops at the open switch and never reaches the internal components, so a faulty heating element sails through with a perfect reading. The instrument displays a pass; the operator has measured nothing.
A Class II appliance tested without the probe lead connected to its accessible metalwork produces the same illusion. The tester reads near-infinite resistance — not because the insulation is sound, but because the measurement circuit was never completed.
An uncalibrated tester drifts. Readings shift over time, and both false passes and false fails become possible without the operator noticing anything wrong.
The wrong test for the appliance class rounds out the list. A standard 500 V d.c. insulation test on IT equipment with EMI filtering gives misleading results; a substitute leakage test is the correct choice there.
Every one of these is an operator error, not an equipment flaw. The difference between a meaningful pass and a meaningless one is the competence of the person holding the tester. The same errors have a mirror image — false fails — covered in what happens when an appliance fails a PAT test.
Can a PAT tester give a false pass? Yes. A test run with the appliance switched off, a disconnected Class II probe lead, an uncalibrated instrument, or the wrong test for the appliance class can each pass a faulty appliance.
Can a PAT test be wrong? Yes, in both directions. Operator error produces false passes on faulty appliances and false fails on sound ones. A calibrated instrument, the correct test sequence and a trained operator keep both to a minimum.
What Covers the Faults PAT Testing Misses
The limits above are managed, not fatal. Each gap maps to a compensating measure, and together they form the maintenance regime that HSE guidance actually describes.
User checks before each use catch what the test date cannot. A thirty-second look at the cable, plug and casing — no instrument, no training beyond basic awareness — is the only measure that finds Wednesday’s trolley damage, and HSE recommends it as part of any electrical maintenance regime.
Risk-based retest frequency shrinks the window in which a post-test fault can go unnoticed. Harsh environments and heavily handled equipment get shorter intervals; stable, low-risk equipment gets longer ones.
Fixed installation testing covers everything behind the socket. An Electrical Installation Condition Report (EICR) examines the building’s wiring, sockets, circuits and protective devices — territory PAT structurally cannot reach. The two are complementary, not interchangeable: PAT tests what plugs in, an EICR tests what it plugs into. The full distinction sits in PAT testing vs electrical installation testing.
Records with measured values close the degradation gap. A log of actual readings, kept in line with the IET Code of Practice, turns individual snapshots into a trend, and a falling insulation figure flags a failing appliance long before it drops below the threshold.
What is the difference between PAT testing and an EICR? PAT testing examines portable appliances that plug in — the appliance, its cable and its plug. An EICR examines the building’s fixed electrical installation: wiring, sockets, circuits and the consumer unit. Most premises need both.
How often should PAT testing be done? No legal interval exists. Frequency follows risk assessment: tools on a construction site may need testing every three months, while double-insulated office equipment may go two to four years between formal tests.
A PAT test is one layer in a maintenance system — a good layer, and the right tool for the faults it targets — but it was never meant to stand alone. The limits set out above are not weaknesses to hide; they are the boundaries a competent tester works within, and the tester who knows exactly where they sit is doing a different job from someone pressing buttons on an instrument. PAT Training Now’s 1-day course covers the full IET 5th Edition Code of Practice, including how to recognise when a result cannot be trusted. Course dates and locations: pat-training-now.co.uk/pat-venue-finder.
