PAT Tester Calibration: Frequency, Checks and Records
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A PAT tester gives you a reading. The question is: is the reading accurate?
Without calibration, you don't know. The instrument measures what it measures, but whether those measurements are within tolerance — whether 0.05 ohms on the screen really is 0.05 ohms, whether 1.2 megohms really is 1.2 megohms — depends on the equipment having been verified against a known standard at appropriate intervals.
This post covers what calibration actually is, why it matters for PAT testing specifically, how often it's needed, what it costs, and what happens if you skip it.
What calibration actually is
Calibration is the process of comparing the readings of your PAT tester to a known reference standard, identifying any drift from accuracy, and either adjusting the tester or documenting the deviation.
It's done by sending the tester to a calibration laboratory (or, for some testers, having an engineer come to you with reference equipment). The laboratory:
- Runs a series of test signals through your tester
- Compares the tester's readings to traceable reference values
- Records the differences
- Either adjusts the tester to bring it back into tolerance, or records the offsets as a calibration certificate
- Issues a calibration certificate showing the readings, dates, traceability and any limitations
The calibration certificate is what you keep. It's the document that proves your tester was performing accurately on the date of calibration.
Why calibration matters for PAT testing
Calibration compares the instrument’s measurements with suitable reference standards. It helps establish accuracy at the time of calibration; it does not guarantee every future measurement or replace checks of leads and instrument condition.
Acceptance limits must match the equipment and the selected test. Protective-conductor resistance and insulation resistance assess different things: a low reading is not a universal pass or fail across every test.
How often calibration is needed
Follow the instrument manufacturer’s instructions, its use and condition, and any applicable scheme or contract. Seaward advises calibration every 12 months for its PAT instruments; that recommendation must not be presented as a universal legal rule.
Heavy use, damage, repair or questionable results can justify earlier checks. There is no basis for promising that 12 months is always sufficient for occasional users.
Why 12 months specifically?
The 12-month interval is supported by manufacturer guidance and industry consensus. PAT testers are robust instruments but they're still electronic devices subject to:
- Component aging and drift
- Temperature variation effects
- Mechanical wear of switches, leads and connectors
- Battery condition affecting analog measurements
- Environmental contamination
Calibration provides evidence about the instrument at the time it is checked. Continue verification and condition checks between calibrations, and investigate damage or suspect results promptly.
What does calibration cost?
UK 2026 calibration prices for typical PAT testers:
- Standard pass/fail PAT tester: £40-£60 per calibration
- Mid-range downloading PAT tester: £50-£80 per calibration
- Advanced multi-function tester: £60-£100 per calibration
Cost typically includes:
- Bench calibration of all measurement functions
- Functional check of all features
- Battery condition check
- Calibration certificate with traceability
- Return shipping
Some calibrators offer collection-and-delivery for £15-30 extra, useful for businesses without postage handling capability.
For multi-tester operations, framework agreements with calibration laboratories often reduce per-unit costs to £30-40 per tester.
What does the calibration certificate look like?
A typical calibration certificate includes:
- The instrument identifier (make, model, serial number)
- Date of calibration
- Next calibration or verification review according to the agreed maintenance plan
- The calibration laboratory's accreditation (typically UKAS Accredited)
- Reference standards used and their own calibration traceability
- Specific readings: actual vs reference for each measurement function
- Any tolerances or notes
- Authorised signature of the calibration engineer
- Reference number
The certificate is your evidence. Keep both physical and electronic copies.
Who calibrates UK PAT testers?
Use the manufacturer or a calibration laboratory with appropriate competence and scope for the instrument. Where UKAS accreditation is required, verify that the relevant calibration is within the laboratory’s accredited scope.
Metrological traceability and laboratory accreditation are related but different. Do not assume every traceable calibration is UKAS accredited or that an unaccredited service is automatically worthless.
When calibration is needed beyond the annual schedule
In addition to the standard annual cycle, calibration is needed:
After equipment damage
If your tester is dropped, bumped hard, or shows signs of physical damage, calibration confirms it's still within tolerance.
After internal repair
Any repair work — replacing leads, fixing battery contacts, replacing internal components — should be followed by calibration to confirm the repair didn't affect accuracy.
After significant environmental exposure
Testers exposed to extreme conditions (very wet sites, high humidity, dusty environments) may benefit from earlier calibration.
Before an audit or major test
If you're being inspected or your work is being reviewed, ensuring you have current calibration evidence avoids the awkward situation of presenting marginally-out-of-date documentation.
The legal position
Keep test equipment suitable and reliable for the work. No universal statutory annual PAT-tester calibration interval is established here. Manufacturer instructions and a particular customer, insurer or certification scheme may impose specific requirements; check them directly.
Records and tracking
Keep:
- All calibration certificates throughout the life of the tester
- A calibration history log showing dates, results, and any adjustments
- Next calibration or verification review according to the agreed maintenance plan
For multiple testers, a tracker spreadsheet showing:
Software-based asset management (PATGuard, SimplyPats etc.) typically handles this automatically.
What happens if you skip calibration?
The realistic consequences:
Insurance implications
Read the policy conditions and ask the insurer about required calibration or verification evidence. An overdue calibration is not, by itself, proof that every policy or claim is invalid.
Client refusal
Most professional clients ask for proof of calibration. Showing them a 18-month-old calibration certificate is a problem; not having one at all means they won't engage you.
Audit failure
In any compliance audit, calibration is one of the first things checked. Out-of-date calibration means the testing carried out using that tester is questioned.
Increased risk of false readings
Beyond the documentation issue, an uncalibrated tester might genuinely be giving false readings. If an appliance is actually faulty but your tester reads it as passing, that's a real safety problem.
Reputational damage
In the small UK PAT testing community, working without calibration develops a reputation. It's a corner that respected testers don't cut.
What about self-calibration?
A few testers offer self-test functions or "auto-calibration" features. These typically:
- Compare internal measurements against internal references
- Useful as a daily check that nothing major is wrong
- NOT a substitute for laboratory calibration
An internal self-test does not automatically replace external calibration or other manufacturer-required verification. Check which functions it covers, follow the instrument’s instructions and keep the appropriate accuracy and maintenance records.
Battery and self-test checks
Beyond formal calibration, daily checks worth doing:
- Battery check: most testers have a battery indicator; check it's adequate before any testing session
- Lead check: visually inspect test leads for damage
- Self-test: where available, run the tester's internal self-test before commencing
- Reference test: some testers come with a reference resistor that can be used to verify earth continuity readings before each session
These don't replace calibration but they catch problems between calibration cycles.
Frequently asked questions
How often should I calibrate my tester?
Follow the manufacturer and any applicable contractual requirements, taking account of use, damage and verification results. A twelve-month recommendation is common for some manufacturers, but is not universal.
Does a check box replace calibration?
Not automatically. Follow the manufacturer’s verification instructions and understand which functions the check device actually verifies.
What if it is out of tolerance?
Stop relying on the affected measurements. Arrange competent advice, repair/calibration and a review of potentially affected earlier results.
What records should I keep?
Keep the calibration results, instrument identity, verification checks and any investigation or corrective action. Record the next planned review using the applicable requirements.
The takeaway
Manage calibration as part of instrument reliability: suitable equipment, sound leads, routine checks and an appropriate calibration plan. Keep the evidence and investigate unexpected readings.
