PAT testers need calibration, and the standard interval is every 12 months. A PAT tester measures resistance and current, and those measurements drift over time. Calibration confirms the instrument reads within its specified tolerances on the day it is checked. It does not prove the tester stays accurate afterwards, so a check box or reference appliance confirms accuracy between calibrations.
The calibration certificate is the document that lets every recorded PAT result stand up. This guide covers what calibration is, how often a PAT tester needs it, how to check accuracy between calibrations, what the certificate proves, and what happens when a tester fails.
Do PAT Testers Need Calibration?
Yes, PAT testers need calibration, because the resistance and current measurements they make drift over time and an inaccurate tester passes appliances that should fail.
Every PAT result depends on the instrument. An earth continuity reading of 0.15 Ω means nothing if the tester reads 0.1 Ω low. A tester that has drifted gives a pass label to an appliance with a failing earth, and the register then records a safe result that is not safe. Calibration is the check that catches that drift before it reaches the register.
What Is PAT Tester Calibration?
PAT tester calibration is the comparison of a tester’s readings against reference values that are traceable to national standards. A calibration technician connects the PAT tester to reference equipment of known value, records each reading, and confirms whether the instrument sits within the manufacturer’s stated tolerance. Where a reading falls outside tolerance, the instrument is adjusted, repaired or reported as failed. Traceability means the reference equipment has itself been checked against a higher standard, so a reading of 1 MΩ on one calibrated PAT tester means the same as 1 MΩ on another. Calibration covers each measurement the tester makes.
What Does Calibration Check on a PAT Tester?
Calibration checks every test function the PAT tester performs, which typically includes:
- Earth continuity, the low resistance reading in ohms (Ω), and the test current delivered.
- Insulation resistance, the high resistance reading in megohms (MΩ), and the test voltage applied, such as 250 V DC or 500 V DC.
- Leakage current, the protective conductor, touch or substitute leakage reading in milliamps (mA).
- Load or power, where the tester measures the current an appliance draws.
- RCD trip time, where the tester includes a portable RCD test.
A tester only needs checking on the functions it has. How often that check happens is the next question.
How Often Should a PAT Tester Be Calibrated?
A PAT tester should be calibrated every 12 months. Seaward, one of the largest UK PAT tester manufacturers, recommends annual calibration, and UK training providers cite the same annual interval from IET Code of Practice guidance. Twelve months is the interval nearly every in-house tester and contractor works to.
Heavy use shortens the practical interval. A tester that travels between sites every day takes more knocks than one used twice a year in the same office, and the check routine between calibrations shows whether that wear is affecting the readings. Twelve months is the default, not the ceiling.
When Is a PAT Tester Due for Calibration Early?
A PAT tester is due for calibration before the 12 months are up when any of the following apply:
- The tester has been dropped or knocked hard enough to damage the case, display or sockets.
- The tester has been misused, such as connected to a live supply it was not designed for.
- The readings are in doubt, for example when a check box result or reference appliance reading has shifted.
- The tester has been repaired, because a repair changes the internal components the readings depend on.
Seaward’s guidance names damage, misuse and any reason to doubt accuracy as triggers for calibration outside the annual cycle.
Does a New PAT Tester Need Calibrating?
A new PAT tester does not need calibrating on purchase. Manufacturers supply new instruments with a declaration or certificate of conformity showing the tester met its specification at production. The first calibration is normally due 12 months after the tester is first put into service. Some PAT testers display a calibration reminder message once the due date passes, and on certain models only the manufacturer or an authorised provider resets that reminder. Between the in-service date and the first calibration, accuracy is checked another way.
What Is a PAT Tester Check Box?
A PAT tester check box is a unit with fixed, known resistance values that confirms a PAT tester still reads accurately between calibrations. The tester is connected to the check box, each test is run, and the readings are compared with the values printed on the check box. A check box does not calibrate the tester, because it does not adjust anything and its own values are only as good as its last check. It proves consistency. Some PAT testers include a built-in check function that does a similar job. A check box is one of three ways to confirm accuracy between calibrations.
How Do You Check a PAT Tester Between Calibrations?
You check a PAT tester between calibrations with a check box, a built-in check function or a reference appliance. The reference appliance method costs nothing and follows these steps:
- Choose a Class I appliance that stays on site and mark it as the reference appliance.
- Mark the exact point on the casing where the earth lead connects.
- Run the same test sequence each time, such as earth continuity and insulation resistance.
- Use the same appliance, the same leads and the same connection point on every check.
- Record each reading with the date in a check log.
- Investigate any reading that moves by more than about 5% before relying on the tester.
The IET Code of Practice recommends checking test instruments at regular intervals between calibrations, and Seaward’s guidance says those check results must be recorded. Weekly or monthly checks suit most in-house testers; a daily check suits a tester in constant use. Those records sit alongside the calibration certificate.
What Does a PAT Tester Calibration Certificate Prove?
A calibration certificate proves the PAT tester measured within its specified tolerances on the date of calibration, and nothing beyond that date. The IET Code of Practice makes the same point: the certificate verifies the instrument at that time only, which is why regular checks follow it.
A certificate identifies the instrument it belongs to, records when it was calibrated, and states that the readings are traceable to national standards. Kept on file with the PAT register and the check log, it shows that every result recorded after that date came from an instrument known to be accurate. An insurer, an auditor or an enforcement officer asking how a result was produced looks for exactly that chain: a certificate, a check log, and a register. Proof of that kind applies equally to every type of tester.
Do Manual and Pass/Fail PAT Testers Need the Same Calibration?
Manual and pass/fail PAT testers need the same calibration, because both types measure resistance and current and both drift. A manual PAT tester shows the actual reading and leaves the pass or fail decision to the person testing. A pass/fail PAT tester compares the reading with a preset limit and shows only the verdict.
The pass/fail type makes calibration more important, not less. A manual tester shows a reading that a competent person notices when it looks wrong. A pass/fail tester hides the number, so a drifted instrument keeps giving confident passes with nothing on screen to question. Downloadable testers that store results are calibrated the same way. The question that follows is what happens when a tester is found outside tolerance.
What Happens If a PAT Tester Fails Calibration?
A PAT tester that fails calibration is repaired and recalibrated, or it is withdrawn from use. The calibration provider reports the readings that fell outside tolerance and quotes for the repair. A tester that cannot be brought back within tolerance is replaced.
The failed result also raises a question about the testing done before it. The duty holder compares the size and direction of the error with the results recorded since the last good check. A tester reading earth continuity low, for example, means recorded passes near the limit need retesting with a calibrated instrument. A tester reading high means some appliances were failed unnecessarily. The check log narrows the window: regular checks mean fewer results to review.
Is PAT Tester Calibration a Legal Requirement?
No law sets a calibration interval for PAT testers. The Electricity at Work Regulations 1989, regulation 4(2), require electrical systems to be maintained so as to prevent danger so far as is reasonably practicable, and PAT testing is one way duty holders show that maintenance is happening. The regulations say nothing about the instruments used.
Calibration matters because of evidence. A PAT result recorded on an uncalibrated tester is a result nobody can stand behind, so the register loses its value as proof of maintenance. The interval comes from manufacturers and the IET Code of Practice, and the certificate is what gives recorded results their weight. Evidence of that kind carries a cost.
How Much Does PAT Tester Calibration Cost?
PAT tester calibration with PAT Training Now costs £65+VAT (£78.00 including VAT), with collection and return carriage included. Prices across the UK market vary with the provider, the instrument and whether carriage is charged separately, so comparing quotes on an all-in basis avoids surprise handling fees. The other cost is time without the tester. A long turnaround stops an in-house testing programme, which is why turnaround time belongs in any comparison alongside price.
How Does the PAT Training Now Calibration Service Work?
The PAT Training Now calibration service works in four steps:
- Book online and pay for the calibration.
- Box up the tester. PAT Training Now emails a courier label and arranges collection.
- Technicians calibrate and check the tester, with readings traceable to national standards.
- The tester comes back by insured courier with its traceable calibration certificate.
Turnaround on the bench is typically 3 days, plus courier time each way. The service covers all makes and models of PAT tester, including Seaward, Kewtech, Martindale and Fluke, and multifunction testers too. If a tester cannot be brought back into tolerance, the repair quote comes first and nothing happens without the owner’s agreement. To book, go to PAT Calibration or call 01793 497181. Calibration is a separate service, so there is no need to have trained with PAT Training Now to use it.
How Does Calibration Fit Into Your In-House Testing Kit?
Calibration keeps the PAT tester accurate, and the rest of the testing kit needs the same attention. Test leads, probes and adaptors wear, and a damaged lead adds resistance that looks like an appliance fault. Storage in a cool, dry place and following the manufacturer’s guidance protect the instrument between calibrations. For the full list of what an in-house tester needs and which items are worth buying, see the guide to In-House PAT Testing Equipment.
Can you calibrate your own PAT tester?
A PAT tester is calibrated by a provider with traceable reference equipment; an in-house tester checks accuracy between calibrations but does not calibrate.
Is a check box the same as calibration?
A check box is not calibration; it confirms the tester still gives consistent readings between calibrations.
Does a calibration sticker replace the certificate?
A calibration sticker is a reminder on the instrument; the traceable calibration certificate is the record kept on file as evidence.
