PAT Tester Calibration

Calibration helps confirm that a PAT tester is measuring within its specified tolerances. The IET Code of Practice recommends that PAT test instruments are calibrated annually.

This article summarises the main calibration options, what to check between calibrations, and what to do about calibration reminder messages on some testers.

Calibration Options at a Glance

• Manufacturer calibration is a good choice where warranty conditions apply or where authorised resets may be required.
• Independent calibration is often lower cost, but check warranty terms and whether any manufacturer-only functions need resetting.
• Onsite calibration can reduce downtime if the tester is used frequently.
• Wholesaler calibration days can be convenient and cost-effective, with minimal time out of service.

What Calibration Confirms

Calibration confirms the tester is measuring correctly at the time of calibration. It does not guarantee accuracy indefinitely, which is why basic periodic checks and good handling (leads, connectors, and avoiding knocks) matter between calibrations.

Manufacturer Calibration

Most PAT tester manufacturers offer a courier pick-up and drop-off service. This is often the most expensive option, but it can be advisable if your tester is within an extended warranty period or needs an authorised reset.

• Fluke
• Kewtech
• Megger
• Metrel
• Seaward

Specialist Calibration Companies

Independent calibration companies can offer a similar pick-up and drop-off service and are often cheaper than the manufacturer. Before booking, check whether your tester has any manufacturer-only features (such as calibration reminder resets) and whether using a non-authorised provider could affect any extended warranty.

• The Calibration Centre

Onsite Calibration

Onsite calibration can be a good option if you want to minimise downtime, especially where the tester is used daily or you have multiple instruments to keep in service.

• On Site Calibration Services

Wholesaler Calibration Days

Some electrical wholesalers offer regular calibration days where you can take a tester in and have it calibrated the same day. These events are sometimes run by manufacturers such as Metrel, Megger, and Seaward. If your tester has a calibration reminder message, check in advance whether it can be reset as part of the service.

• Denmans calibration days
• Re-CAL calibration services
• Bemco
• ACS calibration days (CEF)
• Edmundson Electrical

Accuracy Checking Between Calibrations

As well as annual calibration, it is sensible to carry out periodic checks to confirm the tester is giving consistent results. There are a few common approaches:

• Built-in check facilities on some testers (for example, internal check functions on certain models).
• A separate checkbox (commercial or homemade). We also have an article on making a checkbox.
• A reference appliance, which is often the simplest and lowest-cost method.

Using a reference appliance: designate a Class I appliance as a reference and clearly mark the exact point where the earth lead is connected. Test and record readings for the same test sequence each time (for example, earth continuity and insulation resistance). Use the same appliance, leads, and connection point for each subsequent check. If readings change significantly (a common rule of thumb is more than about ±5%), investigate before relying on the tester.

Calibration certificates and periodic accuracy checks are easiest to manage when they form part of a simple documented system (equipment register, responsibilities, and record keeping). For a practical framework, see our guide to managing PAT testing in the workplace.

New Equipment

New test equipment does not normally need calibration on purchase. Manufacturers typically include a declaration or certificate of conformity to show the instrument has passed production testing and met their specification at the point of manufacture. In most cases, the first calibration is due 12 months after the instrument is first put into service.

Calibration Reminder Messages

Some PAT testers display a calibration reminder message once the due date has passed. On some models this can only be reset by the manufacturer or an authorised calibration provider. The reminder does not usually stop the tester operating, but it can be a prompt to book calibration and it may affect day-to-day usability until calibration is completed.