How Does PAT Testing Work? The Process Explained

If you've been told your workplace or rental property needs PAT testing, you're probably wondering what actually happens when an engineer turns up with a testing box. How does PAT testing work in practice, beyond the sticker left on your kettle or laptop charger? It's a fair question, and the answer matters if you're responsible for keeping staff, tenants, or customers safe.

In short, PAT testing combines a visual inspection of each appliance with a series of electrical safety checks using a calibrated tester. The engineer looks for damage, wear, and incorrect fuses first, then runs tests for earth continuity, insulation resistance, and polarity depending on the appliance class. Anything that fails gets flagged, labelled, and removed from use until it's repaired or replaced.

We've tested thousands of appliances across offices, salons, warehouses, and rented flats throughout London, so we know exactly what clients want to understand before we arrive. This article walks through the full process step by step, from sorting appliances by class to reading the results and what your compliance certificate actually confirms once testing is complete.

Why PAT testing matters for safety and compliance

Faulty appliances start fires, and they injure people. That's the blunt reason PAT testing exists, and it's why landlords, employers, and facilities managers build it into their annual routine. Fire and rescue services attend thousands of electrical fires in homes and workplaces every year, and a good number trace back to appliances that were never checked or were left in service after damage appeared. Regular testing catches the fraying cable, the cracked plug, or the appliance that's been quietly overheating before it becomes a headline. Understanding how does PAT testing work isn't just academic curiosity, it's the difference between spotting a hazard on a Tuesday afternoon and dealing with an insurance claim six months later.

Legal duties under health and safety law

Employers have a duty under the Health and Safety at Work Act 1974 and the Electricity at Work Regulations 1989 to keep equipment safe for use. Nowhere does the law state "PAT test every appliance every 12 months", which surprises a lot of business owners. Instead, the Health and Safety Executive expects a proportionate, risk-based approach, and PAT testing is simply the recognised way most businesses demonstrate they've met that duty. Landlords face a similar expectation under the Electrical Equipment (Safety) Regulations 1994, which requires that anything supplied to tenants is safe when handed over and stays safe throughout the tenancy.

If an appliance injures someone and you can't show you checked it, you're the one explaining that gap to an inspector or a solicitor.

What happens when testing gets skipped

Skipping PAT testing rarely causes an immediate problem, which is exactly why it's tempting to put off. Months pass, nothing goes wrong, and it feels like an unnecessary cost. Then a frayed extension lead in a stockroom shorts out overnight, or a tenant gets a shock from a kettle with a loose earth wire, and suddenly the absence of any testing record becomes the central issue. Businesses that skip testing also run into trouble during insurance disputes, because insurers increasingly ask for evidence of a maintained electrical safety regime before they'll pay out on a fire or injury claim. We've seen claims delayed for months simply because a client couldn't produce a single PAT certificate.

Insurance, tenancies, and due diligence

Commercial leases and insurance policies often reference electrical safety checks directly, sometimes by name. Insurers may treat an EICR and PAT testing together as the baseline for cover on fire and public liability policies, and some policies specify testing intervals as a condition of the contract. Landlords letting under an Assured Shorthold Tenancy also need to show they've taken reasonable steps to keep supplied appliances safe, and a dated PAT certificate is the clearest evidence available if a dispute reaches a tribunal. For commercial tenants and landlords alike, a compliance certificate from a PAT test does more than tick a box, it becomes the paper trail that protects you if something does go wrong.

How the PAT testing process works step by step

Every engineer follows roughly the same sequence, regardless of whether they're testing forty laptop chargers in an office or a handful of appliances in a rented flat. Knowing how does PAT testing work step by step helps you understand why an inspection that looks quick from the outside actually covers a lot of ground. The process moves from a visual check through to electrical measurements, then finishes with labelling and paperwork that proves the work happened.

Visual inspection comes first

Before any tester gets plugged in, the engineer examines the appliance, the plug, and the cable by eye. This step catches a surprising number of faults on its own, and it's the reason experienced engineers never skip straight to the electrical tests.

• Cable condition: checking for fraying, cuts, or exposed wire

• Plug top: confirming the correct fuse rating and no cracks in the casing

• Casing damage: looking for cracks, burn marks, or missing covers on the appliance itself

• Signs of misuse: taped joints, non-standard repairs, or overheating marks

Any appliance that fails this visual check gets pulled from service immediately, because there's no point running electrical tests on something with an exposed live wire.

Running the electrical tests

Once the appliance passes the visual stage, the engineer connects it to a PAT tester, a calibrated device that runs a set of standard checks. These typically include earth continuity, insulation resistance, and, for certain appliance types, a polarity check and a load test under near-normal operating conditions. The exact combination depends on the appliance's class, which we cover in detail next.

A visual check catches the obvious faults, but the electrical tests catch the ones you'd never spot by looking.

Labelling, recording, and certifying results

After testing, every appliance gets a label showing the test date and the next due date, so anyone checking later knows the status at a glance. Results feed into a test record, which lists every appliance, its location, and whether it passed or failed. Finally, the engineer issues a compliance certificate summarising the whole batch, which becomes your evidence for insurers, tenants, or a health and safety inspector. Failed items get flagged separately with a clear note on what needs repairing or replacing before they go back into use.

How appliance classes affect the testing method

Not every appliance gets tested the same way, and this is where a lot of confusion creeps in for people outside the trade. The appliance class determines which electrical tests apply, because Class I and Class II appliances are built with completely different approaches to earthing and insulation. Get the classification wrong, and you either waste time running a test that's irrelevant or, worse, miss a check that matters. Any engineer who understands how does PAT testing work properly starts by sorting appliances by class before touching the tester.

Class I appliances need an earth check

Class I appliances have a metal casing or exposed metal parts, and they rely on an earth connection to stay safe if a fault develops. Kettles, toasters, and most desktop computers fall into this category. For these, the engineer runs an earth continuity test to confirm the earth wire provides a low-resistance path back to the plug, alongside insulation resistance and, where relevant, a load test. Skip the earth check on a Class I appliance and you've missed the single most important safety measure it depends on.

Class II appliances rely on insulation alone

Class II appliances have no earth connection at all. Instead, they're built with double or reinforced insulation, so there's no metal casing for a fault to reach in the first place. Laptop chargers, many power tools, and most modern televisions fall here. Testing focuses on insulation resistance rather than earth continuity, since there's no earth pin to test. Trying to run an earth continuity check on a Class II appliance simply won't produce a meaningful result, because the design doesn't include that path.

Testing the wrong class the wrong way gives you a pass certificate that means nothing.

Comparing Class I and Class II at a glance

Why spotting the symbol matters

Engineers check the rating plate for the double-insulation symbol, a small square inside another square, before deciding which tests to run. It's a two-second check that prevents wasted time and incorrect results further down the line. If you're managing appliances yourself before an engineer arrives, sorting them into rough groups by this symbol speeds up the whole visit and gives you a better sense of what the final compliance certificate is actually confirming for each item.

How often you need PAT testing and who can do it

There's no single legal interval for PAT testing, and that trips up a lot of people who expect a fixed number written into law. Instead, the frequency depends on the type of appliance, how often it's used, and the environment it sits in. A construction site drill gets knocked about far more than an office monitor, so it needs checking more often even though both are electrical appliances doing a job.

Typical testing intervals by environment

Most engineers work from guidance published by trade bodies rather than a legal timetable, adjusting intervals based on risk. Here's a rough guide we use when advising clients across London offices, salons, and rented flats:

There's no fixed legal interval for PAT testing, but there is an expectation that you can justify whatever interval you've chosen.

Who can carry out PAT testing

Anyone can technically buy a PAT tester, but running the tests safely and interpreting the results correctly takes proper training. A competent person, as the Health and Safety Executive describes it, needs enough electrical knowledge to understand what the readings mean, spot a borderline result, and know when an appliance needs deeper investigation rather than a simple pass or fail. That's usually an electrician or a technician who's completed a recognised PAT testing course, not someone reading a manual for the first time on the day.

Hiring a qualified engineer matters for a second reason too: the compliance certificate only carries weight if the person signing it actually had the competence to test properly. An inspector or insurer looking at a certificate wants to see it came from someone who understood what they were checking, not just someone who plugged a box in and wrote down whatever number appeared. Engineers with several years of hands-on experience also spot patterns a newcomer might miss, like a particular brand of extension lead that consistently fails insulation checks, or a batch of appliances from the same supplier showing the same fault.

Setting your own schedule with confidence

Once you understand the risk factors at play, setting a sensible testing schedule for your own premises becomes far more straightforward. Start with the environment, factor in how the appliances actually get used day to day, and adjust the interval upward if you notice more faults turning up at each visit than expected.

Keeping your appliances safe and compliant

Now you know how does PAT testing work: a visual check first, then earth continuity, insulation resistance, and other electrical tests matched to the appliance class, finishing with labelling and a signed certificate. None of that is complicated once you've seen it done, but it does need a competent engineer who understands why a Class I kettle and a Class II laptop charger get tested differently, and who can justify the interval they've set for your premises.

Getting this right protects staff, tenants, and customers, and it gives you a paper trail if an insurer or inspector ever asks questions. If you're due a test, or you've never had one and want to know where you stand, get a quote from our team and we'll sort out the rest.