Class I vs Class II appliances: what gets tested

Why construction class matters

Before any instrument test runs, a competent tester identifies how the appliance is built. UK equipment falls broadly into Class I (earthed metal or exposed conductive parts), Class II (double insulated — no reliance on an earth wire), or Class III (extra-low voltage). The class determines which tests are appropriate and which failures are possible.

Getting this wrong matters: applying a high-current earth bond test to sensitive IT equipment can damage it, while skipping earth continuity on a Class I kettle leaves a serious shock hazard undetected. That is why PAT is not simply “plug it in and press go”.

Class I — earthed appliances

Class I equipment uses basic insulation plus a connection to the protective earth in the plug. Think kettles, toasters, microwaves, most white goods, and many tools with metal bodies. If internal insulation fails, the earth path should carry fault current and blow the fuse rather than leaving the case live.

Combined inspection and testing for Class I typically includes: formal visual inspection, earth continuity (protective conductor resistance), insulation resistance (or an alternative leakage measurement where 500 V DC is unsuitable), and a functional check. These are the failures we most often see on extension leads and kitchen appliances — see our guide on common PAT failures for examples.

Class II — double insulated

Class II appliances do not depend on an earth connection for safety. They use two layers of insulation (basic plus supplementary), so there is no earth pin requirement for safety — though many still have three-pin plugs for other reasons. Look for the double-square symbol (two concentric squares) on the rating plate; if there is no symbol but the appliance clearly has no earth dependency, it is still treated as Class II for testing purposes.

Testing focuses on insulation resistance (or touch/alternative leakage where appropriate) and a functional check — no earth continuity test. Phone chargers, many desk lamps, plastic-cased drills, and a lot of office IT fall here. Class II equipment can still fail: cracked casings, damaged flexes, and failed insulation are common.

Class II FE and Class III (briefly)

Some IT power supplies are Class II FE (“functional earth”) — double insulated for shock protection but with an earth connection for electromagnetic screening or noise suppression. They are tested like Class II with an additional functional earth continuity check where required.

Class III equipment runs at extra-low voltage (typically from a SELV transformer) and usually needs only visual inspection in normal use — think some desk toys and low-voltage lighting systems. Your typical rental or office PAT visit is mostly Class I and Class II.

What this means for your register

A good PAT register records appliance class because it drives retest intervals and explains why two items beside each other on a desk received different tests. Test Harbour’s register captures class, location, and result so auditors see a coherent picture — not just a row of pass stickers.

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Booking or compliance questions for your premises? These pages go deeper on what we test locally.