When performing appliance testing, many technicians prefer leakage testing over traditional insulation resistance tests because the appliance is tested while fully energised and operating under normal load conditions. Motors run, heating elements energise, and control components such as relays or contactors change state — behaviour that may not occur during a static insulation resistance test.
When Leakage Testing Is Mandatory (AS/NZS 3760 Requirement)
AS/NZS 3760 specifically states that if an appliance must be energised to close, operate, or activate a switching device in order to test the insulation, then a leakage current test is the required method.
This means leakage testing is not just a preferred method — it is the only compliant method whenever the insulation cannot be properly evaluated without the equipment operating normally.
Leakage Testing for Class I Appliances (Earthed)
For Class I appliances, testers typically use the differential leakage method.
How Differential Leakage Works
A current transformer (CT) monitors the active and neutral conductors. Any imbalance between them indicates leakage returning through the protective earth. This leakage is displayed in milliamps (mA).
Pass/Fail Limit (AS/NZS 3760)
≤ 5 mA maximum allowable leakage.
Leakage Testing for Class II Appliances (Double-Insulated)
Since Class II appliances have no earth, the tester must create a return path using a test probe applied to exposed conductive parts.
Pass/Fail Limit (AS/NZS 3760) ≤ 1 mA maximum allowable leakage.
The way the leakage current is detected for class II appliances depends on the tester type, make and model.
1. Differential Leakage Method (Basic Testers)
Entry-level testers often use the same differential method for Class II equipment as they do for for Class I equipment. The probe acts as a return path and as a substitute for the earth. Any leakage on exposed conductive parts returns to earth and creates an imbalance between active and neutral. The tester interprets this imbalance as leakage current.
Pros
- Simple instrument design
- Cost-effective to manufacture
Cons
- Can be affected by electrical noise or supply variations
- Less accurate for small leakage levels
2. Touch Leakage Method (Advanced Testers)
Higher-end testers include a dedicated Touch Leakage mode for more precise measurements.
How Touch Leakage Works
The appliance operates normally under mains voltage. The probe connects to a specialised measurement circuit (not the differential CT). The circuit includes simulated human body impedance (typically a few kΩ). Leakage current is measured directly, not inferred.
Why It’s More Accurate than the differential method for Class II equipment
Touch leakage is less influenced by:
- Neutral–earth system noise
- Electrical interference
- Supply fluctuations
This makes it the most accurate and repeatable method for Class II appliances.
Important Safety Warning for Class II Leakage Testing
With no earth path, a faulty Class II appliance may leave exposed parts at live mains voltage until touched by the probe.
Safety Recommendations
- Treat exposed metal as potentially live
- Use insulated gloves or PPE
- Follow workplace safety procedures
Note: Many training organisations specifically advise wearing insulated gloves during Class II leakage testing for operator protection.
