Intermittent Electrical Faults: How to Find the Faults That Come and Go

Every electrician knows the frustration: the customer describes a fault that happens regularly — lights flicker, an RCD trips, a socket stops working — but when you arrive on site, everything tests perfectly. The fault is real, but it is not present at the time of your visit.

Intermittent electrical faults are difficult to find for a simple reason: standard electrical tests give you a snapshot of the installation at a single point in time. If the fault depends on conditions that are not present during the test — a specific temperature, humidity level, load pattern, or vibration — the test results will be normal even though the fault is genuine.

The key to finding intermittent faults is understanding the conditions under which they occur and then either reproducing those conditions during testing or using monitoring equipment to capture the fault when it happens. This requires a combination of good customer communication (to understand the pattern), systematic testing (to eliminate possibilities), and sometimes specialised equipment such as data loggers or thermal imaging cameras.

Elec-Mate's AI fault diagnosis tool is designed to help with exactly this problem. Describe the symptoms — when the fault occurs, what triggers it, what the customer has observed — and the AI suggests probable causes and a testing strategy to confirm them.

Temperature-dependent faults are caused by the physical effects of heating and cooling on conductors, insulation, and connections. They are among the most common types of intermittent fault.

To diagnose temperature-dependent faults, carry out tests both with the circuit cold and after it has been under load for a sustained period. Compare the insulation resistance, continuity, and earth fault loop impedance readings between the cold and warm conditions. Any significant change points to a temperature-dependent fault.

Vibration faults occur when mechanical movement causes a conductor to intermittently lose contact at a terminal, or causes cable insulation to chafe against a sharp edge and intermittently short to earth.

To diagnose vibration faults, try to reproduce the vibration while monitoring the circuit. Gently tap or flex the cable at various points while watching for a change in continuity, insulation resistance, or circuit operation. A "wiggle test" at each accessible connection point while the circuit is energised (with appropriate safety precautions) can reveal a loose terminal.

As covered in the temperature and vibration sections above, loose connections are the root cause of many intermittent faults. The connection may work perfectly most of the time but fail under specific conditions — high load, high temperature, vibration, or a combination.

The challenge is that a circuit may have dozens of connection points — at the consumer unit, at every junction box, at every socket, at every switch, and at every light fitting. Any one of these could be the faulty connection.

Elec-Mate's defect code AI helps classify any faults you find during investigation, ensuring your report accurately reflects the severity and location of each defect.

Cable insulation degrades over time due to heat, UV exposure, chemical exposure, and moisture absorption. This degradation can cause insulation resistance to drop below acceptable levels — but often only under certain environmental conditions.

To detect humidity-dependent insulation faults, test insulation resistance during the conditions that trigger the fault. If the customer reports that the RCD trips every morning but works fine in the afternoon, visit early morning when the humidity is highest. Compare circuit-by-circuit insulation resistance readings taken during the fault condition with readings taken later when the fault is absent. The circuit with the biggest change is your culprit.

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The key to successfully diagnosing intermittent faults is to work systematically rather than randomly testing and hoping to stumble on the answer:

AI-assisted fault diagnosis

Describe the intermittent fault symptoms to Elec-Mate's AI — when it happens, what triggers it…

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Beyond standard test instruments, several specialised tools can help locate intermittent faults:

Elec-Mate's testing calculators help you compare measured values against expected values for each circuit type, making it easier to spot abnormal readings that point to a developing fault.

Intermittent faults present a documentation challenge: if the fault is not present at the time of the inspection, how do you record it on the EICR?

The answer is to use the FI (Further Investigation) code. If a customer reports symptoms consistent with an intermittent fault but you cannot reproduce or confirm the fault during your visit, record an FI observation with a clear description of the reported symptoms and your recommendation for further investigation. This might include data logging, a return visit during the conditions that trigger the fault, or thermal imaging under load.

If you do find evidence of the fault — for example, a loose terminal with signs of overheating, or insulation resistance readings that are borderline — classify it appropriately:

Elec-Mate's defect code AI helps you select the correct observation code and generate a clear, professional description for each finding — even for complex intermittent faults where the classification is not immediately obvious.