HomeCan Electrical Appliances Cause an RCD to Trip?
When an RCD trips, attention often turns immediately to the appliance that was being used at the time. In some cases, that appliance has developed an electrical fault. However, the appliance that appears to trigger the RCD may only be the final contribution to leakage current that is already present from several other items of equipment.
Many modern electrical appliances produce a small amount of leakage current during normal operation. Individually, each appliance may be perfectly serviceable, but their leakage currents can accumulate. If the total becomes high enough, an RCD may operate even though there is no single obvious fault. This can make the problem difficult to identify, particularly where the RCD only trips occasionally or when several appliances are operating together.
What does an RCD detect?
An RCD, or residual current device, monitors the current flowing through the line and neutral conductors of a circuit. Under normal conditions, the current flowing to an appliance through the line conductor should return through the neutral conductor. If some of the current returns by another route, the two currents will no longer be equal. The difference is called the residual current.
The missing current may be flowing:
- through the protective conductor (earth wire);
- through damaged or damp insulation;
- through pipework or other conductive parts;
- directly to earth; or
- in a fault situation, through a person.
When the residual current becomes high enough, the RCD disconnects the supply.
The RCD does not directly measure current in the earth wire. It detects the imbalance between the currents in the live conductors. This means it can respond even where the leakage current is not returning through the appliance’s protective conductor.
Normal leakage from electrical appliances
Leakage current does not always indicate damaged insulation or an electrical fault. Many modern appliances contain electronic power supplies and interference filters designed to reduce electrical noise. These filters may include capacitors connected between the supply and earth. A small alternating current can flow through the capacitors while the appliance is operating. This is a normal consequence of the appliance’s design rather than evidence of insulation failure.
Cables, motors, heating elements and other internal components also have a small amount of capacitance to earth, which can contribute further leakage while the appliance is energised.Computers, monitors, printers, televisions, chargers, washing machines, refrigerators and many other appliances may therefore produce some leakage even when operating correctly.
Leakage can also be caused by a fault, such as deteriorated insulation, moisture, a damaged cable, contamination, a failing heating element or deteriorating motor windings. The important distinction is whether the measured current is expected for the appliance or indicates a defect.
The leakage from one appliance may be small, but the increasing number of electronic appliances used in homes and workplaces means that their combined effect can become significant. The IET identifies filter capacitors in electronic power supplies as a normal source of leakage and notes that the growing use of electronic equipment can contribute to unwanted RCD tripping.
How leakage from several appliances adds together
An RCD responds to the total residual current passing through it. It cannot determine whether that current is coming from one appliance or many different appliances. For example, computers, monitors, refrigerators, printers, chargers and filtered extension leads may each contribute a small amount of leakage. None may produce enough current to operate the RCD individually, but their combined leakage may leave very little margin
When another appliance switches on, its additional leakage may be enough to cause a trip. That appliance may then appear to be faulty even though it is only one contributor to a wider cumulative leakage problem. In many installations, one or two RCDs each protect several circuits. Leakage from appliances connected across those circuits is therefore combined at the relevant RCD, even when the appliances are in different rooms or areas.
A 30 mA RCD may operate below 30 mA
The value marked on an RCD is its rated residual operating current. It should not be assumed that a 30 mA RCD will remain connected until the leakage reaches exactly 30 mA. A 30 mA RCD must not operate at half of its rated current, which is 15 mA, but it is permitted to operate when the residual current rises above that value. In practice, individual devices will have different actual operating points.
The IET’s guidance on avoiding unwanted tripping recommends limiting accumulated leakage downstream of an RCD to 30% of its rated residual operating current. For a 30 mA RCD, 30% is 9 mA. This provides a margin for temporary increases in leakage and prevents the installation from operating too close to the RCD’s possible tripping point. The 9 mA figure is not an appliance pass or fail limit. It relates to the combined leakage downstream of the RCD.
Why does the RCD trip only occasionally?
Intermittent tripping often occurs because appliance leakage is not constant. It may vary with:
- the stage of an operating cycle;
- heating and cooling;
- moisture and humidity;
- vibration or movement;
- an automatic defrost cycle;
- a compressor, pump, or motor starting;
- several appliances switching on together; or
- the number of electronic devices connected at the time.
This is particularly apparent with items such as washing machines, dishwashers, refrigerators, freezers, ovens and similar appliances that contain several electrical parts controlled to operate at different times. Depending on the appliance, these may include motors, pumps, compressors, fans, heating elements, defrost heaters and electronic controls.
A relatively small change can be enough where the standing leakage is already close to the RCD’s operating range. This explains why an RCD may trip one day but not the next, or why an appliance may work normally when tested briefly after the event.
The term nuisance tripping is often used for this type of problem. However, the operation should not be dismissed as a nuisance until the cause has been identified. The RCD may be responding correctly to a genuine fault or excessive cumulative leakage.
The limitations of detecting leakage current during PAT testing
PAT testing can provide useful information when investigating an appliance, but a satisfactory result does not necessarily rule out its involvement in an intermittent RCD trip. There are two important limitations to consider. First, PAT testing assesses one appliance at a time, whereas an RCD responds to the combined leakage from the fixed installation and every appliance connected downstream of it. An appliance may therefore produce an acceptable individual result while still contributing to excessive accumulated leakage.
Second, the test only assesses the appliance under the operating conditions present at the time. It may not reproduce a fault that appears later in the appliance’s cycle. The test most commonly used to assess insulation during PAT testing is the insulation resistance test. This applies a DC test voltage to the appliance and is useful for identifying damaged, damp or contaminated insulation. However, it does not reproduce all the leakage that may occur while the appliance is operating from an AC supply. In particular, it does not include capacitive leakage through interference filters in the same way as a powered leakage test.
A powered leakage test measures the appliance while it is energised. Depending on the equipment and test method, this may involve measuring protective conductor current, differential leakage or touch current. Powered leakage testing is optional, however, and many basic or battery-operated PAT testers do not have the facility to perform it. In these cases, the appliance may receive an insulation resistance test without its actual operating leakage being measured.
Even where a powered leakage test is available, it still has important limitations. The test is typically carried out for only a few seconds and assesses only the components and operating modes active at that time. The IET Code of Practice requires the measured value to remain stable for at least two seconds before it is recorded. This confirms that a stable reading has been obtained under the test conditions present at that moment. It does not prove that the leakage will remain at the same level throughout every stage of the appliance’s operation.
A short test may not energise the heating element, pump, compressor, fan or defrost heater responsible for an intermittent RCD trip. An appliance could therefore produce a satisfactory result during PAT testing but develop excessive leakage later in a complete wash cycle, defrost cycle or heating period.
The current leakage-current limit is 5 mA for both Class I and Class II equipment. An appliance producing less than this limit may be acceptable when assessed individually, but several appliances with measurable leakage can still create excessive accumulated leakage on the circuit.
A PAT result must therefore be considered alongside the appliance’s operating cycle, which components were active during the test, the circumstances in which the RCD operated and any evidence that the fault is intermittent.
Our separate guide to the PAT testing leakage current test explains the test methods and limits in more detail.
Investigating an intermittent RCD trip
Information from the user can be as valuable as the initial test result. Useful details include what the appliance was doing when the RCD operated, how long it had been running and whether the fault is associated with heating, draining, cooling or another stage of operation.
Weather and environmental conditions may also be relevant. Outdoor equipment, appliances in garages and equipment used in damp areas may only develop excessive leakage during rain, condensation or high humidity.
Where safe to do so, a user can unplug portable appliances and reconnect them individually. Automatic appliances should be allowed to pass through their normal operating cycles before they are ruled out.
An appliance that repeatedly appears to operate the RCD should be disconnected and taken out of use until it has been properly inspected and tested.
Could the problem be in the fixed installation?
Not every RCD trip is caused by an appliance. The fault may be within the fixed wiring, a socket, outdoor circuit or permanently connected item of equipment. Neutral-to-earth faults and moisture within the installation can also cause confusing or intermittent symptoms.
If the RCD continues to trip after portable appliances have been unplugged, the installation should be investigated by an electrician.
Conclusion
An appliance can contribute to RCD tripping even though it appears to operate normally and produces satisfactory results during routine PAT testing. Its leakage may be acceptable when considered individually but form part of a larger accumulated leakage current. Alternatively, an intermittent fault may only appear when a heater, pump, compressor or other component operates later in the appliance’s cycle.
A test result must therefore be understood in context. The operating sequence of the appliance, the circumstances of the trip and the limitations of the tests carried out may all be significant. This is one example of why appliance inspection and testing is not always a simple matter of obtaining a pass or fail result. Good training provides an understanding of the principles behind each test, while access to specialist support can be valuable when a fault does not fit the usual pattern.
As a specialist PAT training provider, we deliver PAT testing courses with ongoing support, helping delegates deal confidently with less obvious testing questions and unusual appliance problems when they arise.