Everything You Need to Know about RCD and Loop Testers

With the changes to wiring regulations from 2018, homeowners and commercial businesses are required to regularly have the electrical installations in their property inspected by qualified electricians. This also extends to industrial premises, where the guidelines are more rigorous. A key test is that to the residual current device or RCD in the switchboard.

What Is an RCD?

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A residual current device is a safety switch that disconnects a circuit when there is a fault. It constantly monitors the flow of electricity, and trips, or disconnects a circuit when it senses an imbalance in the power supply. RCDs need to do this within a set timeframe to avoid possible electrocution or electrical fires in the case of a power surge. Regulations state that each building must have at least two RCDs and that are tested within specified timeframes set out by the new guidelines.

What are the types of RCD Tests?

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There are two types of RCD tests. The first is a simple push-button test that can be conducted by homeowners. The second requires specialised RCD testing equipment, or more specifically an RCD loop tester. Push-button tests are designed to find out whether the RCD is operational. It requires testers to simply push the “Test Button”. An RCD in working order should trip the circuit without any delay. If this doesn’t happen, then call an electrician and have the RCD replaced.

The second test is a so-called timed trip test, or an applied current test, which delivers more accurate results and is compulsory for all properties throughout Australia. Here technicians use an RCD loop tester to measure the exact time the RCD takes to trip. For most residential and commercial premises this is usually under 40 milliseconds for type 1 RCDs and 300 milliseconds for type 2 RCDs. This is an extremely short period of time that should prevent injury or damage if there is an electrical fault.

How is a Timed Trip Test Done?

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To test the time it takes for an RCD to trip, electricians plug in the test leads of an RCD loop tester or a multifunctional installation tester into the active or line wire and neutral or earth line anywhere along the installation. This can be a socket anywhere on the premises. To conduct the test the installation should be live, meaning power Is flowing to the premises. A precisely measured current is drawn from the active wire and returns to the earth line, and trips the RCD. The time it takes for this to happen is displayed on the LCD screen on the RCD loop tester. Depending on the type of RCD, and whether the RCD is in working order or faulty, different results can be displayed.

For non-delayed or type 1 RCDs three tests are done to ensure that the device is operational and safe to use. These RCDs are rated at 30 milliamps. The first test involves applying 50 per cent of the tripping current of the RCD for a duration of 2 seconds, and the RCD should not trip. In the second test, the RCD loop tester applies 100 per cent of the rated current for a period no longer than 2 seconds, and the RCD should trip within 200 milliseconds. And the third test involves applying 5 times the rated current, or 150mAmp for no longer than 50 milliseconds, and the RCD should trip within 40 milliseconds. The procedure can be done by manually adjusting the settings with a turn of a knob on the RCD loop tester or automatically, depending on the model.

Homes and businesses require a second RCD, which allows power to be delivered to other circuits in the premises, despite the first RCD tripping and cutting off power to the other half of the electrical installation. This allows you to use things like lights in case of power failure. Tests are also conducted to type 2 RCDs which have a delayed function. They have an operating current three times that of a non-delayed RCD, or 100mAmps. To test a type 2 RCD, the procedure consists of connecting the leads and RCD loop tester to any point in the installation and repeating the testing process. Only here 100 per cent of the rated current or 100mAmps is applied and the RCD should trip within 50 per cent of the rated time delay. An operational type 2 RCD trips within 300 and 500 milliseconds.

The Equipment

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An RCD loop tester is an electronic device that draws power from the lines being tested. Since the current is very small, the current draw doesn’t affect the overall readings. As mentioned, RCD testers can test different types of RCDs and use microprocessors to calculate tripping times. This allows for high accuracy and reliability for consistent results. Units are lightweight, have an IP54 rating and are relatively straightforward to use. The test leads for distribution boards and sockets may be packaged with the unit or sold separately. Besides an RCD loop tester, multifunctional testers which also measure things like loop impedance and the quality of insulation can be used to test RCDs.