RCD Testing: Why RCD Trip Time Matters for Electrical Safety

Having an RCD installed in your home or business is important — but simply having one fitted does not guarantee that it will operate correctly when you need it.

RCDs need to be tested to make sure they operate correctly and, where applicable, within the required operating time. A recent RCD test I carried out demonstrates exactly why checking the actual RCD trip time is so important.

RCD trip-time test showing a result of >300 milliseconds.

What happened during this RCD test?

During this test, I used a Megger MFT1741 multifunction tester to test the operating time of the RCD.

The tester displayed:

The RCD did not operate within the 300 millisecond test limit being assessed, meaning the tester recorded the result as greater than 300 milliseconds.

This is exactly the sort of result that should not simply be ignored. The RCD, the circuit and the test conditions need to be assessed to establish why the expected operation was not achieved.

What is an RCD?

An RCD (Residual Current Device) is a protective device designed to disconnect an electrical circuit when it detects a residual current, such as current flowing to earth through an unintended path.

RCDs provide an important level of additional protection against electric shock and can also help reduce the risk of electrical fires caused by certain types of earth leakage.

A commonly used type of RCD in domestic installations is a 30mA RCD, which provides additional protection against electric shock under the appropriate conditions.

Why does RCD trip time matter?

When an electrical fault occurs, time matters.

The purpose of an RCD is to disconnect the supply quickly when the required residual current is detected. The faster the circuit is disconnected, the shorter the period during which a person may be exposed to a dangerous electrical current.

This is why an RCD should not simply be judged by whether it has a green or red indicator, or whether the TEST button appears to work.

The actual operating time needs to be tested with suitable test equipment.

Does pressing the RCD TEST button mean the RCD is safe?

No — the TEST button is useful, but it does not replace proper RCD testing.

Pressing the TEST button creates a test condition within the RCD and should cause the device to disconnect the supply. It is good practice for the user to operate the TEST button regularly in accordance with the manufacturer’s instructions.

However, a professional electrical test can measure the RCD’s actual operating characteristics and trip time using calibrated test equipment.

This can reveal problems that may not be obvious from simply pressing the TEST button.

What does an RCD reading of >300ms mean?

In the example shown above, the tester recorded >300ms.

For the particular test being carried out, this means the RCD did not disconnect within the 300 millisecond test limit being assessed.

However, RCD trip times must always be interpreted according to the type of RCD, its rated residual operating current, whether it is time-delayed, and the specific test being performed.

Therefore, a result such as this should be investigated by a competent electrician rather than simply assumed to be acceptable or unacceptable without considering the full test conditions.

Why regular RCD testing is important

Electrical equipment and protective devices can deteriorate over time. Connections can become loose, components can fail and protective devices can develop faults.

That is why electrical testing is particularly important in older properties and installations where the condition of the electrical system may not be known.

A professional EICR (Electrical Installation Condition Report) includes appropriate testing of the electrical installation and its protective devices.

Where an RCD is installed as part of the protective measures, its operation should be considered as part of the overall assessment of the installation.

RCD testing as part of an EICR in Plymouth

If you live in Plymouth, Devon, an EICR can provide a useful assessment of the condition and safety of your electrical installation.

During an electrical inspection and testing procedure, an electrician can assess areas such as:

• RCD operation and trip times
• Continuity of protective conductors
• Insulation resistance
• Earth fault loop impedance
• Polarity
• Automatic disconnection of supply
• Condition of the consumer unit and protective devices
• Signs of overheating, damage or deterioration
• Suitability of the installation for continued use

Testing provides information that simply looking at an electrical installation cannot provide.

Don’t wait until an RCD is needed

One of the most important things to remember about an RCD is that you don’t want to find out it isn’t operating correctly when there is already a serious electrical fault.

Test it before you need it.

If an RCD fails to operate correctly, the cause should be investigated and the necessary remedial work carried out by a suitably qualified electrician.

Frequently Asked Questions About RCD Testing

How often should an RCD be tested?

The manufacturer’s instructions should be followed for operating the RCD’s TEST button. Professional electrical testing should be carried out as appropriate for the type and use of the installation, particularly where an EICR is required or recommended.

What is a good RCD trip time?

The required trip time depends on the type of RCD and the test being performed. For many commonly encountered 30mA non-delayed RCDs, the relevant requirements include disconnection within specified maximum times at the applicable test current. A qualified electrician will interpret the result against the correct requirements for the device and installation.

What does >300ms mean on an RCD tester?

It means the tester did not record the RCD disconnecting within 300 milliseconds during that particular test. The result needs to be assessed against the RCD type, test current and applicable requirements.

Can an RCD fail even if the TEST button works?

Yes. The TEST button checks that the RCD’s test mechanism operates, but it does not provide the same information as professional testing with appropriate test equipment.

Why should I have an RCD tested during an EICR?

An EICR involves inspection and testing of the electrical installation. Testing protective devices such as RCDs helps establish whether the protective measures are operating as expected and whether further investigation or remedial work may be required.

Electrical Safety Should Never Be Assumed

The photograph above is a good reminder that you cannot always tell whether an electrical protective device is working correctly just by looking at it.

Proper electrical testing gives you information about what is actually happening within the installation.

If your home or property in Plymouth has RCD protection that hasn’t been tested for some time, consider arranging an electrical inspection or EICR.

When it comes to electrical safety, test — don’t guess.

Dave Prout Electrician Plymouth
EICRs | RCD Testing | Consumer Units | Fault Finding | Electrical Safety