What is RCD Testing?

Whether you’re building a circuit or checking a fuse box, RCDs are essential to protecting electrical systems, appliances, and the people using them. RCD testing is the process of checking that the RCD installed in a circuit works correctly. It can also help electricians or circuit builders identify when an incorrect RCD type has been used. 

RCD testing can be completed using the test button on an appliance or fuse box. It can also be completed using a loop or RCD tester in a circuit. When running the test, a correctly functioning RCD will cut power to the appliance or circuit being checked under certain conditions within a certain time. 

If this isn’t the case, then the RCD may need to be changed or there could be a fault with the appliance or circuit itself. By testing RCDs according to set procedures, under legislative guidelines, and by a qualified electrician, electrical equipment or circuits can be safely operated. 

This article will explain everything you need to know about RCDs and how they are used in testing safety.

RCD stands for Residual Current Device. This describes their function as a tool that monitors the flow of electrical currents through circuits and appliances, detects when residual current is moving somewhere incorrectly, and cuts the power within a short amount of time.

Testers and testing play a crucial role in the safety and reliability of electrical systems, circuits, and appliances.

RCDs can perform different types of tests. A trip time test measures the time taken for the RCD to cut power off when a fault occurs. This is a key indicator that the RCD is fast enough to prevent safety hazards like shocks. A ramp test instead gradually increases the current flow to identify if it’s functioning correctly and providing an adequate level of protection. 

These processes are vital to accident and injury prevention. In using RCDs, testers can and do save lives and protect valuable infrastructure.

The process of working with RCDs requires a methodical approach to testing procedures. Here is an example of some of the steps usually involved in this process:

1. Visual inspection: Typically an RCD tester would first examine the device for signs of damage or wear and tear. This would include looking for signs that connections are insecure or that the device is properly installed
2. Testing method selection: A tester will need to select the appropriate method for testing based on the type of RCD and any specific system requirements
3. Conduct testing: The tester will perform the appropriate testing procedure using their required training and PPE safety equipment. For example, if trip testing was conducted, a fault would be applied to the current of the RCD. The time taken for the device to trip and cut off the power would be measured and recorded
4. Evaluate the results: Finally, the tester will analyse the resulting data and determine if further testing or action is required

RCDs are installed into a circuit with both the live and neutral wire passing through it. Both wires are connected to coils that are wrapped around the iron core of the RCD. As such, when power is passing through these wires, a magnetic field is created by each of them. 

When the electrical current passing through each wire is the same, the magnetic fields are also equal and neutralise each other. As a result, the iron-panelled switchgear next to the device’s iron core doesn’t react and the appliance or circuit can function as normal. 

However, if the current in the returning neutral wire drops, then the magnetic field will also reduce. This will cause an imbalance in the fields, attracting the iron panel of the switchgear towards the iron core and breaking the circuit automatically. 

To reset the RCD, a switch or button has to be pressed to reset the switchgear before the appliance or circuit is made live again. If the fault continues, the RCD will continue to be triggered by the magnetic field’s imbalance. 

It should be noted that though the basic function of all RCDs is the same, there are different types of RCDs which are designed to detect different types of currents or be used in particular circuits or appliances. 

• Type AC devices can only detect sinusoidal alternating currents (therefore can only be used in AC circuits)
• Type A can monitor pulsating direct currents as well as sinusoidal alternating ones making them suitable for use in single-phase appliances or loads
• Type F RCDs can be used in AC or DC circuits and are specifically designed to be installed in circuits where single-phase variable speed drives are used, meaning they won’t be triggered by changes in frequency
• Type B devices can cover all the above needs plus be used in circuits alongside three-phase rectifiers. This makes these RCDs the most useful in complex or commercial applications
• Fixed devices are, as the name suggests, permanently put into a circuit such as in a fuse box or consumer unit
• Socket-outlet RCDs are placed in specially-designed sockets to give extra protection to the appliances plugged into them and therefore the people using the appliance
• Portable devices are installed in appliances and devices, meaning they help keep this equipment and those using it safe when its plugged into a standard socket

When installing or testing an RCD, it’s important to keep the type of device in mind to ensure the circuit complies with regulations and will work effectively.

Though both loop and RCD testers are used by qualified electricians to analyse the current flow and assess any faults in a circuit, they test different operations within a system. A loop tester is designed to check the function of the earth connection and ensure any fault current passing through the system will be strong enough to trigger the circuit protection. 

In contrast, an RCD tester checks the performance of an RCD device at different power levels (50%, 100% and 500% at both 0 and 180 degrees) to ensure it will function correctly in the case of a fault. 

Both devices can be used to determine whether the trip time and function of the RCD fall in line with regulations. However, RCD test results need to fall in line with BS 7671 regulations for a circuit to be certified as safe to use. Therefore it’s essential that you use an RCD tester to check a circuit’s safety, with a loop tester as a backup if required.