RCD Selection: How to Choose a Residual Current Device?

Whether buying RCDs to install in your home or specifying RCDs for a project, it is important to select the right type of device for the job. RCD selection involves a careful analysis of both the application and the associated wiring system to ensure that the RCD will operate correctly and reliably. This RCD selection guide highlights the key considerations.

RCD Selection

RCD selection is the process of choosing the right type of residual current device (RCD) for a given application. RCDs are used in electrical circuits to protect against shock hazards and fire risks posed by faulty or damaged wiring.

In order to function correctly, an RCD protection device must be matched to the characteristics of the circuit it is protecting. Done correctly, RCD selection can help ensure the safety of users and the integrity of the electrical circuit.

The correct choice of residual current device also helps to ensure that the RCD will operate correctly in the event of an electrical fault, minimizing the risk of injury or property damage.

RCD Selection Guide

Electricians must consider several factors when choosing an RCD, whether for a new installation or as part of an upgrade to an existing circuit. The following is a list of key considerations for RCD selection:

1. RCD Sensitivity

The first step in RCD selection is to determine the required level of protection. That, in turn, will help you choose the correct RCD sensitivity level. The criteria for selection will usually be based on the type of loads that will be protected, as well as any specific regulations that apply to the installation.

Smaller RCDs are suitable for protecting low-current circuits, such as those used for lighting, while larger RCDs are required for heavy-duty appliances and industrial machinery. There are generally 3 RCD sensitivity levels: low, medium, and high.

High RCD Sensitivity

These are rated to trip at 10 mA to 30 mA and are used to protect against electric shocks. They are typically used in installations where there is a risk of electrocution, such as in wet areas or where children are present. Use these for homes and other residential buildings.

Medium RCD Sensitivity

These RCD devices have a rated trip current of between 100 and 300mA and are commonly used to protect against shock and fire risks. They are often used in commercial buildings, such as retail stores and offices, where there is a higher risk of fire from electrical faults.

Low RCD Sensitivity

These RCDs have a rated trip for residual currents above 500mA and are designed for use in industrial applications. They offer a high level of protection against both shock and fire hazards and can protect industrial equipment such as motors.

2. RCD Type

The next step of the RCD selection process is to determine the type of device required. Depending on the application, you may choose between a DIN rail-mounted RCD or a plug-in RCD. There is also the portable residual current device that can be used for temporary protection, such as during outdoor work.

Fixed RCD

DIN rail-mounted RCDs are installed in the electrical distribution panel and offer permanent protection for circuits. These are mandatory in many regions and may be used for whole installations or selected circuits. The often protect the different circuits.

Plug-in RCD

These come in the form of a plug-in RCD adaptor and are designed for easy installation. They are often used in domestic premises. The plug-in RCD protection is similar to that of rail-mounted RCDs, but less expensive and easier to install.

Portable RCD

Portable RCDs are plugging types that can be used for temporary protection, such as during construction work where they are often used in conjunction with power tools. These devices can be plugged into any standard electrical outlet and offer protection against electric shock.

There are also residual current device types based on the types of loads they are built for and as defined by the IEC 60755 RCD standard. The most common is type AC, type A, type F, type B, and type S.

Type AC RCD

Type AC RCDs are meant for AC or sinusoidal currents and circuits that produce minimal disturbances. Applications for these residual current device types include lighting circuits, immersion heaters, and other types of heaters that use AC circuits.

Type A RCD

These are designed to protect in both AC circuits and pulsating DC circuits. Such applications include electronic loads that contain rectifying circuits and components. Use these RCD types for circuits involving computers, TVs, induction cookers, and similar appliances.

Type F RCD

Type F residual current device is for single-phase installations and has a high level of immunity to false tripping. As such, electricians must choose these for circuits with equipment that use speed devices, Examples include air conditioning equipment, heat pumps, newer washing machines, and other appliances that use variable drive motors.

Type B RCD

This type of RCD is for three-phase installations and has the same high immunity to false tripping as type F. However, they can only be used on three-phase supplies that have a common return path. These are often found in commercial and industrial applications where there are many electric motor-driven loads. Common applications are lifts, HVAC, and a majority of industrial motors.

Type S RCD

The type S RCD device incorporates a time delay feature. They are used on circuits that have inrush currents, such as electric motors and transformer circuits. This RCD is not suitable for protection from electric shock since it takes time to trip. Choose it your application requires more of the fire protection that an RCD offers.

3. RCD Ratings

RCD ratings indicate the specified current and voltage and other values that the device can handle including frequencies within which it can operate. These specifications are important when sizing your devices for the circuit.

RCD Voltage Rating

Most residual current devices are designed for 240V AC circuits, but some may be rated for 110V or 415V three-phase supplies. During the RCD selection procedure, this is one of the key specifications that you must check.

RCD Current Rating

The RCD amp rating is usually specified in milliamperes (mA) and indicates the maximum current that the device can handle before it trips. Sizing your RCD properly means choosing a device with a current rating that is greater than the prospective short-circuit current of the circuit.

RCD current ratings include rated current and rated residual current. The rated current is the maximum current that the device can handle before it trips.

The rated residual current is the amount of current that causes the RCD to trip and disconnect the circuit. This is usually around 30mA and less for domestic RCD application and up to 500mA for industrial uses.

RCD Frequency Rating

RCDs are designed to operate within a specific frequency range. The most common is 50-60Hz, but some devices may have a wider range. This specification is important for equipment that uses variable speed drives since the current harmonics produced by these devices can cause false tripping if the RCD isn’t designed to filter them out.

After you’ve selected the proper RCD type for your circuit it’s now time to find a manufacturer or supplier. Different RCD manufacturers offer the device at different prices.

When buying RCDs in bulk, you may be able to get a discount from the manufacturer. This is especially true for industrial applications where large numbers of devices are required. Therefore, consider the RCD cost, and compare different offers from different suppliers to get the best price.

Conclusion

RCD selection requires careful consideration of the application and the type of supply. These devices are available in a variety of ratings and types to suit different applications. Be sure to check the RCD voltage rating, current rating, and frequency range to ensure that the device is compatible with your circuit. Additionally, compare the prices from different suppliers to get the best deal on RCDs.