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Different Residual Current Device Types and Their Uses

Different RCD types to serve different electrical circuits and loads
Different RCD types to serve different electrical circuits and loads
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To cater to the specific needs of various applications, manufacturers have developed different residual current device types based on the IEC 60755 standard and other requirements. These range from the regular ones that are commonly used to the more specialized versions based on the type of load. Here is an overview of the different RCD types as well as their uses:

Residual Current Device Meaning

Before delving into the types of RCDs, it is important to first define RCD or residual current devices. These are life-saving devices that are designed to quickly disconnect a circuit whenever they detect that the current flowing through it is not balanced.

The imbalance usually occurs when there is a leakage of current to the earth or ground. When that happens, the RCD trips and cuts off the power supply, thus preventing electrocution or injury.

There are different types of RCDs, and each is suitable for specific applications. The choice of which one to use will depend on the particular needs of the circuit that it will be protecting. Below is a rundown of the different types of residual current devices as well as their applications.

Residual current devices
Residual current devices
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Residual Current Device Types

Because protection needs are varied, so are the types of residual current device types. These offer different capabilities, such as the ability to handle different loads and currents, as well as different response times. The following are the most common types of RCDs:

Type AC RCD

The type A RCD is only used for circuits that have a residual alternating current and the fault current is sinusoidal. It is the most common type of RCD for most households, though, and usually a general type of residual current device.

Because it only works for alternating residual current, these types of RCDs are not the most suitable for modern electrical equipment that contains electronic components and produces DC fault currents. You can use them for different types of loads.

Common Applications:

  • Immersion heater circuits
  • Electric showers
  • Electric ovens
  • Tungsten filament Light bulbs

Type A RCD

These residual current device types are built to detect the same fault currents as type AC RCDs. The only difference is that they are designed to work with residual pulsating direct current as well. These include single phase systems with rectifying circuits such as those used for heating plate loads.

Since you can use them for direct current faults, type A RCDs are suited for circuits that contain electronic equipment such as solar photovoltaic systems. Additionally, A type RCD can also be used residual pulsating currents that are superimposed by smooth direct currents of magnitude not above 6mA.

Common Applications:

  • Solar power inverters
  • UPS systems
  • Motor drives
  • EV charging stations
  • LED lighting systems

Type F RCD

The F in these RCD types stands for frequency. As such, these devices are designed to work with circuits for frequency controlled electrical equipment. These include the motors of washing machines and air conditioners.

Because type F RCDs are designed for circuits with inrush currents, they are designed to withstand surge currents without tripping. Additionally, type F RCDs can also be used to protect against pulsating DC faults or pure DC superimposed on a sinusoidal current..

In other words, these residual current device types can function as type A and AC RCDs. Common uses of the type F residual current device are given below.

Common Applications:

  • Washing machines
  • Dishwashers
  • Refrigerators
  • Air conditioners
  • Tumble driers
  • Some class 1 tools (powered)

Type B RCD

The type B RCD is meant for single and triple phase systems. In addition, you can use this type of RCD for type AC circuits, Type A and type F RCD circuits. When it comes to the specifics, you can use these RCD types in the flowing situations or circuits:

For residual AC currents of frequency up to 1000Hz, residual sinusoidal currents superimposed by smooth DC currents and residual pulsating currents, regardless of polarity. Applications of B type RCDs are listed below.

Common Applications:

  • UPS systems
  • Solar energy systems
  • EV charging units (with smooth residual DC currents)
  • Welding machines
  • Electric lifts in buildings

Type S RCD

These RCD types are also called time delay RCD devices as they incorporate a time delay function. They take longer to detect a fault and to trip the circuit than other types of RCDs. This is because they are made to avoid nuisance tripping.

The main benefit of using Type S RCD devices is that they offer greater immunity to electrical transients on the supply that could cause false trips of fires. Nuisance tripping happens when an RCD device trips due to a momentary power fluctuation or power outage. Type S RCDs have a delayed reaction time so they can avoid tripping in these situations.

But that also makes the S type RCD unsuitable for shock protection, as it could allow fatal currents to pass through the body for longer that the required duration of 40 mS before tripping. As such, these RCD types are commonly used to protect circuits from fires.

Which RCD to Use?

The type of RCD you use should be based on the application or circuit you want to protect. Different types of residual current devices are made for different purposes. For example, if you want to protect a circuit with electronic equipment, you would use a type A or type B RCD.

If you want to protect a motor drive circuit, you would use a Type F RCD. When choosing an RCD, you should also consider the level of protection you need.

For example, if you want to protect a circuit from serious faults, you would use a fast-acting RCD with high sensitivity. If you want to protect a circuit from nuisance tripping, you would use a slow-acting RCD with low sensitivity.

Conclusion

There are several different residual current device types available on the market. The type of RCD you need depends on the circuit you want to protect and the equipment you are using. In general, AC RCDs are the most common type of RCDs used in households. For industrial applications, however, there are other types of RCDs that are more suitable.

When choosing an RCD, make sure to select one with a higher rating than the maximum prospective short-circuit current of the circuit it is protecting. This will ensure that the RCD will trip in the event of a fault and adequately protect you.

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William

I am William, Electrical Engineering Author. Dedicated to writing technical articles on Timer Relay, Monitoring Relay, Surge Protection Device and other electrical devices. With 7 years of writing experience, I am committed to providing accurate and in-depth expertise to my readers.

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