AC electrical systems are designed to protect people from power shocks. Appliances are built using insulation or other methods to create a barrier so that people don’t come into contact with a live device. Sometimes, this contact can happen accidentally, which can cause a ‘residual’ current or ‘earth leakage’ to flow through the individual’s body. Just 30 MA of current passing through a human body can cause death if the current is not interrupted quickly.
RCDs (residential current devices), known as GFCI in North America, are built to provide protection against this danger. But new challenges have come up because of the wide use of electrical platform applications like variable speed drives to control motors, photovoltaic installations, and charging stations for electric vehicles. These applications create residual currents which are not wholly sinusoidal. In this article, we detail the main types of RCDs used today and the kinds of hazards each is built to shield against.
Matching RCDsTo Loads
The IEC 60755 regulation presents definitions of four kinds of RCDs for AC systems. Each addresses different combinations or types of residual current waveforms.
- Type AC
This RCD type is utilized where the fault current is sinusoidal and at a similar frequency to the supply. For instance, this is when a fault happens on the supply conductor or if the load is linear or resistive. In many nations, these are used as general-purpose RCDs.
- Type A
Like those with a single-phase rectifying circuit such as a heating plate, some load types will produce a pulsating DC residual current. A Type A RCD is needed for these systems as it can detect this type of residential current. It can also tolerate a ‘smooth’ superimposed DC up to 6 mA. Further, it can also catch sinusoidal residual currents similar to the Type AC RCD.
- Type F
There could be ‘composite’ residual currents in the range of a few Hz to 1000 Hz in some systems. Examples are circuits with single-phase motors that are controlled by a variable-speed drive, such as an air conditioner or heat pump. A Type F RCD is needed for these systems. The RCD can also withstand smooth superimposed DC residual current up to 10 mA. In addition, it can catch all the residual currents identified by a Type A RCD.
- Type B
Let’s now consider circuits with a 3-phase motor that is controlled by a variable-speed drive. These include medical equipment needing high precision movement and certain kinds of electric vehicle chargers, pumps, and air conditioners.
These applications can have residual current frequencies higher than 50 or 60 Hz, and also non-sinusoidal elements – waveforms created from six-pulse-bridge rectifying circuits, and smooth DC as well. For this use case, a Type B RCD is required. Like a Type F, the Type B RCD can tolerate smooth superimposed DC residual current up to 10 mA and detect all the residual currents caught by a Type F.
Schneider Electric presents a full array of RCDs, including add-on devices for circuit breakers as well as RCCBs (residual current circuit breakers), including type SI and Type B models. We offer highly efficient and affordable products in this category that deliver great value for money.
We hope this article can help you choose the right RCD type for your specific industry or business requirements. However, we recommend that you talk to a dependable provider with a proven track record or qualified and experienced professionals to get suggestions on suitable electrical devices and systems for your particular needs. Experts have the right knowledge and technical know-how to guide you competently and help you select the best solutions for your use cases based on your budget and preferences.