Why Four Types of Residual Current Devices? It’s all About the Load.
Protecting people from shocks has always been a priority in AC electrical system design. Equipment is typically designed to provide a barrier to anyone making physical contact with a live conductor, either through the use of insulation or other means. However, such contact can sometimes accidentally occur, causing what’s termed an ‘earth leakage’ or ‘residual’ fault current to pass through the person’s body. And it only takes 30 mA of current through a hand to the feet to cause death, unless the current is interrupted within a very short amount of time.
Residual current devices (RCDs), or GFCI in North America, are designed to protect again this risk. However, there are new challenges due to the growth of electrical system applications, such as electrical vehicle charging stations, photovoltaic installations, and the wider use of variable speed drives for motor control. This has resulted in residual currents that are not purely sinusoidal. In this two-part blog post series, I’ll provide a quick overview of the principle types of RCDs on the market today, and the types of risks each is designed to protect against.
Matching the RCD to the load
The IEC 60755 standard defines four types of RCDs for AC applications. Each of these addresses different types, or combinations, of residual current waveforms.
- Type AC. These type of RCDs are used where the fault current is expected to be sinusoidal and at the same frequency as the supply. For example, this will be the case when a fault occurs on the supply conductors or when the load is resistive or linear. These are considered general-purpose RCDs in many countries.
- Type A. Some kinds of loads, such as ones with a single-phase rectifying circuit like a heating plate, will create a pulsating DC residual current. For these applications, a Type A RCD is required, which can detect this kind of residual current. These can also withstand a superimposed ‘smooth’ DC current up to 6 mA. They will also detect the sinusoidal residual currents that the Type AC RCD can.
- Type F. In some applications there may be ‘composite’ residual currents, ranging from a few to 1000 Hz. For example, circuits with single-phase motors being controlled by a variable-speed drive, like a heat pump or air conditioner. For these applications, a Type F RCD is required. These can also withstand superimposed smooth DC residual current up to 10 mA. And they can also detect all of the residual currents detected by a Type A RCD.
- Type B. Now let’s look at circuits with a 3-phase motor being controlled by a variable-speed drive. This can include certain types of air conditioners, pumps, electric vehicle chargers, or medical equipment requiring high precision movement. In these cases, there can be residual current frequencies greater than 50 or 60 Hz, as well as non-sinusoidal components, waveforms resulting from six-pulse-bridge rectifying circuits, and even smooth DC. In this case, a Type B RCD is needed. Similar to a Type F, the Type B can withstand superimposed smooth DC residual current up to 10 mA, as well as detecting all the residual currents detected by a Type F.
In my next post, we’ll have a closer look at Type B RCDs, including how they work, their applications, and coordinating with other RCD types. We’ll also have a look at a special ‘super immunity’ Type SI RCD for high frequency earth leakage applications.
Schneider Electric offers a complete range of RCDs, from add-on devices for circuit breakers to complete residual current circuit breakers (RCCB), including Type B and Type SI models. For more information about choosing the right RCD type, including relevant standards, download the white paper “Why to Choose Type B Earth Leakage Protection for Safe and Efficient People Protection”.
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