Residual current device protection (RCD) in EV charging

A residual current device (RCD) is an electrical safety device that quickly disconnects a circuit when it detects an imbalance in the electric current, indicating a leakage or fault to the ground. In an EV charger, an RCD is crucial for safety and enhances the overall safety of the EV charging process, protecting users and equipment.

RCDs come in different types, starting from a basic to an advanced level. The levels in each RCD represent the types of fault handled. Figure 1 illustrates the RCDs type and how each RCD is unique in working and detecting the faults.

Figure 1. Types of RCD, the applicable fault and load type. (Image: Electrical Installation Guide)

Type AC: This is the most basic type of RCD used in most applications. The Type AC RCD is designed to detect and respond to ac faults. Therefore, they have a sine wave symbol representation.

Type A: This category of RCD is designed to detect ac and pulsating dc faults. It offers a higher level of protection, making it suitable for a broader range of applications, including those involving electronic or semiconductor devices. They carry a sine wave and a pulsating dc symbol for representation.

Type F: In addition to the faults covered by Types A and AC, Type F RCD covers the residual current with multiple frequencies up to 1 kHz. However, the residual fault current should limit the pure dc component to 10mA or lesser. Their representation includes a sine wave, a pulsating dc, and a high-frequency sine wave.

Type B: This is the most advanced type of RCD, which also covers the smooth residual dc current and the previously mentioned fault components. They’re used in high-power applications such as EV charging systems, PV systems, etc., where a three-phase rectification is employed.

Which type of RCD is ideal for EV charging?
The IEC 60364 standards state: “Each ac connecting point shall be individually protected by a residual current device (RCD) with a rated residual operating current not exceeding 30 mA.”

Having mentioned the different types of RCDs, it’s clear that ac RCD is not the ideal choice for EV charging. This is because EV charging takes place with dc power. The type F RCD can also be avoided as EV charging does not strictly need protection against the high-frequency residual current.

We’re left out with Types A and B RCDs suitable for EV charging. Yet, type B is a straightforward choice. However, there are instances where a Type A is needed.

Figure 2 shows the line diagram of an EV charging system where Type B RCD is used for protection. In an EV charging station where multiple EVs are charged, numerous electric vehicle supply equipment (EVSE) are used. Therefore, each EVSE is equipped with a Type B RCD.

Figure 2. Single line diagram of a EV charging station incorporating Type B RCD. (Image: Schneider Electric)

The Type B RCD is used where a three-phase rectifier is used for high-power applications such as the EV charging system. But there’s still a lot of infrastructure dependent on single-phase supply. In such cases, type A RCD, as shown in Figure 3, can be used but with a condition.

A separate device called residual direct current detecting device (RDC-DD) must be used with the type A RCD. The RDC-DD detects dc residual current of 6mA or less. It’s a monitoring device and not a protection device. Therefore, the protection is carried out by the 30 mA type A RCD.

Figure 3. A single-line diagram of a EV charging station incorporating Type A RCD. (Image: Schneider Electric)

Summary
A RCD is a crucial component for protection against electric shocks and faults. There are many types of RCDs in the market, but the type B is the best suitable for high-power EV charging. This is because type B can detect ac, pulsating dc, multiple frequencies, and smooth residual dc. However, the modest type A can also be used in conduction with a RDC-DD to monitor any residual dc of 6mA or more for protection.

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