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A residual current is a leakage current that goes from the electrical system to the ground. It occurs due to a ground fault. It is hazardous as it can result in injury or burns, electric shocks and overheating that can lead to a fire. A residual current device or a residual current circuit breaker is used to detect the currents and then disconnect them automatically when the value has exceeded the set limit. A residual current monitoring unit is similar to an RCD and will activate the alarm. However, this does not include the disconnection function.


RCD types: These different categories of RCDs under IEC 60755 are defined based on their ability to trip, sense, and withstand the different currents.

Type AC: This is sensitive to the residual sinusoidal alternating current. (AC)

Type A: This is sensitive to AC and DC or pulsed direct current.

Type B: Type B RCCB is sensitive to residual AC, pulsed AC and smooth DC currents.

Photovoltaic systems require many regulations that have to be provided along with the residual current detection or monitoring. To fulfil these functions, RCD is integrated into photovoltaic inverters. The residual current device is integrated into the photovoltaic inverter for PV systems inverters. They are typically installed into non-isolated grids and require a continuous detector.

The RCCB cannot protect the circuit between the PV inverter and the mains. The protection will have to be at the main source or end of the circuit. For regulations that need protection through the RCD of the AC, the circuit will also require an external RCD.


Understanding how the PV Inverter will affect the selection of RCD

The current path may or may not be needed depending on the way that the inverter is designed. For example, an inverter with no isolation can pass through DC to the AC side unless the design of the inverter will prevent the same.

Wherever a residual current device is used on the AC side of the circuit, the selection of type A, C or B will be based on the knowledge of the type of residual currents present in the circuit on the AC side. During the installation of an inverter, the possibility is present that the residual current is on the AC side as the inverter is designed so that it is specifically limited to the DC residual current.

The proper operation of the RCD can only be ensured when a type BRCD is selected. The ICD in a photovoltaic inverter will protect the photovoltaic array and will not be replaced by the ICD on the AC side of the photovoltaic inverter.


Things to know before selecting a Residual Current Device: 

How do they work?

Residual current devices work by observing the flow of the current in a circuit. If the flow is imbalanced because of a leakage, the device will trip, and the supply will be cut off to the circuit. RCDs usually have a nominal current. Therefore, they can trip beyond the nominal value as well. However, residual current devices cannot limit the current or the voltage. Through this, they will protect by limiting the time where a specific current can flow towards. On the other hand, in a healthy circuit, the current flowing in conductors should come to 0. When a fault occurs and causes a current to flow to earth, it creates an imbalance that leads to the device’s tripping.

What are the hazards involved? 

In your installations, it is evident that a consumer unit is installed that was already supplied with the connected RCDs. Unfortunately, these are usually the type of AC that can be ineffective because of the different kinds of electrical equipment created by the residual DC fault. Unfortunately, many installers misunderstand the limitations for RCDs, specifically for an AC type; they think it is easy to install; however, it is not.

Modern appliances are becoming more sophisticated by the day and feature newer technology that emphasizes energy reduction. Because of this, RCD manufacturers incorporate measures to save energy, for example, speed control which is also an element of DC residual fault current.


How to select the correct Type of Residual Current Device?

It is vital to choose the correct type of residual current device for the equipment to be used. The kind of RCD depends on the characteristics that the equipment holds. However, equipment manufacturers must specify the RCD type that will be required. If the equipment contains inverters, different safety requirements should be followed. There are guidelines to get the correct RCDs for the PV inverters. However, if the information cannot be reached, a solar panel type B RCD is advised to be installed to make up for the worst-case scenario. Hence, it is very important to clearly understand the requirements to ensure that the money is not wasted. The solar energy system in RCD will also be quite expensive.


How to test different Types of RCDs?

No additional requirements are needed for the installer to check for the ongoing operation. The testing will be carried out during the manufacturing process itself and is known as type testing, which is the same way we rely on circuit breakers under faulty conditions. Type A, B RCD is also tested in the same way as other types.

It is not correct to assume that an AC-type residual current device will be perfect for installation. The residual current devices should be chosen according to the guidelines and instructions given by the manufacturer.