What Is a Residual Current Device?
When you plug in an appliance, it consumes energy and causes an electrical current to flow. Ideally, the current entering the appliance should be identical to the current leaving the appliance (this is the case with resistive appliances such as heaters).
If the incoming and outgoing currents differ, this indicates that a fault current has occurred. The fault current will then be discharged either to earth through a person or directly to earth (in IT, TT and TN-S neutral earthing systems - domestic installations in France use TT systems).
The Different Types of Residual Current Devices
The simplest residual current devices use coils to detect magnetic fields. Current flowing through a wire generates a magnetic field. If the incoming and outgoing currents are identical, the total magnetic field created by both wires will cancel out, resulting in zero net magnetic field.
If the currents are not identical, the coil will detect the resulting magnetic field and convert it proportionally into an electrical current within the residual current device's coil. This measurement can then be used to determine the magnitude of the fault current.
AC Type
This basic device is called an AC-type differential device and is used in standard applications.
An AC-type device operates with alternating current (AC), which is the most common type of electrical current used in homes and industries. AC devices include a wide variety of appliances such as household equipment, power tools, lighting, air conditioners, computers and televisions. It's important to note that alternating current regularly changes direction, meaning the electrical current reverses continuously, typically 50 or 60 times per second.
Type A
To prevent false tripping, it's necessary to upgrade to a measuring device that accounts for the rectified and DC components (up to 6 mA DC) that can be generated. This device is called Type A.
This is why the logo features a rectified waveform symbol.
Applications include: computer outlets, freezers, etc.
Type B
In certain applications, the output current contains a DC component due to the generation of direct current and other current types (such as electric vehicle charging stations). These applications produce output currents with DC (or other) components that cannot be measured by Type A and AC residual current devices. Therefore, a Type B device must be installed to handle these types of output currents.
Applications include: electric vehicle charging stations, high-power rectified power supplies, etc.
Type Si, Hi, HPI
Certain equipment, such as variable speed drives for motors and compressors, generate high-frequency and electromagnetic interference.
These external disturbances can be detected by the measurement system and misinterpreted as faults.
The differential protection system must therefore be shielded against these disturbances. The 'SI' (Super Immunized) differential device provides immunity against electromagnetic interference.
It's typically used in conjunction with Type A or Type B devices. When combined with Type A, it may be designated as Type F.
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