Residual Current Devices Selection Guide
A residual current device measures the difference between phase and neutral currents and trips the circuit above a threshold. AC, A and B types, 30/300 mA, pole choice.
What is a residual current device? A residual current device (RCD, RCCB) is a protective device that measures the difference between the currents flowing through the phase and neutral conductors of a line and opens the circuit when that difference exceeds its set threshold. 30 mA types protect people against electric shock, and 300 mA types protect the installation against the fire risk that leakage current can cause. It provides no overcurrent or short-circuit protection; for that you need a fuse or a residual current circuit breaker with overcurrent protection (RCBO).
This guide covers what to look at when choosing products in the Residual Current Devices category.
How a residual current device works
All live conductors of the line, including the neutral, pass through the summation current transformer inside the device. In a healthy circuit the instantaneous sum of these currents is zero; if part of the current flows to earth through the device housing or a person, the balance is upset and the tripping mechanism is activated. For this reason, on lines where the neutral does not pass through the device, or is joined to the neutral of another circuit on the output side, the device trips without any fault being present.
For a sinusoidal residual current, the device must trip at a value between 50% and 100% of its rated residual current. A 30 mA device therefore trips between 15 mA and 30 mA; when the small operating leakage currents of the appliances connected to the same device add up and approach this level, apparently unexplained trips begin.
The device does not treat current flowing from phase to neutral or from phase to phase as leakage; it provides no protection against current flowing along that path.
Types of residual current devices
- Residual current switch (RCCB): DIN rail devices that provide residual current protection only. ABB F200, Schneider Electric Acti9 iID and Chint NL1 are of this type.
- Combined type with overcurrent protection (RCBO): the residual current and miniature circuit breaker functions are in a single housing; the product name shows the tripping curve and a fuse rated current such as 4 A, 6 A or 16 A (Siemens 5SV1313). The main category for this group is the Miniature Circuit Breakers with Residual Current Protection page.
- Residual current blocks: modules that are fitted next to a miniature circuit breaker to add residual current protection to it. ABB DDA200 and Schneider Electric Vigi iC60 blocks are not used on their own; they are selected together with a compatible breaker series.
- Residual current detection relays: the threshold is adjustable in the Tense TAR series; the product names state adjustment ranges of 30 mA – 3 A and 30 mA – 30 A. These relays measure leakage with a toroidal current transformer and do not interrupt the line themselves; tripping is done by a contactor connected to the relay output according to the manufacturer's diagram, or by the shunt trip coil of a circuit breaker. In the TAR-35D and TAR-70D models the toroid is inside the housing.
What the AC, A, B and S types mean
| RCD Type / Threshold | Detected Residual Current Waveform | Typical / Mandatory Application | Catalog Link |
|---|---|---|---|
| Type AC | Sinusoidal alternating current (AC) only | Purely resistive loads and standard non-electronic circuits | Residual Current Relays |
| Type A | Sinusoidal AC + pulsating DC (up to 6 mA smooth DC) | Single-phase rectifier loads, appliances, IT power supplies and LED drivers | Residual Current Relays |
| Type B | AC + pulsating DC + smooth DC + high frequency (up to 1 kHz) | Three-phase VFDs, EV charging stations, PV solar inverters and UPS | Residual Current Relays |
| RCCB vs RCBO | Combined overload + short-circuit + earth leakage | Dedicated critical circuits use RCBOs to isolate faults per line | RCCB vs RCBO Difference Guide |
The type letter shows which form of residual current the device detects:
- Type AC: sinusoidal alternating residual currents.
- Type A: in addition to those, pulsating direct residual currents. Its function is assured as long as the smooth direct current superimposed on it does not exceed 6 mA. It is used on lines with single-phase electronic equipment containing rectifiers.
- Type B: also detects smooth direct residual currents and alternating residual currents up to 1 kHz. For equipment such as three-phase frequency converters, electric vehicle charging units and photovoltaic inverters, the equipment manufacturer states the type required. ABB F202 B and F204 B models are listed in this category.
Devices marked S (selective) trip with a delay and are used upstream of non-delayed devices, on the incoming side. ABB F202 A S and F204 A S, Schneider Electric iID 300mA-S and the 5SV models that Siemens product data lists as selective belong to this group. ABB's models with the AP-R suffix are the short-delay type; the product data describes them as having high immunity to unwanted tripping.
Residual current threshold and number of poles
Thresholds fall into two groups. 10 mA and 30 mA are high-sensitivity thresholds; they are used for additional protection against direct contact (life protection), and the highest threshold accepted for this purpose is 30 mA. Medium-sensitivity thresholds such as 300 mA are for fire protection; a 300 mA device cannot take the place of a 30 mA device. Which threshold is mandatory on which circuit is determined by the applicable installation regulations and the project.
A 2-pole device is used on a single-phase line, and a 4-pole device on a three-phase line with neutral; in both cases the neutral passes through the device.
How to read the product name and label
The label carries two current values: the rated current (In) that the device can carry continuously, and the rated residual current (IΔn), which is the tripping threshold. In ABB naming, F204 AC-80/0.3 is a device from the F200 series that is 4-pole, type AC, with an 80 A rated current and a 0.3 A, that is 300 mA, threshold; F202 A-63/0.03 is 2-pole, type A, 63 A and 30 mA. In Schneider Electric product names the number of poles, rated current, threshold and type are given; in Chint names the number of poles, rated current and threshold are stated explicitly. Siemens 5SV names do not include the threshold; check the rated residual current value in the product information.
The rated current is not a tripping value: a 40 A device does not trip at currents above 40 A, it is only suitable for carrying current up to that value. The device is therefore selected together with the fuse on the same line; the manufacturer data states which rated current of fuse can be used to protect it.
Brands in this category
- ABB: F200, FH200 and FMF residual current switches and DDA200 residual current blocks; AC, A and B types.
- Schneider Electric: Acti9 iID and Easy9 residual current devices, Vigi iC60 blocks; 10 mA, 30 mA and 300 mA thresholds.
- Siemens: 2- and 4-pole 5SV residual current devices and 5SV1313 combined types.
- Chint: NL1 series 2-pole, 30 mA and 300 mA devices.
- Tense: TAR series adjustable residual current detection relays.
Ordering notes
The devices are sold per piece. For models shown as made to order, the delivery time is confirmed before the order is placed. Codes such as A9R41463 or 5SV3642-8 take you straight to the product in site search. Schneider Electric ARA automatic reclosing modules and the Siemens 5ST38 latch lock and spacer set are accessories, not residual current devices.
Frequently asked questions
Why does a residual current device trip so often?
There are three common causes. The first is a genuine insulation fault: a damp appliance, a crushed cable or a faulty heater. The second is total leakage current; because the device can trip from half of its rated threshold, the small leakage currents of many electronic devices connected to one device add up and cause a trip even without a fault. The third is a wiring error: the neutral on the output being joined to the neutral of another circuit. The source is found by separating the circuits one by one; the device must not be taken out of service without eliminating the cause.
Is a 300 mA residual current device enough for personal protection?
No. Medium-sensitivity thresholds such as 300 mA are used to prevent leakage current arising from an insulation fault from starting a fire, and they are mostly placed at the main incoming supply. For additional protection against electric shock, high-sensitivity thresholds such as 30 mA or 10 mA are used. The two thresholds are not alternatives to each other; a 300 mA selective device at the incoming supply and 30 mA devices on socket and wet-area lines can be used together. The regulations and the project determine the mandatory threshold.
When is a type A residual current device needed instead of type AC?
Type AC detects only sinusoidal alternating residual current. A fault in equipment containing rectifiers, such as washing machines, dishwashers, computer power supplies and LED drivers, can produce leakage in the form of pulsating direct current; type A detects this. Type B may be required for equipment that can produce smooth direct current leakage, such as three-phase drives, vehicle charging units and inverters. Base your choice on the residual current device type given in the equipment's user manual.
What is the difference between a residual current device and a residual current circuit breaker?
A residual current device (RCCB) detects only residual current; for overload and short circuit, the line must also have a separate miniature circuit breaker, and a single device often protects several lines at once. A residual current circuit breaker with overcurrent protection (RCBO) combines the two functions in one housing and is fitted separately on each line; this way a leak on one line does not cut power to the other lines. The RCBO product name also includes a tripping curve such as B or C.
What does it mean if the device does not trip when the test button is pressed?
The test button creates an artificial residual current inside the device and shows that the tripping mechanism works. If it does not trip when the button is pressed while the device is energized, the mechanism is not doing its job and the device must be replaced. Because the test circuit operates on mains voltage, a test made while the device is not energized gives no result. The button gives no information about the condition of the installation and the earthing; these are the subject of periodic checks made with measuring instruments. For the test interval, see the manufacturer's instructions.
What is a selective (type S) residual current device for?
A type S device trips after a short delay. When a type S device is at the main incoming supply and non-delayed devices are on the downstream outputs, a leak on one output first trips that output's device and the rest of the installation stays energized. If two non-delayed devices are connected in series, it is not certain which one will trip. For full selectivity, the rated residual current of the incoming device is chosen to be at least three times that of the device on the output; for example, a 300 mA selective device is placed upstream of 30 mA outputs. The pairing is verified from the manufacturer data.