Key takeaways in 30 seconds
- An RCD compares the current entering the circuit with the returning current; when a part "escapes" sideways, it trips the circuit.
- 40 A is the rated current (load capacity), and 30 mA is the rated residual operating current IΔn at which the RCD will trip - two different values.
- The letter AC, A, F, or B indicates what shape of leakage current the device detects; this is not the B/C/D characteristic of an overcurrent circuit breaker.
- An RCCB alone does not protect against overload and short circuit, nor does it replace the PE protective conductor and earthing.
- The TEST button checks the mechanism but does not replace periodic measurements by an electrician.
What it does and how it works
A circuit is a closed loop: current flows through the phase conductor L to the receiver and returns through the neutral N - what went in should come back. A residual current device (RCD) compares both values. When part of the current escapes via another path, to the ground or the protective conductor (e.g. through damaged insulation), a residual current (leakage) occurs, and once the threshold is exceeded, the device trips the circuit.
Hence its limitation: an RCD only detects current that has somewhere to "escape". If a person touches both L and N simultaneously, the current will flow through them within the loop and will look like a normal load to the device - the RCD will not trip. This is additional (supplementary) protection, not absolute protection.
RCD, RCCB and RCBO
RCD is the name of a group of devices that react to residual current. An RCCB reacts only to leakage (standard PN-EN 61008-1) and will not protect the cable against overload or short circuit, so it works in tandem with a separate miniature circuit breaker (MCB). An RCBO combines both functions in a single module (standard PN-EN 61009-1) - an RCCB and an MCB in one housing.
40 A versus 30 mA - two different numbers
The rated current (In), e.g. 40 A, is the load capacity - how much operating current can continuously flow through the device. The rated residual operating current (IΔn), e.g. 30 mA, is the leakage level at which the device will trip. "40 A / 30 mA" means: I conduct up to 40 amps, and I trip at a leakage of around 30 milliamperes.
IΔn is a rated value, not a rigid point: the device will trip at IΔn at the latest, but it may already trip above half of this value (for a 30 mA threshold, even around 15 mA) - this behaviour complies with the standard, it is not a fault.
IΔn values are standardised: 30 mA is the most common level of additional protection (life protection), 10 mA is sometimes used for increased risk, and higher values (100, 300, 500 mA) have a different purpose, as described below. Standard PN-HD 60364-4-41 requires supplementary RCD protection up to 30 mA, among others, for socket-outlets and portable equipment up to 32 A - this is a selection requirement implemented in the design.
Types AC, A, F and B - what shape of current
The letter indicates what type of leakage current the device will recognise. Receivers with electronics (switched-mode power supplies, frequency converters, chargers) produce leakages other than a pure sine wave, and a small direct current can "blind" a simpler device. The type is therefore selected to match the receiver, following a circuit analysis.
Subsequent letters extend the range - from AC (pure sine wave) to B (additionally smooth direct current); details are summarised in Table 1. Product standards: PN-EN 61008-1 and PN-EN 61009-1, and for types F and B additionally PN-EN 62423. "B+" is not a type from IEC or European standards - it is described by the German standard DIN VDE 0664-400, and in Poland it is merely a trade name, not a normative type from PN.
Time-delay versions and other markings
RCDs also differ in operating speed: instantaneous ones trip immediately, short-time delay ones have a slight delay and higher resistance to nuisance tripping from surges, and selective ones (letter S) have a longer delay and act as an upstream device. From the housing, you can also read the number of poles, voltage and frequency (Table 2), as well as short-circuit coordination - which overcurrent protection the device must cooperate with to survive a short circuit. All of this is done visually, without removing the covers.
Most common misunderstandings
"A type B RCD is the same as a B characteristic". These are two different things sharing a letter. Characteristic B (alongside C and D) describes a miniature circuit breaker (MCB) and indicates at what multiple of the rated current its short-circuit release will operate. Type B in a residual current device describes the shape of the detected leakage current, including smooth direct current. In short: characteristic B refers to overcurrents in an MCB, type B refers to leakage in an RCD.
"The higher the IΔn, the safer it is". In terms of human protection, it is the opposite. 30 mA is the level of additional protection for life, while values of 100, 300 or 500 mA are mainly used for fire protection and selectivity - they do not replace 30 mA where such protection is required.
"An RCD will protect in every situation". No. The device only detects current escaping outside the loop, to the ground or the protective conductor. If someone touches both the phase and neutral conductors simultaneously, the current will flow within the loop and will look like a normal load to the device. Therefore, an RCD is additional protection, not absolute protection.
The TEST button - what it checks
The TEST button forces an artificial leakage inside the device greater than IΔn and checks whether the tripping mechanism works. This is a user action in accordance with the manufacturer's instructions (often periodic, e.g. once a month). However, the test does not measure the actual operating current and time, nor does it check the earthing and correctness of the connection - this is determined by an electrician through protective measurements.
What an RCD does not do
An RCCB alone does not protect the cable against overload and short circuit (that is what an MCB or RCBO is for). It does not detect an L-N short circuit or a series arc, which is handled by an arc fault detection device (AFDD). It does not replace the PE protective conductor, earthing, or basic protection. For disconnecting the installation for work, a main switch-disconnector is used.
What the letter says and what the housing says
Table 1. RCD types simplified.
| RCD Type | What waveforms it detects (simplified) | Where it is considered | What the letter alone does not resolve |
|---|---|---|---|
| AC | alternating sinusoidal current | simple circuits without electronics generating pulsating DC | whether pulsating current will actually not occur in the circuit |
| A | as AC + pulsating rectified current | common domestic circuits with typical electronics | whether smooth DC or mixed frequency currents will not appear |
| F | as A + mixed frequency currents (up to ~1 kHz) | circuits with certain drives and equipment with frequency converters | whether pure smooth DC will occur (then type B is required) |
| B | like F + smooth DC and high-frequency currents | circuits where smooth direct current may occur | IΔn values, polarity, selection for a specific installation |
Table 2. Parameters on the casing.
| Parameter on the casing | Technical meaning | In plain English |
|---|---|---|
| In (e.g. 40 A) | rated current, device load capacity | how much operating current can continuously flow through it |
| IΔn (e.g. 30 mA) | rated residual operating current | at what leakage level the device trips the circuit |
| Type (AC/A/F/B) | class according to the shape of the detected current | what "type of current leakage" the device will recognise |
| S or delay symbol | selective/short-time delay version | whether it should trip first or act as a master device |
| Number of poles | how many conductors it disconnects (2P, 4P...) | matching a 1- or 3-phase circuit |
| Un / frequency | rated voltage and frequency | which grid the device is designed for |
When should you ask an electrician to check the installation?
It is worth scheduling an inspection when the RCD trips repeatedly for no apparent reason, when it does not react after pressing the TEST button, when it is unknown what type and IΔn value are installed, or when more electronic devices have been added (photovoltaics, EV charger, inverters) and the installation is older. Repeated tripping is a sign that something requires diagnosis, not that the device is "getting in the way". Never bypass, block or bridge an RCD - this removes the protection, not the fault.
In case of alarm symptoms (burning smell, smoke, sparking, hot casing), disconnect the power supply, provided you can do so without touching the damaged component, and call an electrician. Do not open the distribution board or carry out repairs yourself.
Summary
40 A is the load capacity, 30 mA is the leakage threshold, and the letter AC, A, F or B indicates what shape of current the RCD will detect. An RCCB reacts only to leakage, while an RCBO combines this function with overcurrent protection. The device does not replace a protective conductor or earthing and protects only as part of a coordinated system.
Want to be sure? Not sure what type of RCD is operating in your installation, whether the IΔn value matches the circuits, or if the selection corresponds to today's electronic appliances? During an inspection, we carry out visual checks and protective measurements (including RCD tripping time and current) and provide a documented assessment with recommendations. Order an inspection with measurements: book a visit online or call +48 459 566 991.
FAQ: frequently asked questions
Are 30 mA and 40 A the same value expressed differently?
No. 40 A (amperes) is the operating current that the device can conduct. 30 mA (milliamperes) is the leakage level at which it trips the circuit. These are two independent values indicated on a single casing.
Can I replace an AC-type RCD with an A or B type myself?
This is not a task for a user. The choice of type depends on the appliances and circuit configuration, and replacing a device in the distribution board is a job for an electrician. The user can visually read the markings and report the need to verify the selection.
How often should I press the TEST button?
According to the manufacturer's instructions for the specific device; it is often recommended periodically, for example once a month. The test confirms the operation of the mechanism, but does not replace measurements carried out by an electrician.
The RCD trips every few days. Can I block it or install one with a higher rating?
No. Repeated tripping is a symptom of a problem in the installation or an appliance and requires diagnosis. Blocking, bridging or increasing the IΔn on your own removes the protection, not the cause.
Is an RCD alone enough to make an installation safe?
No. An RCCB does not protect against overload and short circuit (that is what an MCB or RCBO is for), it does not detect series arc faults (AFDD), and it does not replace a protective conductor or earthing. Safety is ensured by a coordinated set of protective devices, not a single unit.
What does the letter S next to the value mean?
It stands for selective type, which is a device with an intentional time delay acting as a master device. It is designed to trip only if the device closer to the fault fails to do so, which prevents shutting down the entire installation in the event of a local leakage.
Sources and legal basis
Verification date: 18 July 2026.
- PKN - PN-EN 61008-1:2013-05 - Residual current operated circuit-breakers without integral overcurrent protection (RCCBs)
- PKN - PN-EN 61009-1:2013-06 - Residual current operated circuit-breakers with integral overcurrent protection (RCBOs)
- PKN - PN-EN 62423:2013-06/A11:2022-02 - Type F and Type B residual current operated circuit-breakers
- Eaton - Residual current device guide (PL)
- elektro.info - Rules of electric shock protection in the light of the PN-HD 60364-4-41 standard
- Doepke Polska - RCD type A, AC or B? Selection of RCDs
- VDE-Verlag - DIN VDE 0664-400 (VDE 0664-400) 2012-05 - RCCB Type B+
- Kanał elektryczny - Testing and inspection of residual current devices
