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 on its own does not protect against overload and short circuit, nor does it replace a PE protective conductor and earthing.
- The TEST button checks the mechanism, but it 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 appliance and returns through the neutral conductor N - as much as flowed in should return. A residual current device (RCD) compares both values. When part of the current escapes via another path, to the ground or a protective conductor (e.g. through damaged insulation), a residual current (leakage) is created, and after exceeding the threshold, the device disconnects the circuit.
Hence its limitation: an RCD only detects current that has somewhere to "escape". When a person touches both L and N simultaneously, the current will flow through them within the loop and looks 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 reacting 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 casing.
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 approximately 30 milliamperes.
IΔn is a rated value, not a rigid point: the device will trip at the latest at IΔn, but it may already trip above half of this value (for a 30 mA threshold, even around 15 mA) - this is compliant with the standard, not a fault.
The IΔn values are standardised: 30 mA is the most common level of additional protection (life protection), 10 mA is sometimes used in high-risk areas, and higher values (100, 300, 500 mA) have a different purpose, as described below. The PN-HD 60364-4-41 standard requires supplementary RCD protection up to 30 mA, among others, for socket-outlets and mobile equipment up to 32 A - this is a selection requirement implemented in the design.
Types AC, A, F and B - what current shape
The letter indicates what type of leakage current the device will recognise. Appliances with electronics (switched-mode power supplies, inverters, 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 appliance, after analysing the circuit.
Subsequent letters extend the range - from AC (pure sine wave) to B (additionally smooth direct current); details are gathered 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 only a commercial designation, not a normative type from PN.
Time-delay versions and other markings
RCDs also differ in their operating speed: instantaneous ones trip immediately, short-time delay ones have a slight delay and greater resistance to nuisance tripping from surges, and selective ones (letter S) have a longer delay and act as an upstream device. From the casing, you can also read the number of poles, voltage and frequency (Table 2), as well as short-circuit coordination - with which overcurrent protection device the RCD must cooperate to survive a short circuit. All of this visually, without removing the covers.
Most common misunderstandings
"Type B RCD is the same as B characteristic". These are two different things sharing a letter. B characteristic (alongside C and D) describes a miniature circuit breaker (MCB) and indicates at what multiple of the rated current its short-circuit trip will operate. Type B in an RCD describes the waveform of the detected leakage current, including smooth DC. In short: B characteristic is for overcurrent in MCBs, type B is for leakage in RCDs.
"The higher the IΔn, the safer". In terms of human protection, the opposite is true. 30 mA is the level of additional protection for life, while values of 100, 300 or 500 mA are used mainly 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 earth or to the protective conductor. If someone touches both the phase and neutral conductors simultaneously, the current will flow within the loop and look like a normal load to the device. Therefore, an RCD is additional protection, not absolute.
TEST button - what does it check
The TEST button forces an artificial leakage greater than IΔn inside the device and checks whether the tripping mechanism works. This is a user action in accordance with the manufacturer's instructions (often periodically, e.g. once a month). However, the test does not measure the actual operating current and time, nor does it check the earthing and correct connection - this is determined by an electrician using protective measurements.
What an RCD does not do
An RCCB on its own 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 a 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 casing says
Table 1. RCD types simplified.
| RCD Type | What waveforms it detects (simplified) | Where it is considered | What the letter alone does not determine |
|---|---|---|---|
| AC | sinusoidal alternating current | simple circuits without electronics generating pulsating DC | whether pulsating current will actually not occur in the circuit |
| A | like AC + pulsating rectified current | common domestic circuits with typical electronics | whether smooth DC or mixed frequency currents will not appear |
| F | like A + mixed frequency currents (up to ~1 kHz) | circuits with certain drives and inverter-equipped appliances | whether pure smooth DC will not occur (then B is needed) |
| 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 from the casing.
| Parameter from the casing | Technical meaning | In plain English |
|---|---|---|
| In (e.g. 40 A) | rated current, current-carrying capacity of the device | 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 recognize |
| S or time-delay symbol | selective/short-time delay design | whether it should operate first or as a master device |
| Number of poles | how many conductors it interrupts (2P, 4P...) | matching to a 1- or 3-phase circuit |
| Un / frequency | rated voltage and frequency | which network the device is designed for |
When to ask an electrician to check the installation?
It is worth booking an inspection when the residual current device trips repeatedly for no apparent reason, when it does not react after pressing TEST, when it is unknown what type and value of IΔn is installed, or when more electronic devices have been added (photovoltaics, EV charger, inverters) and the installation is older. Repeated tripping is a signal that something requires diagnosis, not that the device is "getting in the way". You must never bypass, block or bridge a residual current device - this removes the protection, not the fault.
In case of emergency 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 only protects as part of a coordinated system.
Want to be sure? Not sure what type of residual current device is operating in your installation, whether the IΔn value matches the circuits, or whether the selection corresponds to today's electronic appliances? During an inspection, we perform 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: most common 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 specified on a single casing.
Can I replace an AC type residual current device 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 a 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 performed by an electrician.
The residual current device trips every few days. Can I block it or install a higher one?
No. Repeated tripping is a symptom of a problem in the installation or appliance and requires diagnosis. Blocking, bridging or raising 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), does not detect series arcs (AFDD) and does not replace a protective conductor or earthing. Safety is provided by a coordinated set of protections, not a single device.
What does the letter S next to the value mean?
This is a selective design, i.e. a device with an intentional time delay acting as a master device. It is meant to operate only if the device closer to the fault fails to do so, which prevents disconnecting 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 - Wyłączniki różnicowoprądowe bez wbudowanego zabezpieczenia nadprądowego (RCCB) (opens in new tab)
- PKN - PN-EN 61009-1:2013-06 - Wyłączniki różnicowoprądowe ze zintegrowanym zabezpieczeniem nadprądowym (RCBO) (opens in new tab)
- PKN - PN-EN 62423:2013-06/A11:2022-02 - Wyłączniki różnicowoprądowe typu F i typu B (opens in new tab)
- Eaton - Residual current device guide (PL) (opens in new tab)
- elektro.info - Zasady ochrony przeciwporażeniowej w świetle normy PN-HD 60364-4-41 (opens in new tab)
- Doepke Polska - RCD typ A, AC czy B? Dobór różnicówek (opens in new tab)
- VDE-Verlag - DIN VDE 0664-400 (VDE 0664-400) 2012-05 - RCCB Type B+ (opens in new tab)
- Kanał elektryczny - Pomiary i sprawdzanie wyłączników różnicowoprądowych (opens in new tab)
