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About FI switches
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What was an RCD?
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How do FI circuit breakers work?
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What types and types are there?
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What standards and regulations are there for residual current circuit breakers?
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What is important when purchasing an FI circuit breaker?
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Our practical tip: Function test
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FAQ - Frequently asked questions about RCD circuit breakers
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Conclusion: This way you get the right FI-circuit breaker
What was an RCD?
A residual current circuit breaker, also known as RCD, residual current circuit breaker or, in short, RCD, is used to detect residual currents and, if the permissible residual current is exceeded, to disconnect the connected circuits and electrical devices from the mains.
Fault currents are all currents that do not flow in the cables intended for this purpose, but rather take other, undesirable paths. In conventional networks, this is the way over the ground potential that (in the case of electrical devices of protection class I, i.e. with protective earth conductor) can lead over the protective earth conductor or – much less favorably – when a live component is touched over the human body.
In order to detect such fault currents, all current-carrying cables (L1 and N, in single-pole networks or L1, L2, L3 and N in three-phase systems) are routed via a so-called summation current transformer in the case of residual current operated circuit breakers.
The summation current transformer compares the operating currents for symmetry, thus monitoring whether the sum of the currents flowing in and out is exactly the same. Deviations cause an imbalance of the magnetic alternating fields in the converter core, which induces a voltage in its secondary winding.
As a result, an electromechanical actuating mechanism triggers the contacts, the residual current circuit breaker separates all-pole cables and connected consumers from the mains in a very short time.
How do FI circuit breakers work?
As already mentioned, it measures the current flowing in and out in a circuit. For this purpose, the return conductors, i.e. P and N, are guided through a ferromagnetic toroidal core, which functions as a total current transformer. If both conductors carry the same current, but in opposite directions, the magnetic alternating fields are lifted. A secondary winding is also applied to the Kern, which only releases a voltage when a magnetic field is induced. This is only the case if the respective currents differ in P and N. The voltage generated in the secondary winding triggers the switching mechanism in the event of a fault, which interrupts the circuit on all poles. To ensure that the FI switch also functions properly, a current corresponding to the rated differential current can be generated via a resistor using a test button. A functioning residual current circuit breaker must then trip.
Switching principle of a single-phase FI circuit breaker.
Principle of residual current circuit breakers:
The return and return lines (L, N) pass through a ferromagnetic toroidal core (3). If the currents are the same, the magnetic fields will pick up. In the case of a current difference, a voltage is induced in the secondary winding (2) which triggers the mechanism for switching off (1). Via the test button (4), the current through the resistor triggers the protection function manually.
What types and types are there?
Residual current circuit breakers are manufactured in two- and four-pole versions and with different current carrying capacity. The most commonly used four-pole types have a current rating of 25, 40 or 63 amps.
The most common are FI switches with a measured residual current ("residual current") of 30 mA. A total of 10, 30, 100, 300 and 500 mA versions are offered.
If you look away from the so-called PRCD circuit breakers (the "P" stands for "Portable"), which are available as intermediate connectors or integrated in socket strips, FI circuit breakers are usually installed permanently in power distribution units.
Like other built-in devices, they are suitable for snapping on mounting rails of type TS 35. FI switches are also available with integrated miniature circuit breakers. For different purposes, FI circuit breakers with different sensitivities are offered.
Currently the most common type is "A", which is sufficient for normal households and offices. Type "F" is also able to detect fault currents with mixtures of different frequencies up to 1 kHz, and type "B" also covers DC fault currents.
The "B+" version extends the sensitivity by a fault current detection at higher frequencies up to 20 kHz.
What standards and regulations are there for residual current circuit breakers?
Because currents above 30 mA (0.03 A) in humans cause heart chamber flickering and are therefore fatal, the rated differential current of FI switches is a maximum of 30 mA.
FI-switches of type A without LS function are specified in DIN EN 61008-1 (VDE 0664 Part 10):2000-09 and DIN EN 61008-2-1 (VDE 0664 Part 11):1999-12 and those with LS function in DIN EN 61009-1 (VDE 0664 Part 20):2000-09 and DIN EN 61009-2-1 (VDE 0664 Part 21):1999-12.
For type B, only the standard design DIN VDE 0664-100 (VDE 0664 part 100):2002-05 exists.
Since May 1, 1984, according to DIN VDE 0100-701:2008-10, it has been stipulated that circuits in new buildings must be protected in rooms with bathtubs and showers with residual current circuit breakers. Fixed boilers are excluded.
As of February 1, 2009, according to DIN VDE 0100-410:2007 (section 411.3.3), all socket circuits used by non-specialists must be protected with residual current circuit breakers in new buildings.
Although there is no obligation to retrofit, it is recommended to supplement older installations with residual current circuit breakers. These can be installed in fuse boxes and sub-distributions with little effort.
RCCB has a test button. This should be exercised at least twice a year. This not only checks the function, but also moves the contact mechanism, which could be mechanically jammed over a longer period of time or the contacts may be stuck together.
What is important when purchasing an FI circuit breaker?
Depending on the intended use, additional sensitivities regarding the residual current form may be required. Especially in the industrial environment fault currents with higher frequencies occur, which are caused by frequency converters, for example. However, appliances that can potentially cause such leakage are increasingly being used in households and offices. These include modern washing machines, air conditioning systems, dishwashers and similar appliances that have integrated a frequency converter for speed control.
Switching power supplies in the form of external or integrated power supplies can also be used in the event of a fault as the cause of a wide range of frequency mixtures and DC voltage components. If there are uncertainties regarding the selection of the appropriate RCD, it is advisable to consult an electrician or to use a FI-circuit breaker of type "B" or "B+" if there are high demands on personnel and fire protection.
Practical advice
RESIDUAL CURRENT circuit breakers are reliable and durable. They do not require maintenance, but should be regularly checked for proper operation. This is done via the test button, which each FI switch has. Pressing the test button internally generates a residual current over a resistor slightly above the rated tripping current. The FI switch must trigger immediately upon pressing a button.
This makes it easy to check both electrical and mechanical functionality. In private households, the function should best be tested approximately every six months; in commercial areas, shorter test intervals may be required.
Professionals only switch off residual current operated circuit breakers via the test button, so that the correct function is checked in the same move.
FAQ - Frequently asked questions about RCD circuit breakers
Where do FI circuit breakers need to be installed?
In Germany, installation in rooms with shower or bathtub has been mandatory for new buildings since 1984, from 2009 all circuits must be equipped with a rated current of up to 20 ampere with a residual current circuit breaker. Although existing buildings are protected against inventory, retrofitting is always recommended, and is the most effective protection against dangerous electrical accidents. The cost and effort of retrofitting are low, but the added security is quite significant.
How many residual current operated circuit breakers are required in the house?
In the commercial sector, the rule applies that in the event of a fault, the entire electrical system must not fail. The distribution to at least two residual current operated circuit breakers is therefore essential. However, the use of several residual current circuit breakers and the splitting of the circuits makes absolutely sense in private residential buildings too, so that triggering, for example by a defective electrical device, does not lead to a complete power failure in the entire house.
An extension, modification or refurbishment of the existing electrical installation is planned, will this make an FI switch mandatory?
Generally yes. Only very simple changes to the inventory system, such as the replacement of a defective socket, do not require retrofitting. For all other changes or expansions, the relevant circuit must be adapted to the current state of the art.
Where is the FI switch installed?
Residual current circuit breakers are installed, such as miniature circuit breakers, in sub-distribution boards (distribution cabinets) and behind the main fuses. The miniature circuit breakers are then connected to the ground fault circuit interrupter. It is recommended that this work be carried out by a qualified electrician.
Conclusion: This way you get the right FI-circuit breaker
Single-phase power supply networks require 2-pole residual current circuit breakers, 4-pole residual current operated switches are used in three-phase power supply systems. The current carrying capacity must be equal to or higher than the upstream main fuses. RESIDUAL CURRENT operated circuit breakers designed for personal protection must not have a rated trip current higher than 30 mA. When it comes to pure fire protection, a residual current circuit breaker with a maximum trip current of 300 mA must be selected.
GROUND FAULT circuit breakers with integrated circuit breakers are space-saving and are suitable for fully protecting individual circuits. Note the type class (sensitivity to the type of fault currents). FI switches of type "AC" are no longer permitted in Germany, therefore for general standard applications the type "A" must be selected, which also detects pulsating DC fault currents. All-current sensitive residual current circuit breakers of type "B" and "B+" currently offer the highest possible safety in the most varied residual current forms.