300 mA and 30 mA RCDs: which protects what

Two sensitivities, two roles. In a domestic installation, Book 1 puts an RCD (residual current device) of 300 mA at most at the origin of the installation, then, immediately downstream, at least one high or very high sensitivity RCD, that is 30 mA at most, for a list of circuits. Confusing them leads to an infringement, or to nuisance tripping. Time to set things straight.

The principle: measuring leakage current

An RCD constantly compares the current flowing out to the installation with the current coming back. As long as the two are equal, all is well. As soon as part of the current escapes elsewhere, to earth through an insulation fault or through a person, the difference appears and the device cuts off. Its sensitivity, the rated residual operating current, tells you at what leakage current it trips: a 30 mA device cuts off at a leakage ten times smaller than a 300 mA one.

Section 2.6.4 of Book 1 classifies devices by sensitivity. For alternating current, high sensitivity runs from above 10 mA up to 30 mA, very high sensitivity stops at 10 mA, and medium sensitivity runs from above 30 mA up to 1000 mA. A 300 mA device is therefore a medium sensitivity device.

The main RCD at the origin of the installation

According to subsection 4.2.4.3 of Book 1, at least one RCD with a rated residual operating current of 300 mA at most is placed at the origin of the domestic installation. It also provides the isolation function. By way of derogation, it may be placed in the main distribution board if the wiring supplying that board is class II or has equivalent insulation. Until 31 October 2026, its input terminals are made inaccessible by a seal fitted by the person in charge of the conformity inspection before commissioning. The royal decree of 13 August 2026 removes this seal from 1 November 2026.

Subsection 5.3.5.3 adds that RCDs in domestic installations are at least type A, and that those at the origin have a rated current of at least 40 A.

The 30 mA devices: where they are mandatory

Immediately downstream of the main RCD, subsection 4.2.4.3 requires at least one high or very high sensitivity RCD, to which at least the following are connected:

• socket outlets not intended for fixed or stationary appliances;
• lighting;
• locations containing a bath or shower (chapter 7.1);
• washing machines, tumble dryers and dishwashers.

For the bathroom, subsection 7.1.4.3 specifies that these RCDs are installed outside the location containing the bath or shower. The detail of the volumes is covered in our article on volumes 0, 1 and 2.

Distribution behind the 30 mA devices

Subsection 4.2.4.3 allows at most eight final circuits per high or very high sensitivity RCD. If the earth electrode resistance exceeds 30 ohms, at least two are needed, and each takes at most sixteen single or multiple socket outlets, a fixed appliance or a group of fixed appliances controlled by a common switching device counting as one socket outlet.

The other circuits, for example fixed appliances such as refrigeration, cookers or electric heating, may be connected directly downstream of the main RCD, or on additional RCDs at the origin. If the earth electrode resistance exceeds 30 ohms, those RCDs at the origin are also supplemented, for these circuits, by at least one RCD of 100 mA at most per circuit or group of circuits. And electric heating using resistance elements embedded in a wall, supplied at a voltage above 25 V AC, 36 V unsmoothed DC or 60 V smooth DC, is protected separately by an RCD of 100 mA at most (subsection 4.2.4.3). Finally, subsection 5.3.5.3 requires RCDs to be chosen and the load distributed so that the normal leakage currents of appliances do not cause nuisance tripping.

The type of RCD

Sensitivity is not the whole story: the type of RCD refers to the shapes of fault current it detects. In domestic installations, Book 1 requires at least type A (subsection 5.3.5.3). When equipment likely to produce DC components, such as certain semiconductor devices, is installed downstream, one of the listed measures is taken: for example an RCD whose operation remains guaranteed in the presence of a disturbing DC component, or an RCD combined with a residual direct current detection device.

For a charging point protected by automatic disconnection of supply, subsection 7.22.4.1 requires each AC dedicated circuit to be individually protected by an RCD of 30 mA at most (a derogation is possible in an IT system), and one of those two solutions against DC components, in the fixed installation or in the charging point. For a domestic photovoltaic installation, section 7.112.3 provides that the tests by the approved body also cover the operation of the RCDs. Check Book 1 and the manufacturer’s instructions before ordering the device.

The points to check before inspection

• The test button. In a distribution board, it must be easily accessible without special means and without danger of accidental contact with live parts (subsection 5.3.5.3). When the manufacturer provides for periodic testing, for example monthly, the check must ensure that the supply is actually cut off.
• A forgotten sensitive circuit. A bathroom towel rail wired onto a circuit that is not behind a high or very high sensitivity RCD: subsection 7.1.4.3 requires one for the circuits supplying that location, except SELV, electrical separation or safety installations.
• The diagram that does not tell the whole story. For domestic work, subsection 3.1.2.2 requires the single-line diagram to show the type and characteristics of the RCDs.
• Too many circuits on one 30 mA device. Beyond eight final circuits per device, the limit of subsection 4.2.4.3 is exceeded.

This circuit-by-circuit reasoning is best done while drawing the diagram: in Amperio, each circuit is checked against the RGIE (Book 1) as you enter it, RCDs included, and a missing 30 mA RCD is flagged at the exact spot on the diagram. What Book 1 provides for the inspection by the accredited body, measurements and checks included, is covered in our article on how the RGIE inspection unfolds.

This article is informative. It replaces neither the RGIE (Book 1) nor the advice of your inspection body.