30 mA or 300 mA RCD? Sensitivity, selectivity and auto-reclosing RCDs

Short answer: A 30 mA RCD protects people against electric shock; it is the value used in homes on socket, bathroom, kitchen and outdoor circuits. A 300 mA RCD is fire protection: it disconnects leakage from insulation faults that could start a fire, but it is not sensitive enough to protect a person. So 300 mA never replaces 30 mA; it is usually installed at the building entrance or in larger installations, time-delayed (type S) and with 30 mA devices downstream. In a home, if in doubt, 30 mA is the right choice.

What does the mA rating mean?

An RCD compares the current flowing out on the live conductor with the current returning on the neutral. When the difference (current escaping to earth or through a person) reaches the rated value, it trips. This value is the rated residual operating current (IΔn).

• 10 mA: very sensitive. Commonly chosen for some wet-area and special applications; prone to nuisance tripping on circuits with many appliances.
• 30 mA: personal (additional) protection. Disconnects before the current reaches a level that can harm the heart.
• 100 mA and 300 mA: fire protection and selectivity. Not enough to protect people.
• 500 mA and above: at the main incomer of large installations, again for fire and plant protection.

An RCD does not protect against overload or short circuit; every circuit also needs a suitable circuit breaker.

Where is 30 mA used?

• Socket circuits, indoors and outdoors
• Bathrooms and wet rooms
• Kitchen, washing machine and dishwasher circuits
• Garden, balcony and outdoor lighting
• EV charger circuit (at least type A, on its own RCD)

If a single 30 mA RCD covers the whole home, small leakages add up and cause nuisance tripping. Splitting the circuits across two or more 30 mA devices reduces tripping and keeps the rest of the house powered when one circuit has a fault.

Where is 300 mA used?

• At the building entrance or main board, to monitor the whole installation for fire risk
• Large buildings, workshops and warehouses with long cable runs
• As the upstream stage of a distribution with 30 mA RCDs below

Fitting a 300 mA RCD on a socket or bathroom circuit as personal protection is the wrong choice.

Selectivity: coordinating main and sub-boards

With a 300 mA RCD on the main board and a 30 mA RCD on a sub-board, both may trip on the same fault and the whole building goes dark. Selectivity prevents this:

1. The upstream RCD is time-delayed, i.e. selective (type S). It waits briefly so the downstream device can trip first.
2. The upstream sensitivity is clearly higher than the downstream one, usually at least three times (e.g. 300 mA S above, 30 mA below).
3. Never put two non-delayed RCDs in series on the same circuit.

That way only the faulty circuit is disconnected. Coordination is set out in the installation design. The Legrand XC10 devices in our shop are not offered as type S; if a selective incoming RCD is needed, it has to be specified separately for the project.

What does an auto-reclosing RCD do?

An auto-reclosing (self-resetting, smart) RCD waits after tripping for a temporary reason such as lightning or a mains surge and then switches the circuit back on by itself. Some models test the insulation of the circuit before reclosing.

When is it useful?

• Holiday homes and villas that stand empty, with fridges, freezers, boilers, alarms and cameras that must stay on
• Remote, unattended systems such as pumps, booster sets and irrigation
• Telecom sites and automation panels where downtime is costly

Limits:

• If the fault is permanent (real leakage, damaged cable, damp junction box), it locks out after a few attempts and needs manual intervention.
• It does not remove the cause; a circuit that keeps tripping must be investigated.
• It does not change the sensitivity or type; 30 mA or 300 mA, AC or A are still chosen by the same rules.
• It costs more than a standard RCD and may take more modules in the board.

The Legrand XC10 devices in our shop are not auto-reclosing; they are standard RCDs reset by hand.

Test button: once a month

The T or Test button checks that the mechanism works. Manufacturers usually recommend pressing it once a month; if the RCD does not trip, replace it. The button does not measure trip time or current; that is done by a qualified person with an RCD tester. Some auto-reclosing devices run this test themselves.

Type AC or type A?

Sensitivity (30 mA, 300 mA) and type (AC, A, F, B) are different things. The type shows which waveforms of residual current the RCD detects. In today's homes with many electronic appliances type A is recommended, and EV charging needs at least type A. See our guides to RCD types A, B, F and the type A RCD for an EV charger.

Legrand XC10 RCDs in our shop

Legrand XC10 type A and type AC RCDs are available as 2-pole (single-phase) and 4-pole (three-phase), 25, 40 and 63 A, 30 mA and 300 mA, and comply with EN/IEC 61008-1. Local rules apply: in the UK BS 7671, in Turkey the Electrical Internal Installations Regulation; which device goes on which circuit is decided by the installation design. ENGN plans distribution boards according to the design on electrical contracting projects in Bursa.

• → RCD types A, B, F
• → Type A RCD for an EV charger
• → Home circuit breakers and fuse box
• → Type B vs type C circuit breaker
• → Earthing explained
• → Residual current devices
• → Circuit breakers