RCD 30 mA or 300 mA - which rated current fits?

What does the rated residual current IΔn mean?

The rated residual current IΔn is the fault current at which an RCD (residual current device) reliably trips. It is printed on the device and defines its purpose: small values such as 30 mA protect people, larger values such as 300 mA protect against fire in the wiring.

An RCD constantly measures the sum of all currents flowing in and out. During normal operation this sum is zero. If current leaks away through a fault, for example through the human body or damaged insulation to earth, a difference appears. Once it exceeds IΔn, the RCD disconnects the circuit within milliseconds.

• IΔn 10 mA: enhanced protection in special areas such as medical rooms.
• IΔn 30 mA: standard personal protection for sockets and final circuits.
• IΔn 100 mA / 300 mA: fire and system protection for whole circuit groups.
• IΔn 500 mA and above: pure system protection with no personal protection.

Why do 30 mA protect people?

The value 30 mA is derived from the physiological effect of current on the body. Currents above roughly 30 mA can trigger ventricular fibrillation if they persist. An RCD with IΔn 30 mA disconnects so quickly that a dangerous current through the body never has time to build up.

According to IEC 60364‑4‑41 (and the national DIN VDE 0100‑410), additional protection by an RCD with IΔn ≤ 30 mA is mandatory for socket outlets up to 32 A used by ordinary persons and for final circuits outdoors. The 30 mA RCD thus delivers classic personal protection: it prevents electric shock from direct or indirect contact.

What are 300 mA used for?

An RCD with IΔn 300 mA does not protect people, it prevents fires caused by insulation faults. If a small fault current leaks continuously to earth through damaged wiring, the resulting heat can ignite insulation material. Fault currents of only a few hundred milliamperes are enough to start a smouldering fire at the fault location.

The 300 mA RCD trips before the fault current can build up this critical heat over time. It is therefore often used upstream for whole distribution boards, for timber constructions, agricultural buildings or fire-risk premises per IEC 60364‑4‑42. For fire protection, RCDs with IΔn ≤ 300 mA are specified.

• Fire protection in fire-risk locations (timber, straw, dust).
• Agricultural and horticultural installations per IEC 60364‑7‑705.
• Upstream protection of long cable runs with high leakage current.
• Selective (short-time delayed) type S version for grading.

How do you combine 30 mA and 300 mA?

Many installations use both values in a graded arrangement. An upstream selective 300 mA RCD of type S protects the main distribution against fire, while downstream 30 mA RCDs secure the individual final circuits for personal protection. The type S trips with a short delay, so a fault first clears the nearer 30 mA device and only the affected circuit switches off.

This selectivity per IEC 60364‑5‑53 keeps a single fault from shutting down the whole installation. Equally important is choosing the RCD type (A, F, B) to match the fault-current waveform of the connected loads, for example with frequency inverters or charging equipment.

Frequently asked questions