RCD Trip Sensitivity: 30mA vs 100mA vs 300mA Explained

The mA rating on an RCD is its trip sensitivity — the leakage current at which it will disconnect. 30mA protects people from electric shock. 100mA protects against fire. 300mA provides upstream discrimination without disconnecting individual circuits.

Not all RCDs are rated at 30mA. The sensitivity rating determines the threshold at which the device trips, and choosing the right rating for each application is as important as choosing the right type (A, B, F) or the right current rating (40A, 63A, 100A). Using the wrong sensitivity can result in inadequate personal protection, nuisance tripping, or a loss of discrimination in the system.

The three main sensitivity ratings

Sensitivity	Trip threshold	Primary purpose	Common application
30 mA	30 milliamps	Personal protection — electric shock prevention	Final circuits in all domestic and commercial installations
100 mA	100 milliamps	Fire protection and upstream discrimination	Upstream RCDs in TT systems, time delay discrimination
300 mA	300 milliamps	Fire protection and upstream selectivity	Main incomer RCDs in larger installations, upstream of 30mA devices

Why 30mA protects against electric shock

The threshold for ventricular fibrillation (cardiac arrest caused by electric shock) is generally accepted as around 30–50mA through the heart. At currents below 30mA, the physiological effect is typically painful but not immediately life-threatening for a healthy adult. At currents above 50mA the risk of cardiac arrest increases rapidly.

A 30mA RCD trips in under 40 milliseconds — fast enough to prevent a lethal dose of charge passing through the body even if contact is made with a live conductor. This is why 30mA is the standard for all final circuits supplying sockets, outdoor equipment, and circuits in bathrooms where shock risk is elevated.

Important: a 30mA RCD does not prevent all electric shocks — it does not disconnect fast enough to prevent the initial shock, and contact with 230V at 30mA is still dangerous. It prevents the shock from becoming fatal.

Why 100mA and 300mA devices exist

If 30mA is the safe threshold, why use anything higher? The answer is discrimination — the principle that only the protective device closest to the fault should trip, leaving the rest of the installation energised.

In a layered system, a 300mA upstream RCD will not trip when a 30mA downstream RCD operates. This ensures that a single circuit fault disconnects that circuit only, not the entire upstream supply. Without this selectivity, a fault on any one circuit in a large installation could take down the entire distribution board.

The 100mA rating also serves a fire protection role: BS 7671 recognises that leakage currents above around 300–500mA sustained over time can cause heating and ignition in certain materials even without tripping a 300mA device — 100mA upstream RCDs add an additional layer of fire protection in high-risk installations.

Sensitivity ratings in practice

Location / application	Correct sensitivity	Reason
Domestic socket circuits	30 mA	BS 7671 Reg 411.3.3 — personal protection
Domestic lighting circuits	30 mA	Required under BS 7671 for most installations
Bathroom and outdoor circuits	30 mA	Enhanced shock risk locations
EV charging circuits	30 mA	Personal protection on accessible equipment
Upstream of 30mA RCDs (TT system)	100 mA or 300 mA (S-type time delay)	Selectivity — upstream must not trip on 30mA operation
Main incomer in large distribution boards	300 mA	Upstream fire protection with full selectivity

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Frequently Asked Questions

Can I use a 100mA RCD for personal protection?

No. A 100mA RCD will not provide reliable protection against electric shock — it trips at more than three times the threshold where cardiac arrest becomes a serious risk. 30mA is the required rating for personal protection on all final circuits accessible to users.

What is a 300mA S-type RCD?

An S-type (Selective or time-delay) RCD is intentionally delayed — it will not trip immediately when a fault current at its threshold is detected. This allows the downstream 30mA device to trip first. S-type devices are used upstream of 30mA devices to maintain selectivity without requiring a higher sensitivity rating. See our full guide: Time delay RCDs — when and why

Does a 30mA RCD protect against fire?

Partially. A 30mA RCD will trip on small leakage currents that could cause fire through sustained heating of insulation. However, it is not specifically designed as a fire protection device — it is designed for shock protection and will trip much faster than is needed for fire protection. Upstream 300mA RCDs provide broader fire protection by responding to leakage currents below the shock threshold but above what sustained operation would be safe.

Further reading:
Time delay RCDs — when and why
What is an RCBO? MCB vs RCD vs RCBO explained
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