RCD Sensitivity: 10mA, 30mA, 100mA and 300mA Explained
What does RCD sensitivity mean? Sensitivity is the rated residual operating current IΔn — the leakage current, in milliamps, at which an RCD is required to trip within the time limits set by IEC 61008 and IEC 61009. Get IΔn wrong and the device either nuisance-trips on routine cable leakage or fails to clear a fault fast enough to stop a shock injury or an arcing fire, so the number stamped on the front is a safety decision, not a spec-sheet footnote. This article works through the common sensitivity bands (10 mA, 30 mA, 100 mA and 300 mA), the split between personal protection and fire/equipment protection, how IΔn sizes the earth electrode resistance on a TT system, and how to choose sensitivity for a real circuit.
What "Sensitivity" Means: IΔn and the Trip Threshold
An RCD sums the current flowing in the live and neutral conductors through a toroidal core. In a healthy circuit that sum is zero. Earth leakage — current returning through a fault path instead of the neutral — unbalances the core and induces a current in the sense winding. Once that induced signal crosses the trip threshold, the device opens. IΔn is simply the label for that threshold, expressed as the residual current at which the manufacturer guarantees a trip within the standard's time limit.
General (non-delayed) devices are built to break well inside a few hundred milliseconds at IΔn, and much faster at higher multiples of IΔn. That speed matters because both electrocution risk and ignition risk are functions of current magnitude and duration together, not current alone. A 30 mA RCD that takes a full second to open is not doing the job a 30 mA rating implies.
10 mA and 30 mA: Personal Protection Thresholds
30 mA is the sensitivity most codes treat as the human-safety threshold: additional protection against direct contact with a live part, on top of basic insulation and enclosure protection. It is the default IΔn for socket outlets, portable tools, and any circuit a person can touch without specialist training. 10 mA sits below that, used where installation practice calls for extra margin — a single dedicated appliance circuit, or a location where the consequences of a slow trip are worse than the inconvenience of a lower nuisance-trip threshold.
Neither value protects against overload or short-circuit current. That's a separate device (an MCB) or a separate function inside the same housing. For the distinction, see MCB vs RCBO vs RCD vs RCCB differences.
100 mA, 300 mA and 500 mA: Fire and Equipment Protection
Above 100 mA, the job changes. A leakage current in this band is well past the personal-protection threshold but still capable of heating insulation, arcing across a damaged cable, or igniting dust and fibre near a fault. 100 mA, 300 mA and 500 mA devices exist to interrupt that leakage before it becomes a fire, not before it becomes a shock — the shock risk at that point is already handled by 30 mA (or 10 mA) devices further downstream.
What we see in the field: a distribution board feeding a bank of VFDs and switch-mode control gear can leak several milliamps continuously through EMI filter capacitors on every drive, even with no fault present. Fit a single 30 mA RCD ahead of that whole board and it nuisance-trips within days. Fit a 300 mA device at the incomer, with 30 mA RCDs or RCBOs on the individual final circuits that actually have exposed live parts, and the board stops tripping while personal protection stays intact where it's needed.
Why Sensitivity Also Sizes the Earth Electrode Resistance
On a TT system, the fault current path back to the source runs through the earth electrode, not through a bonded neutral conductor. The touch voltage a person could experience during a fault is bounded by how much resistance sits in that path and how much current flows through it. Sensitivity and earthing design are therefore the same calculation, not two separate ones.
Formula: Maximum earth electrode resistance for touch-voltage limit (TT systems) — Source: general TT earthing/disconnection principle applied in IEC 60364
RA × IΔn ≤ 50 V
| Symbol | Description | Unit |
|---|---|---|
| RA | Resistance of the earth electrode and protective conductor circuit | Ω |
| IΔn | Rated residual operating current of the RCD protecting the circuit | A |
| 50 V | Conventional touch voltage limit for normal (dry) conditions | V |
Rearranged, this puts a hard ceiling on the earth electrode resistance you can get away with for a given IΔn: roughly 1,667 Ω at 30 mA, 500 Ω at 100 mA, and 167 Ω at 300 mA. A rod-and-earth installation that measures a few hundred ohms is routine; getting under 1,667 Ω is usually easy, which is one reason 30 mA became the practical default for personal protection on TT systems rather than something tighter. Push toward 10 mA and the allowable resistance rises further, but the marginal safety gain has to be weighed against a lower nuisance-trip threshold on the same electrode.
How IEC 61008 and IEC 61009 Define Sensitivity Classes
IEC 61008 covers RCCBs (residual current only, no overcurrent function); IEC 61009 covers RCBOs (RCCB plus MCB in one module). Both standards work from the same set of preferred IΔn values — 10, 30, 100, 300 and 500 mA — and both distinguish general (instantaneous) devices from time-delayed variants used for selectivity. An S-type or selective device carries the same IΔn as its downstream general-type counterpart but trips more slowly and at a higher multiple of IΔn, so it stays closed while the downstream device clears the fault first. Sensitivity and time-delay class are independent settings on the same device; changing one doesn't change the other.
This is also where waveform type and sensitivity intersect but stay distinct. Type AC, A, F and B describe what shape of residual current the device can detect — pure AC, pulsating DC, mixed frequencies, or smooth DC. IΔn describes how much of that current it takes to trip. A Type A device at 30 mA and a Type B device at 30 mA trip at the same current magnitude; they differ in which waveforms they can even see. See how those types work in practice in our overview of RCD types AC, A, F and B.
Choosing Sensitivity for a Specific Circuit
Four questions settle most sensitivity decisions. Can a person contact a live part on this circuit without tools? If yes, 30 mA is the floor, 10 mA the option for higher-risk single appliances. What's the earthing system? On TT, check the electrode resistance against the formula above; on TN, the MCB or MCCB may clear line-earth faults on its own, but 30 mA is still standard practice for socket circuits. How much cumulative background leakage does the load side already produce? A board full of drives, filters and long cable runs pushes toward 300 mA at the incomer with 30 mA downstream, not a single blanket 30 mA device. Does the circuit need Type B protection for smooth DC fault current, as with an EV charger or a transformerless PV inverter? If so, sensitivity and type get selected together, not sensitivity first and type as an afterthought. Our checklist for selecting RCD sensitivity, type and poles walks through this in more detail circuit by circuit.
Common Mistakes with Sensitivity Selection
The most frequent error is treating 300 mA as a drop-in upgrade from 30 mA on a circuit that still has exposed live parts — a socket outlet or hand tool feed doesn't get safer with a higher IΔn, it gets less protected. The second is the reverse: fitting 10 mA everywhere "to be safe" on a board with normal SMPS and drive leakage, which produces nuisance trips that eventually get worked around with a bypass, which is worse than no upgrade at all. The third is skipping the earth electrode check on a TT installation and assuming any 30 mA RCD satisfies the touch-voltage rule regardless of the measured resistance. None of these are exotic failures; they show up on ordinary panel jobs. If nuisance tripping is already happening on an installed board, our guide on why an RCD keeps tripping and how to fix it covers the diagnostic steps.
Frequently Asked Questions
What is the difference between a 30 mA and a 100 mA RCD?
30 mA is a personal-protection threshold, intended to trip before a shock through a person becomes lethal. 100 mA is a fire/equipment-protection threshold, intended to interrupt leakage before it heats insulation or ignites material near a fault. A 100 mA device does not provide the same additional protection against direct contact that a 30 mA device does.
Can a 300 mA RCD be used on a socket outlet circuit?
Not as the only protection. A socket circuit where a person can contact live parts needs a 30 mA (or 10 mA) device for personal protection. A 300 mA device is normally placed further upstream, at a board incomer, for fire and cumulative-leakage protection, with 30 mA devices on the individual final circuits.
Does a lower IΔn always mean better protection?
Not automatically. A lower IΔn reduces the shock energy at trip, but it also lowers the nuisance-trip threshold against ordinary background leakage from filters, long cables and multiple appliances. Below a certain point the practical result is more unwanted trips rather than meaningfully better safety.
Why does my 30 mA RCD keep tripping on a VFD-fed motor?
VFDs and their EMI filters leak a small continuous current to earth even when healthy, and that leakage adds up with every other electronic load sharing the same RCD. A 30 mA device protecting several drives or a long board can trip on cumulative background leakage rather than an actual fault; segregating the load or moving to a higher IΔn upstream, backed by 30 mA on the parts that need personal protection, usually resolves it.
What sensitivity is needed on a TT earthing system?
30 mA is standard for personal protection on TT circuits, but the earth electrode resistance has to be checked against the touch-voltage rule RA × IΔn ≤ 50 V for the actual IΔn installed. If the measured electrode resistance is too high for the chosen sensitivity, either the electrode needs improving or a lower IΔn has to be used.
Is 10 mA overkill for a normal domestic circuit?
For a general socket or lighting circuit, 30 mA is the usual choice and 10 mA adds little beyond a lower nuisance-trip margin. 10 mA earns its place on specific higher-risk single-appliance circuits where codes or risk assessment call for the extra margin, not as a default across a whole installation.
Conclusion
IΔn is a single number, but it encodes three separate decisions: how much shock or fire risk the circuit presents, how much background leakage the load side already produces, and how much resistance sits in the earth fault path. 30 mA covers most personal-protection cases, 10 mA is for the specific higher-risk exceptions, and 100–500 mA belong upstream for fire and equipment protection, not on circuits with exposed live parts. Check the earth electrode resistance against the touch-voltage rule before assuming any IΔn is compliant on a TT system, and size sensitivity together with waveform type and discrimination rather than in isolation. For the full picture of RCD construction, types and earthing interaction, see the RCD protection guide, and browse Stoklink's residual current devices and RCBOs across the full IΔn range from ABB, Schneider Electric and Siemens.
