How to Select an RCD: Sensitivity, Type and Poles Checklist
How do you select the right RCD? You match four parameters to the circuit, in order: sensitivity (IΔn) to the protection goal, type (AC/A/F/B/SI) to the load's residual waveform, poles and rated current (In) to the supply and cable, and the housing form — RCCB, RCBO, or relay — to the board layout and earthing system, per IEC 61008/61009. Get any one of these wrong and the device either nuisance-trips on healthy leakage or, worse, fails to see a real fault because the waveform blinds the sensing coil. This checklist covers sensitivity, type, poles and current, earthing-system checks, RCCB vs RCBO vs relay, and special cases such as EV chargers and VFDs.
Step 1 — Match Sensitivity (IΔn) to the Protection Goal
Sensitivity comes first because it defines what the device is protecting against, not just how fast it trips. An RCD sums the current in line and neutral through a toroidal core; any current returning outside that loop — through a person, a faulty cable, or a wet enclosure — unbalances the sum and trips the device at its rated residual operating current, IΔn.
10 mA and 30 mA — personal protection
30 mA is the standard human-safety threshold in most wiring codes for socket outlets, bathrooms, and outdoor circuits. It must trip within the time limits IEC 61008 sets at IΔn and again at 5x IΔn. 10 mA devices exist for higher-risk circuits — swimming pool equipment, some medical locations — where the margin to a dangerous body current needs to be tighter.
100 mA, 300 mA, 500 mA — fire and equipment protection
These sensitivities do not protect a person from a lethal shock. They protect the building. Leakage current in that range can ignite insulation or overheat a poor connection well before it reaches a level that would stop a heart, which is why these values sit on distribution boards and large motor feeders, not on sockets.
What we see in the field: a single 100 mA main RCD fitted "for protection" upstream of socket circuits, with no downstream 30 mA device. That satisfies fire protection. It does nothing for shock protection, and an inspector will reject it.
Step 2 — Match the Type to the Load's Residual Waveform
Sensitivity decides whether the device trips. Type decides whether it can see the fault at all. IEC 62423 defines the waveform each type detects, and getting this wrong is worse than a wrong IΔn — a Type AC device on the wrong load can sit there while current leaks to earth. It won't trip.
| Criteria | Type AC | Type A | Type F | Type B |
|---|---|---|---|---|
| Waveform detected | Pure sinusoidal AC only | AC + pulsating DC | Type A + mixed frequencies | Type A/F + smooth (pure) DC |
| Typical load | Resistive circuits, incandescent lighting | Electronics, SMPS, most general sockets | Single-phase VFDs, frequency-controlled loads | Three-phase VFDs, EV chargers, transformerless PV inverters |
| Selection note | Restricted for electronic loads in most codes | Modern default for general circuits | Specify where a single-phase drive sits on the circuit | Mandatory wherever smooth DC fault current is possible |
Adding "SI" (super-immunized) to any of these types does not change what the device detects — it raises the threshold for transient leakage (surges, high-frequency noise) that the device will ignore before deciding a trip is warranted. Fit it where nuisance tripping already has a history: corrosive or humid enclosures, long runs with cumulative capacitive leakage. See RCD types AC, A, F, B and SI explained for the full waveform reference and the standards behind each class.
Step 3 — Choose Poles and Rated Current (In)
Poles follow the supply, not the load rating. A single-phase circuit takes a 2P (line + neutral) RCCB or RCBO; a three-phase circuit — a machine, a VFD, a three-phase distribution board — takes 4P. Never switch neutral through a separate single-pole device on a TT or TN-S system where the neutral must disconnect together with the lines; an unswitched neutral defeats the point of fitting an RCD at all.
Rated current In (25, 40, 63, 80, 100 A and up) is sized to the load and cable exactly as an MCB would be, with one addition: check the rated conditional short-circuit current Inc. An RCCB carries no overcurrent trip of its own — it relies on an upstream or associated MCB to clear a short circuit within its Inc rating, commonly 6 or 10 kA, backed by a specific breaker type and rating. Undersize In and the device nuisance-trips on ordinary load current; skip the Inc check and a downstream fault can weld the contacts before the backup device opens.
Step 4 — Check the Earthing System and Selectivity
The earthing system sets a hard limit on how insensitive the RCD is allowed to be. On a TT system, the earth-fault loop runs through the local electrode rather than a solid metallic return, so overcurrent devices alone often cannot clear a line-earth fault fast enough. The RCD does that job instead, and the sensitivity you're allowed depends on the electrode resistance.
Formula: Maximum touch-voltage limit for TT systems — Source: IEC 60364-4-41, disconnection-time rule
RA × IΔn ≤ 50 V
| Symbol | Description | Unit |
|---|---|---|
| RA | Resistance of the earth electrode plus protective conductor to the exposed-conductive-part | Ω |
| IΔn | Rated residual operating current of the RCD protecting the circuit | A |
| 50 V | Conventional touch-voltage limit under normal, dry conditions | V |
With a typical rural earth electrode of 200 Ω, the rule allows up to 250 mA — comfortably above the 30 mA already specified for socket circuits. That's why TT installations default to 30 mA on every socket and portable-equipment circuit rather than relying on the arithmetic minimum: the extra margin covers electrode resistance that drifts upward over time. On TN systems the MCB or MCCB may clear a line-earth fault on its own, but 30 mA RCDs are still required for socket circuits and additional protection. See RCDs in TT, TN and IT earthing systems for the full breakdown by system type, including IT systems, where insulation monitoring comes first and RCDs sit on sub-circuits.
Selectivity matters once RCDs are staged — a main-board device upstream of sub-board devices. The upstream device should be S-type (time-delayed, roughly 2x the downstream IΔn), so a fault on one final circuit trips only that circuit, not the whole board. Fit two instantaneous devices in series on the same fault path and both can trip together, taking out the entire board for one socket fault.
Step 5 — RCCB, RCBO, or Residual Current Relay?
An RCCB detects earth leakage only; it has no overcurrent protection and must sit behind its own MCB or fuse. An RCBO combines the RCD and the MCB in one module — earth leakage, overload, and short-circuit protection per pole-way. Use RCCB-plus-MCB where one RCD can economically cover several circuits, a sub-board of lighting circuits, for example. Use RCBO where each final circuit needs independent protection, so one earth fault doesn't take the neighboring circuit down with it. The fuller RCCB vs RCBO differences guide and the MCB vs RCBO vs RCD vs RCCB comparison cover the terminology and wiring differences in more depth.
Above roughly 125 A, neither a fixed RCCB nor an RCBO is normally available. Move to a residual current relay (Siemens 5SM2, ABB RD3, Schneider Vigirex) wired to a separate toroid around the feeder conductors, tripping a shunt-trip breaker or contactor. Schneider's modular range covers the lower end with the Acti9 iID RCCB up to 100 A, plus the Vigi add-on block that turns an iC60 MCB into an RCBO; ABB's DS201/DS202C and Siemens' 5SU1/5SV1 follow the same one-piece RCBO logic. Browse the current residual current devices range or the RCBOs collection to check what's in stock at the poles and In you need.
Step 6 — Special Cases: EV Chargers, VFDs, and Nuisance Tripping
An EV charge point can produce a smooth DC fault current downstream of its rectifier, and a Type A device cannot detect that waveform — it will sit closed through a real fault. The circuit needs either a full Type B RCD, or a Type A RCD combined with a 6 mA DC residual current detection device (RDC-DD, per IEC 62955) built into the charger or its RCBO. Confirm which approach the charge point manufacturer specifies before you order; retrofitting the wrong one after commissioning means re-terminating the circuit. See Type A vs Type B RCD for VFD, EV and solar for the decision criteria.
Variable frequency drives follow a similar logic but aren't identical: a single-phase VFD generally needs Type F for its mixed-frequency residual, while a three-phase VFD's DC bus can produce the same smooth DC residual as an EV charger, needing Type B. What we see in the field is Type A devices fitted on VFD feeders because that's the assumed-safe default for electronics — they hold up fine until the drive develops a fault mode the Type A can't see, and then the RCD does nothing while the fault persists.
If nuisance tripping is a genuine risk at the site — long cable runs with cumulative capacitive leakage, a surge-prone supply, a humid or corrosive environment — specify the super-immunized variant of whichever type you've already selected, rather than dropping to a lower sensitivity to "solve" the tripping. Lowering sensitivity trades away shock protection; SI does not.
Frequently Asked Questions
What sensitivity RCD do I need for a socket outlet?
30 mA, Type A, is the standard baseline in most wiring codes for socket and portable-equipment circuits. Confirm local regulations, but 30 mA is the threshold used for additional protection against electric shock.
Can I use a Type AC RCD on a modern consumer unit?
Generally no, for circuits feeding electronics. Type AC only detects pure sinusoidal residual current, and most modern loads — LED drivers, variable-speed pumps, chargers — produce pulsating DC that a Type AC device cannot see, which is why Type A is now the default minimum.
Do I need a full Type B RCD for a normal EV charger?
Only if the charger doesn't already include an RDC-DD. Many domestic EV chargers integrate a 6 mA DC residual current detector alongside a Type A RCD, which satisfies IEC 62955 without a separate Type B device. Check the charger's documentation before specifying.
Should every final circuit have its own RCBO?
It's the more resilient design: one earth fault trips one circuit, not the whole board. A shared RCCB across several circuits is cheaper to install but takes down every circuit on it when any one of them faults.
What happens if I fit the wrong poles, 2P instead of 4P?
A 2P device won't connect correctly to a three-phase supply, and running a three-phase load through a 2P RCD leaves two phases unmonitored for earth leakage. Match poles to the supply first, before touching sensitivity or type.
Why does a new RCD trip when the old one didn't?
It's often correctly detecting leakage an older, less sensitive, or Type AC device couldn't see, not a fault with the new device. Cumulative cable and equipment leakage on an aging installation can already sit close to the 30 mA threshold before the new device is even fitted.
Conclusion
Selecting an RCD is an ordered checklist, not a single spec-sheet lookup: sensitivity for the protection goal, type for the load's waveform, poles and In for the supply and cable, then earthing system and selectivity, then the RCCB/RCBO/relay form factor — with EV and VFD circuits checked separately for smooth DC content. Get the order right and the device does what it's rated for. Skip a step and the result is either a nuisance-tripping board or a fault the RCD never saw. For the underlying detection principle and the full type and standard reference, see the RCD protection guide.
