RCD Selectivity: S-Type vs Instantaneous Discrimination
What is RCD selectivity? Selectivity (also called discrimination) is the deliberate coordination of two or more residual current devices on the same feeder so that only the RCD closest to a fault trips, leaving the rest of the installation energized. Without it, a single earth fault on one final circuit can drop the main incomer along with every other circuit on the board, an outcome the standard hierarchy in IEC 60947-2 and IEC 61008-1 exists specifically to prevent. This article covers the current-ratio and time-delay rules behind selective (S-type) RCDs, how they differ from an ordinary instantaneous device, where G-type short-delay units fit between the two, and how Schneider, ABB, and Siemens implement selective tripping across their ranges.
Why RCD Selectivity Matters
Put an ordinary 30 mA RCD on the main incomer of a board that also has 30 mA RCDs on individual final circuits, and a fault on one circuit will very likely trip both. The upstream and downstream devices see nearly the same leakage current at nearly the same instant, and an instantaneous device reacts in well under 300 ms regardless of where it sits in the hierarchy. The board goes dark, not just the faulted circuit.
What we see in the field: this shows up most often on refit jobs, where an electrician adds a second RCD upstream of an existing consumer unit "for extra protection" without checking what's already downstream. The result is two instantaneous devices racing each other, and whichever happens to be marginally faster wins — not a designed outcome, just device tolerance.
Instantaneous vs S-Type: How the Trip Delay Differs
Instantaneous devices
A standard general-purpose RCCB or RCBO is built to trip as fast as the mechanism allows once residual current crosses IΔn — typically well inside the IEC 61008 limits (under 300 ms at IΔn, faster at 5x IΔn). That speed is exactly what you want at the final circuit closest to the fault, where the priority is stopping shock current before it does harm.
S-type (selective) devices
An S-type RCD deliberately trips slower. It carries a built-in time delay so an upstream device waits long enough for a downstream instantaneous device to clear the fault first. The S designation is a time-delay class defined alongside the standard classes in IEC 61008-1; the device still has the same IΔn rating options (10/30/100/300/500 mA), it just responds on a longer curve.
This delay is not a defect to be tuned out. It's the entire mechanism that makes selectivity possible, and removing it (or substituting an instantaneous device upstream) defeats the coordination the board was designed around.
The Discrimination Rule: Current Ratio and Time Delay
Two conditions have to hold together, not separately. Current alone doesn't guarantee discrimination, and neither does time delay alone — a fast fault at high multiples of IΔn can still race past an under-sized time margin.
Formula: Selective (Vertical) Discrimination — Source: IEC 60947-2 Annex M / IEC 61008-1
IΔn(upstream) ≥ 2 × IΔn(downstream), and t(upstream) > t(downstream) at the fault current
| Symbol | Description | Unit |
|---|---|---|
| IΔn(upstream) | Rated residual operating current of the upstream (S-type) device | mA or A |
| IΔn(downstream) | Rated residual operating current of the downstream (final-circuit) device | mA |
| t(upstream) | Trip time of the upstream device at the fault current | ms |
| t(downstream) | Trip time of the downstream device at the fault current | ms |
A common example: 300 mA S-type on the main incomer, 30 mA instantaneous devices on final circuits. The current ratio is 10:1, comfortably clear of the 2:1 minimum, and the S-type's built-in delay adds the time margin on top. Cut the ratio down to something closer to 2:1 and you're relying on the time delay alone to do the separating work — check the manufacturer's time/current curve at the actual fault current expected, not just at IΔn.
G-Type: The Short-Delay Class Between Instantaneous and S
G-type sits between the two. It carries a shorter time delay than S-type — enough to ride through brief transient leakage without acting as a full discrimination device further up the chain. It's less common than S-type in distribution board design and mostly shows up in equipment-level applications where a brief delay reduces nuisance operation without needing a two-stage selectivity scheme. If the goal is main-board-to-final-circuit discrimination, S-type is the class to specify, not G.
Where S-Type Devices Belong in the Distribution
S-type belongs upstream: main incomer, distribution board feeding sub-boards, or any point where multiple downstream RCDs already exist and a single fault shouldn't take out the whole group. It does not belong on a final circuit feeding a socket outlet or a piece of fixed equipment — put an instantaneous device there, because that's the point closest to a person and the trip needs to be as fast as the standard allows.
For larger feeders — above roughly 125 A, where a modular RCCB frame runs out of current rating — the upstream selective function usually moves to a standalone residual current relay with an external toroid rather than a fixed-frame RCCB. The relay reads the same vector-sum principle through a separate core clamped around the feeder conductors and switches an associated circuit breaker's shunt trip; it's not a self-contained switching device the way an RCCB or RCBO is.
This depends on the cumulative cable leakage on the installation too — long cable runs and large numbers of EMI filters on connected loads add background leakage that a tightly-set S-type main can start reading as a fault. Some designers set the main at 300 mA specifically to leave margin for this, accepting a slower response in exchange for fewer nuisance trips.
Brand Approaches to Selective RCDs
Schneider Electric offers S-type discrimination within the Acti9 iID RCCB range and pairs it with Vigi add-on blocks downstream on iC60 MCBs — the upstream/downstream split is built from standard modular parts rather than a dedicated relay for board-level jobs. ABB's F200 series includes S-type variants across its 25-125 A range, keeping the same modular form factor as the general-purpose F200/F204 devices. Siemens covers the same board-level role with S-type 5SV RCCBs, and steps up to the 5SM2 residual current relay with external toroid for feeders beyond modular RCCB current ratings.
| Criteria | Instantaneous | G-Type | S-Type |
|---|---|---|---|
| Typical position | Final circuit | Equipment-level | Main incomer / upstream board |
| Trip delay | None (standard curve) | Short delay | Deliberate delay for discrimination |
| Used for selectivity with a downstream RCD | No | Limited | Yes, when paired with the 2x current rule |
| Typical IΔn on a main incomer | Not recommended upstream of other RCDs | N/A | 100/300/500 mA |
Common Selectivity Mistakes
The most frequent error is fitting two instantaneous devices in series and calling it protection in depth. It isn't — it's an unresolved race condition. The second is under-sizing the ratio: 30 mA upstream over 30 mA downstream gives zero discrimination margin regardless of device class. The third is assuming an S-type main makes downstream RCDs unnecessary; it doesn't, the S-type only coordinates with them, it doesn't replace the fast trip a final circuit needs for shock protection.
It won't. A single well-chosen RCD, no matter how it's specified, cannot do the job of both tiers at once.
For a full breakdown of sensitivity ratings and where 10/30/100 mA fits into a design, see RCD sensitivity ratings, and for the waveform classes that determine whether a device even sees the fault current in the first place, see RCD types AC, A, F and B. Selectivity also interacts with the earthing arrangement — TT systems in particular depend on RCDs for fault clearance in a way TN systems don't; see RCDs in TT, TN and IT systems. For a step-by-step checklist covering sensitivity, type, and pole count together, see how to select an RCD.
Frequently Asked Questions
What does S-type mean on an RCD?
S-type marks a time-delay class defined in IEC 61008-1. The device is built to trip slower than a general-purpose instantaneous RCD at the same IΔn, so it can be placed upstream of other RCDs without pre-empting their trip.
Is a 2:1 current ratio always enough for selectivity?
It's the minimum stated in IEC 60947-2 Annex M, but it should be checked against the manufacturer's time/current curve at the expected fault current, not assumed automatically. Higher ratios (10:1 is common in practice) give more margin.
Can I use two instantaneous RCDs and get selectivity by luck?
No. Two instantaneous devices exposed to the same fault current will trip within milliseconds of each other, and which one wins depends on manufacturing tolerance, not design. Use an S-type upstream if discrimination is required.
Does an S-type RCD protect people as well as an instantaneous one?
Not on its own, and it's not meant to. S-type devices are for board-level coordination; the final circuit still needs an instantaneous RCD at 10 or 30 mA for direct-contact protection.
What replaces a modular S-type RCCB above 125 A?
A standalone residual current relay with an external toroid, such as Siemens 5SM2, ABB RD3, or Schneider Vigirex, switching a separate circuit breaker rather than acting as the switching device itself.
Is G-type the same as S-type?
No. G-type carries a shorter delay intended to ride through transient leakage at equipment level; it isn't specified as a discrimination class against downstream RCDs the way S-type is.
Conclusion
Selectivity is a design decision, not a device feature you get by accident. It needs an S-type device upstream, a current ratio of at least 2:1 against every downstream RCD, and a time margin confirmed at the actual fault current — not just at IΔn. Get any one of the three wrong and the board behaves like it has no coordination at all. For the broader picture of RCD construction, ratings, and standards, see the RCD protection guide, and browse Stoklink's stock of residual current devices and RCBOs across the Schneider, ABB, and Siemens ranges covered above. For how RCCB, RCBO, RCD, and MCB terminology maps together, see MCB vs RCBO vs RCD vs RCCB differences.
