Selectivity — also called discrimination — means that when a fault occurs on one final circuit, only the protective device immediately upstream of that fault opens, and the rest of the installation stays energised. IEC 60947-2 Annex A defines selectivity and backup protection between circuit-breakers in series, and IEC 60364-5-53 clause 536 requires protective devices to be coordinated. Achieving it requires comparing time-current curves and, in many cases, a selectivity limit current Is quoted by the manufacturer — a rating ratio alone proves nothing.
Total Selectivity, Partial Selectivity and the Selectivity Limit Current
IEC 60947-2 Annex A distinguishes two outcomes:
- Total selectivity — for every fault current up to the maximum prospective short-circuit current at the downstream device, only the downstream device trips.
- Partial selectivity — selectivity holds only up to a stated current. Above that value, both devices may trip. The boundary is the selectivity limit current Is, quoted by the manufacturer for that specific pair of devices.
The practical rule: compare Is against the prospective short-circuit current at the downstream device. If Is is greater than or equal to that fault current, you have total selectivity for that location. If it is lower, you have partial selectivity, and you must decide whether losing the upstream device under a high-current fault is acceptable for that circuit.
Backup protection (cascading) is the opposite trade
IEC 60947-2 Annex A also covers backup protection, sometimes called cascading: an upstream current-limiting breaker reduces the energy let through to a downstream breaker whose own rated breaking capacity would otherwise be too low. This permits a lower-rated downstream device, but it deliberately trades away selectivity at high fault currents. Cascading must be backed by the manufacturer’s tested combination tables — it is not a design assumption. Where a project requires both coordination and cascading, the two claims must be checked against each other, since they pull in opposite directions.
What the Time-Current Curves Have to Show
Coordination is verified by overlaying trip curves. Three mechanisms create selectivity:
1. Current selectivity
The upstream device’s instantaneous trip threshold is higher than the fault current seen at the downstream location. With MCBs to IEC 60898-1, the instantaneous ranges are: type B 3–5 × In, type C 5–10 × In, type D 10–20 × In. A device with a higher instantaneous threshold upstream buys separation, but only if the fault current at the downstream device stays below it.
2. Time selectivity
The upstream device delays its trip. Adjustable electronic-trip MCCBs can set a short-time delay, typically in the tens to hundreds of milliseconds range, with an I²t or a definite-time characteristic. Time selectivity is the most robust method because it does not depend on the precise fault magnitude.
3. Energy selectivity
With current-limiting breakers, the upstream device may sense the fault but not complete its trip before the downstream device clears it, because the downstream device limits I²t more effectively. This depends on tested device pairs and is manufacturer-specific.
Comparison Table: Coordination by Device Type
| Upstream device | Downstream device | Typical coordination method | What must be verified |
|---|---|---|---|
| MCCB, thermomagnetic, fixed | MCB type C | Current selectivity via instantaneous threshold | Manufacturer selectivity table for the exact pair; check Is against the fault level |
| MCCB, electronic trip with short-time delay | MCB or MCCB | Time selectivity; delay set above downstream total clearing time | Delay setting, and that the upstream Icw withstands the delay without damage |
| Fuse (gG / gL, IEC 60269) | MCB or MCCB | Time/current coordination from fuse time-current band | Fuse characteristic band, nominal current ratio, and the applicable national practice |
| RCD, time-delayed (selective, S type) | RCD, general type | Time selectivity on earth fault only | Break-time bands per IEC 61008 / IEC 61009; S-type delay against general-type maximum break time |
| Current-limiting MCCB | Lower-rated MCCB | Backup protection (cascading) | Tested combination table; accepted loss of selectivity above the stated current |
A word on rules of thumb: the familiar “upstream rating at least 1.6 to 2 times the downstream rating” is a screening heuristic, not a proof. It ignores curve type, fault level and I²t characteristics. Use it to shortlist, then confirm with manufacturer tables.
RCD Selectivity: A Separate Calculation
Residual current devices need their own coordination study. IEC 61008 (RCCBs) and IEC 61009 (RCBOs) define maximum break times; selective “S” type devices are intended to delay so a downstream general-type device clears first. Layering is normally done as: a time-delayed or higher-threshold device upstream, and 30 mA general-type devices on final circuits where additional protection is required by the applicable wiring rules. Verify against the device’s stated break-time band and non-actuating time rather than assuming the time delay is automatic.
Application Scenario / Case Analysis: Commercial Floor Distribution Board
Scenario constraints
A floor distribution board in a multi-tenant commercial building feeds lighting circuits, small-power socket circuits, a rooftop AHU, and a tenant sub-board. The supply arrives via a long feeder from the basement switchboard, so the prospective short-circuit current at the floor board is substantially lower than at the origin. Tenants require continuity: losing the whole floor because of a fault in one socket circuit is not acceptable. Emergency lighting on the floor must remain available.
Selection approach
- Establish the prospective short-circuit current at each board by calculation from the source, transformer impedance, feeder length and cable data. This number governs everything else, and it changes if the supply arrangement changes later.
- Choose a main device: an MCCB with an electronic trip and an adjustable short-time delay gives time selectivity that a thermomagnetic device cannot.
- Select downstream MCBs by curve type and load: type C for general small power with moderate inrush, type D where transformer or motor inrush is significant, and confirm the resulting Is values against the fault level at the board.
- Provide a time-delayed RCD or RCBO upstream of general-type 30 mA devices where the wiring rules require additional protection on socket circuits, and document the break-time coordination.
- Separate emergency circuits on a distinct protective device group so a fault on general services does not remove emergency lighting.
- Record the whole study: single-line diagram, device schedule, fault levels, TCC overlays and manufacturer references.
Common mistakes
- Assuming selectivity from the rating ratio alone without checking Is against the actual fault level.
- Using the same curve type and adjacent ratings upstream and downstream, so the curves overlap.
- Designing on the fault current at the origin instead of the reduced value at the floor board — which can make an over-conservative design, or the reverse after a supply upgrade.
- Applying cascading to save cost, then discovering during commissioning that a single fault removes the whole floor.
- Ignoring RCD coordination entirely, so an earth fault trips the upstream RCD and darkens the floor.
- Failing to re-check coordination after a transformer replacement or a feeder modification. Any change to source impedance invalidates the original study.
Acceptance checkpoints
- Obtain the coordination study: single-line diagram with device types, ratings and settings, and the manufacturer’s selectivity tables or TCC overlays for each upstream/downstream pair.
- Confirm every Is value exceeds the calculated prospective fault current at that point, or record the accepted partial-selectivity decision.
- Verify installed devices on site against the schedule — type, rating, curve and trip unit settings — and photograph each trip unit setting.
- Confirm the assembly’s ratings: the distribution board must meet IEC 61439-3 (or the applicable national equivalent), including its rated short-time withstand where a delayed upstream trip is used.
- Complete the verification required by IEC 60364-6: continuity of protective conductors, insulation resistance, and earth-fault loop impedance measured and compared against the values the coordination study assumed.
- Functional check of each RCD with its test button and, where specified, an injected-current test at the rated residual current; record measured trip times.
Selection Key Points
- Calculate the prospective short-circuit current at every board before selecting devices.
- Prefer time selectivity where the design allows it; it is less sensitive to fault-level error.
- Use manufacturer selectivity tables for the exact device pair — generic ratios are not evidence.
- Coordinate RCDs separately from overcurrent devices.
- Treat cascading as a documented trade-off, never a silent default.
- Confirm the board assembly rating under IEC 61439-3 matches the fault level and any delayed-trip duty.
- Re-issue the study after any change to the supply.
Quality Inspection Points
- Device schedule verified against installed hardware: manufacturer, model, rating, curve, breaking capacity.
- Trip unit settings verified and photographed.
- Torque check on all terminations against the manufacturer’s stated values.
- Insulation resistance and dielectric verification of the assembled board.
- Earth-fault loop impedance measurement, compared against the design assumption.
- RCD trip-time measurement recorded for each device.
- Labeling: every way identified, with the circuit schedule fixed inside the door.
Supplier Evaluation
Ask whether the supplier can provide selectivity tables for the actual device combinations proposed, whether the board is verified to IEC 61439-3 with the declared ratings, and whether they will issue a coordination study as part of the deliverable. Ask specifically how they handle a change in supply impedance after handover, because that is when coordination studies silently expire. A supplier who provides device boxes but no coordination data is handing you the liability for a design decision they are better placed to make.
BANGE Electric builds distribution boards to IEC 61439-3 and can supply the device schedule, fault-level calculation and coordination tables with the assembly. See the distribution boxes range or contact the engineering team with your single-line diagram.
FAQ
1. What is the difference between selectivity and discrimination?
They describe the same concept; “discrimination” is the term used in BS 7671 and much of the European literature, “selectivity” in IEC 60947-2 and IEC 60364 texts.
2. What is the selectivity limit current Is?
The highest fault current, for a given pair of devices, at which only the downstream device trips. Above Is, both may operate. Compare Is with the prospective short-circuit current at the downstream device.
3. Is a 2:1 rating ratio enough for selectivity?
No. It is a screening heuristic only. Real coordination depends on curve type, instantaneous thresholds, I²t characteristics and the fault level. Confirm with manufacturer tables.
4. Does cascading conflict with selectivity?
Yes, at high fault currents. Backup protection deliberately allows the upstream device to assist the downstream one, which can mean both operate. Where continuity matters, verify selectivity separately from the cascading claim.
5. How do I coordinate RCDs?
Use a time-delayed selective (S type) device upstream of general-type devices, and verify the break-time bands per IEC 61008 or IEC 61009 rather than assuming separation from the rating alone.
6. Which standard governs coordination in the installation?
IEC 60364-5-53 clause 536 covers coordination of protective devices; IEC 60947-2 Annex A defines selectivity and backup protection between circuit-breakers in series.
7. Do I need to re-check coordination after a supply change?
Yes. Replacing a transformer, changing the feeder, or adding a generator changes the prospective fault current and possibly the source impedance. The original study no longer applies until it is re-issued.
Talk to BANGE About Distribution Board Coordination
Send your single-line diagram, load schedule and available fault level, and we will return a device schedule with selectivity tables and the assembled board to match. Contact BANGE Electric to begin.
Requirements may vary by application, market, and applicable standard.
