RCD and RCBO Testing in KNX Panels: Trip Current, Trip Time and Selectivity Verification
KNX distribution panels contain multiple RCCBs and RCBOs protecting circuits with electronic loads — actuators, SMPS power supplies and capacitive inputs — that behave differently from resistive loads under earth fault conditions. Correct RCD type selection and verified trip performance at commissioning is mandatory under BS 7671 and IEC 60364-6.
Why RCD testing is mandatory
BS 7671 (UK Wiring Regulations) and its European equivalent IEC 60364-6 both require RCD testing as part of initial verification of every new electrical installation. Testing proves that the protection device actually operates within its rated parameters — a mechanically intact RCD that trips outside its rated time or current provides inadequate protection and is a latent life-safety risk.
Protection functions of RCDs
- 30mA RCD: protection against electric shock (lethal current threshold ~30mA for vulnerable individuals)
- 100–300mA RCD: fire protection (arcing fault current too low to trip MCB)
- S-type (selective): upstream device with 200–300ms delay for discrimination
- Time-delayed RCDs: allow downstream RCDs to clear first
Mandatory test intervals
- Initial commissioning: 100% of all RCDs and RCBOs
- Residential: every 6 months (mechanical test button), annual instrument test
- Commercial and industrial: every 6 months instrument test
- After any fault event or modification: retest affected circuits
RCD types in KNX panels
RCD type must match the waveform of earth fault current that could occur on the protected circuit. KNX actuators and their associated power supplies produce fault current components that Type AC devices cannot detect — making Type AC RCDs inappropriate for KNX actuator circuits.
| RCD type | Detects | KNX application |
|---|---|---|
| Type AC | Sinusoidal AC residual current only | Not suitable for KNX actuator circuits — cannot detect pulsating DC from switched-mode power supplies |
| Type A | AC + pulsating DC residual current | Minimum type for circuits with KNX switch actuators, DALI drivers, LED power supplies — required by IEC 60755 |
| Type F | Type A + high-frequency (50Hz–1kHz) residual current | Recommended for KNX circuits with VFD-controlled HVAC motors, variable speed pumps |
| Type B | AC + pulsating DC + smooth DC up to 1000 Hz | Required for EV charger circuits, PV inverters, VFD drives — overkill for standard KNX lighting circuits |
Specify Type A as the minimum for all KNX circuits: IEC 60755 Amendment 2 (2020) prohibits Type AC RCDs in new installations where the protected circuit contains equipment with electronic components (including KNX actuators). Type A is now the effective minimum for any circuit with a switched-mode power supply, LED driver, or KNX device.
Test instrument and connections
RCD testing requires a dedicated installation tester with RCD test function. The instrument injects a precisely controlled earth fault current and measures the time to trip. Standard multimeters cannot perform this function — they lack the controlled current injection and timing circuitry required.
Test instrument setup and connections
Approved instruments:
Fluke 1660 series (1662, 1664 FC) — recommended
Megger MFT1741 / MFT1741+ — recommended
Seaward Primetest 350 — suitable
Test lead connections:
L (line) terminal → line conductor at test point
PE (earth) terminal → protective earth at test point
Test point → typically at socket outlet or
load terminal of RCBO
Critical: always test from the FURTHEST POINT of the
circuit (highest earth fault loop impedance Zs) — this
is the worst-case condition for RCD operation.
RCD test mode selection:
0° phase angle: positive half-cycle injection (standard)
180° phase angle: negative half-cycle injection
BS 7671 requires testing at BOTH phase angles for
Type A RCDs — some may exhibit different trip times
depending on fault current waveform phaseBS 7671 Table 41.1 test criteria
BS 7671 Table 41.1 defines the test sequence and acceptance criteria for RCDs. The same criteria are referenced by IEC 60364-6 for continental European installations. All three tests must be performed and recorded for each RCD or RCBO.
RCD test sequence and pass criteria
Test 1 — Half rated current (I△n / 2): 30mA RCD: inject 15mA 100mA RCD: inject 50mA Pass criterion: NO TRIP in 300ms Purpose: verify RCD does not nuisance trip below threshold Test 2 — Rated current (I△n): 30mA RCD: inject 30mA → trip in ≤ 300ms 100mA RCD: inject 100mA → trip in ≤ 300ms Pass criterion: TRIP within 300ms Record measured trip time (e.g. 42ms) Test 3 — Five times rated current (5 × I△n): 30mA RCD: inject 150mA → trip in ≤ 40ms 100mA RCD: inject 500mA → trip in ≤ 40ms Pass criterion: TRIP within 40ms Purpose: verify fast clearance for high earth fault currents Test 4 — Mechanical test button: Press test button on RCD face Pass criterion: RCD trips and latches open Purpose: verify mechanical trip mechanism operable S-type (time-delayed) RCDs — additional criteria: Rated current test: trip time between 130ms and 500ms (IEC 61008-1 for S-type RCCBs) Ensures S-type clears AFTER standard 30mA RCDs downstream
RCBO additional tests
RCBOs combine residual current protection (RCCB function) with overcurrent protection (MCB function) in a single module. Both protection functions must be verified. The residual current test is identical to RCCB testing — the overcurrent element requires different verification.
RCBO residual current test
Identical to RCCB test procedure. Test RCD part using installation tester RCD test function at I△n/2, I△n, and 5×I△n as for standard RCCBs.
Also verify: earth continuity through the RCBO PE terminal — some RCBO designs have a separate PE terminal that requires a bridging link. Measure continuity between PE in and PE out terminals with a low-resistance ohmmeter. Resistance should be below 0.5Ω.
RCBO overcurrent verification
Do not apply 6× rated current on site to verify overcurrent trip — this is destructive to the RCBO and unsafe without proper current injection equipment.
Instead: verify overcurrent characteristic from manufacturer datasheet (time-current curve). Confirm MCB type (B, C, or D) matches the design specification for the protected circuit type. Document datasheet reference in test record.
Selectivity between upstream and downstream RCDs
Where RCDs are installed at multiple levels (upstream incomer and downstream final circuits), selectivity (discrimination) must be achieved. Without selectivity, a downstream fault trips the upstream incomer RCD, de-energising the entire distribution board rather than just the faulted circuit.
RCD selectivity design and verification
Selective (S-type) design: Upstream: 100mA S-type RCCB (time delay 200–300ms) Downstream: 30mA standard RCCBs / RCBOs Selectivity verification test: Step 1: Induce 30mA earth fault on a downstream circuit Expected: only the downstream 30mA RCBO trips Upstream 100mA S-type remains closed → Test PASS if only downstream device trips Step 2: Induce 100mA fault (via instrument 5×I△n on 30mA RCD) Expected: both downstream 30mA RCBO AND upstream S-type trip (selective discrimination breaks down at 100mA — both trip) → This is acceptable and expected behaviour Step 3: Reset downstream RCD, verify upstream S-type reset Expected: both reset and circuit restored normally Selectivity NOT achieved if: Upstream S-type trips during 30mA downstream fault → cause: S-type has too short a delay or incorrect type → remedy: verify S-type is IEC 61008-1 S-classified device
Electronic vs. electromechanical RCDs
Two distinct internal designs of RCD exist with significantly different behaviour characteristics in KNX panel applications. The choice affects reliability, suitability for critical circuits, and nuisance trip behaviour.
Electronic RCDs (VIGI modules)
Examples: Schneider Acti9 VIGI CB, ABB RCQ/RCBO-E modules. Require auxiliary power supply (typically from the line conductor). If the auxiliary power supply fails — power cut, blown fuse, connector fault — the RCD function may be lost or the module may lock in tripped position.
Not suitable for: fire shutters, emergency circuits, UPS-backed critical loads where loss of auxiliary power during a fault is possible.
Electromechanical RCDs
Examples: Schneider iID, Hager CDA series, ABB F200 series. Trip mechanism is a toroidal transformer — operates without auxiliary power supply. Trip energy comes from the fault current itself. Operate normally during supply interruptions and power-up transients.
Specify for: KNX PS640 circuit (prevents nuisance trip during bus startup capacitive inrush), fire shutter circuits, emergency lighting, critical HVAC circuits.
Nuisance tripping in KNX panels
Nuisance RCD tripping is one of the most common commissioning issues in KNX panels. The cause is the capacitive input characteristics of KNX bus power supplies and actuators, which create transient earth fault currents during power-up that can exceed the 30mA RCD trip threshold momentarily.
Nuisance tripping causes and mitigations
Root cause: KNX PS640 input capacitance: typically 400µF per TP segment On power-up: capacitor charges from mains → brief current spike Spike has DC component → Type AC RCD cannot discriminate Type A RCD detects DC component → may trip at inrush peak Mitigation 1: Use Type A RCDs (not Type AC) Type A RCDs have higher immunity to capacitive inrush compared to Type AC which is sensitive to DC components Mitigation 2: Interlock relay to sequence KNX PS640 startup Delay KNX PS640 energisation by 500ms after main RCD settles (use time-delay relay or KNX relay with startup delay) By the time PS640 energises, RCD has stabilised Mitigation 3: Omit 30mA RCCB on KNX PS640 circuit Where nuisance tripping persists despite Type A RCDs: Install upstream 100mA fire protection RCCB only KNX bus cable (YCYM 2×2×0.8mm²) is Class II insulated SELV bus voltage (29V DC) — Class II cable provides equivalent shock protection for the bus conductors (Note: 230V output circuits from KNX actuators still need standard 30mA RCBO protection) Mitigation 4: Electromechanical RCBO for PS640 circuit Electromechanical type has longer time constant Less sensitive to capacitive transient peaks
Test documentation
BS 7671 requires RCD test results to be recorded on the Schedule of Test Results (part of the Electrical Installation Certificate or Minor Works Certificate). Records must be retained for a minimum of 10 years and provided to the building owner.
Schedule of Test Results — RCD section
For each RCD / RCBO — record: Circuit reference (from circuit schedule) RCD type: AC / A / F / B Rated residual operating current I△n (mA): 30 / 100 / 300 I△n/2 test result: NO TRIP in 300ms — PASS / FAIL I△n trip time (ms): measured value, e.g. "42ms" 5×I△n trip time (ms): measured value, e.g. "18ms" Test button mechanical trip: PASS / FAIL Phase angle tested: 0° and 180° (for Type A) Overall result: PASS / FAIL S-type RCDs — additional columns: S-type delay trip time at I△n (ms): e.g. "245ms" S-type non-trip at 50mA (15ms): PASS / FAIL Format: IEC 60364-6 Annex C (or BS 7671 equivalent) Retention: minimum 10 years (commercial), life of installation (residential) Provided to: building owner, local authority (if required), insurer
Need KNX panels with verified RCD performance and full test documentation?
We commission KNX distribution panels with complete RCD and RCBO testing, selectivity verification, nuisance trip analysis and Schedule of Test Results — fully compliant with BS 7671 and IEC 60364-6.
Request a quote →