Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
๐ก๏ธ IEC 61009: RCBO Selection, Installation, and Consumer Unit Design โ A Practical Engineering Guide
In the landscape of low-voltage final distribution for household and similar premises, the device governed by IEC 61009 — the Residual current operated Circuit-Breaker with integral Overcurrent protection (RCBO) — has quietly become the defining component of modern consumer unit design. A single RCBO combines residual current protection (the RCD function, detecting earth leakage) and overcurrent protection (the MCB function, responding to overloads and short-circuits) within one modular enclosure. It is, in every meaningful sense, two protective devices fused into one. For residential switchboards where every DIN-rail module counts, the RCBO unlocks a leaner panel layout, cleaner circuit segregation, and surgical fault isolation that the traditional “one RCD feeding multiple MCBs” architecture cannot match. But there is a catch — one that has tripped up electricians for decades: the neutral connection. Get it wrong and the RCBO either nuisance-trips endlessly or, far worse, fails to trip when a person’s life depends on it. This article, grounded in the IEC 61009 series (which traces back to the 1991 IEC 61009-2-2 supplement and includes the current IEC 61009-1:2013), unpacks the engineering essentials of RCBO selection, installation, and system-level coordination.
2-in-1
Integrated RCD + MCB
≤ 125 A
Maximum Rated Current
1P+N / 2P
Standard Modular Formats
Type AC/A/B
Residual Current Detection Types
💡 1. How an RCBO Works: Two Independent Protection Systems in One Compact Body
1.1 The Dual Detection Architecture
Inside every IEC 61009-compliant RCBO, two entirely independent detection mechanisms share a single main contact system and tripping mechanism. Understanding their physical separation is key to understanding why the neutral connection matters so profoundly:
- Residual current detection channel (RCD function): At the heart of this channel sits a toroidal core-balance current transformer (CBCT). All live conductors — the line and the neutral — pass together through the core. Under normal conditions, the algebraic sum of currents through the core is zero: whatever current leaves on the line conductor returns on the neutral, their magnetic fluxes cancel perfectly, and the secondary winding produces no signal. When an earth fault occurs — a person touching a live part, or insulation breakdown creating a leakage path to earth — some current bypasses the neutral and flows to earth instead. The instantaneous vector sum of line and neutral currents is no longer zero. A differential flux appears in the CBCT core, the secondary winding develops a voltage proportional to this residual current, the signal is amplified by a sensitive electronic circuit (powered from the line supply in most designs), and the trip coil fires, mechanically unlatching the main contacts. This is precisely what “residual current operated” means: the device responds only to the difference between outgoing and returning currents, not to their absolute magnitudes.
- Overcurrent detection channel (MCB function): A thermal bimetallic strip (for overload protection, with an inverse-time characteristic) and a magnetic solenoid plunger (for short-circuit protection, with an instantaneous characteristic) are placed in series with the line conductor only. The thermal element deflects predictably with I2R heating — the greater the overload, the faster it bends and trips. The magnetic element responds to the peak instantaneous current, providing near-instantaneous disconnection on a short-circuit. Both operate through the same mechanical latch as the RCD channel, so either an earth fault or an overcurrent will open both the line and (in 1P+N or 2P designs) the neutral contacts.
These two detection channels converge onto a single trip mechanism. The beauty of the arrangement is that it provides per-circuit granularity: every final circuit in the installation gets its own independent earth-leakage and overcurrent watchdog. This is the technical foundation of the “all-RCBO consumer unit” — the topology that has become the gold standard in modern residential wiring practice.
💡 Engineering Insight — Why Per-Circuit RCBOs Beat One Big RCD + Multiple MCBs
The traditional split-load consumer unit places a single 30 mA RCD upstream of, say, six MCBs covering lighting, sockets, kitchen, and outdoor circuits. The architecture has two fatal flaws. First, a single earth fault on any one circuit kills power to all six — the fridge stops, the WiFi goes dark, the hallway lights vanish, and the homeowner is left fumbling in the dark trying to deduce which circuit caused the trip. Second, and more sinister, frustrated occupants who experience repeated whole-house blackouts are documented to have bypassed the RCD entirely or replaced it with a non-RCD isolation switch, removing the sole earth-leakage protection from the entire installation. Per-circuit RCBOs confine any fault to a single circuit only. The freezer stays on, the lights stay on, and the fault circuit is immediately identifiable: it is the one whose RCBO lever is in the OFF position. IEC 60364 and most national wiring codes are moving decisively toward mandating per-circuit RCBO protection in new residential builds.
The traditional split-load consumer unit places a single 30 mA RCD upstream of, say, six MCBs covering lighting, sockets, kitchen, and outdoor circuits. The architecture has two fatal flaws. First, a single earth fault on any one circuit kills power to all six — the fridge stops, the WiFi goes dark, the hallway lights vanish, and the homeowner is left fumbling in the dark trying to deduce which circuit caused the trip. Second, and more sinister, frustrated occupants who experience repeated whole-house blackouts are documented to have bypassed the RCD entirely or replaced it with a non-RCD isolation switch, removing the sole earth-leakage protection from the entire installation. Per-circuit RCBOs confine any fault to a single circuit only. The freezer stays on, the lights stay on, and the fault circuit is immediately identifiable: it is the one whose RCBO lever is in the OFF position. IEC 60364 and most national wiring codes are moving decisively toward mandating per-circuit RCBO protection in new residential builds.
1.2 Two Physical Forms of RCBO — And Why the Difference Matters
RCBOs come in two fundamental physical configurations defined by how the internal neutral path is handled. Choosing between them shapes the entire consumer unit layout:
| Configuration | Internal Topology | Module Width | Key Advantage | Limitation / Pitfall |
|---|---|---|---|---|
| 1P+N (single-pole + switched neutral, 1 module) | The line conductor passes through both the thermal/magnetic overcurrent elements and the CBCT. The neutral conductor passes through the CBCT only, with a switch contact that opens only on RCD trip — it has no independent short-circuit breaking capacity. | 1 standard module (~18 mm) | Maximum space efficiency; fits more circuits into a standard DIN-rail consumer unit; the dominant form factor in European residential installations | The neutral terminal must never carry short-circuit current — this is the single most common installation error (see Section 2). The RCBO body is clearly marked with “N” or a blue indicator for the neutral terminals. |
| 2P (double-pole, 2 modules) | Both the line and neutral poles feature independent overcurrent/short-circuit trip elements and contacts with full rated breaking capacity. Either pole detecting a fault opens both simultaneously. | 2 standard modules (~36 mm) | Higher safety redundancy — even if L and N are accidentally reversed, protection remains intact. Preferred for critical circuits (medical equipment, fire pump controls) where the penalty of a protection failure is unacceptable. | Twice the panel footprint. For the vast majority of residential circuits, a properly wired 1P+N RCBO meets all IEC 61009 safety requirements. |
⚠️ Specification Alert — Do Not Confuse “Pole Count” With “Protection Completeness”
Some low-cost products on the market are labelled “1P+N” but do not pass a full IEC 61009 type test — their neutral terminal may be nothing more than a pass-through screw connector that does not route the conductor through the CBCT and does not switch off on trip. A genuine IEC 61009-certified RCBO must route the neutral through the summation transformer and must open the neutral contact when the RCD function trips. Always verify the presence of an internationally recognised certification mark (VDE, SEMKO, KEMA-KEUR, UKCA, or a valid CB Test Certificate) and explicit reference to “IEC 61009” on the product label.
Some low-cost products on the market are labelled “1P+N” but do not pass a full IEC 61009 type test — their neutral terminal may be nothing more than a pass-through screw connector that does not route the conductor through the CBCT and does not switch off on trip. A genuine IEC 61009-certified RCBO must route the neutral through the summation transformer and must open the neutral contact when the RCD function trips. Always verify the presence of an internationally recognised certification mark (VDE, SEMKO, KEMA-KEUR, UKCA, or a valid CB Test Certificate) and explicit reference to “IEC 61009” on the product label.
🏗️ 2. The Neutral Connection: The Most Dangerous RCBO Installation Pitfall
2.1 Why the Neutral Wires Are Where RCBOs Win or Lose
Of all the wiring mistakes made in low-voltage distribution assemblies, the misconnection of the neutral conductor in an RCBO installation ranks as the most common, the most consequential, and the most difficult to catch during an initial energisation test. The root cause is the structural asymmetry inside a 1P+N RCBO: the neutral path does not carry overcurrent protection — it is a sensing path, essential to the functioning of the CBCT. If the neutral conductor from the downstream load does not return to the correct RCBO’s neutral terminal, the CBCT sees an incomplete current loop, leading to one of two dangerous failure modes:
- Nuisance tripping: During normal circuit operation, the current flowing in the line conductor passes through the CBCT. But the returning current does not pass through this RCBO’s neutral terminal — it finds a path through another RCBO’s neutral terminal or a shared neutral busbar. The CBCT registers a non-zero residual current (equal to the full load current, which far exceeds the 30 mA trip threshold) and the RCBO trips instantly on energisation, or intermittently as loads on other circuits switch on and off. This is frequently misdiagnosed as a “faulty RCBO” and leads to unnecessary product returns.
- Failure to trip (loss of protection): In a more insidious scenario, a genuine earth fault occurs on the protected circuit. The fault current flows from line to the earthed metallic enclosure, then back to the supply earthing terminal. But if the neutral path has been miswired, some or all of the returning current may bypass the RCBO’s CBCT entirely. The residual current seen by the CBCT is less than the actual leakage — potentially below the nominal trip threshold. The RCBO remains silently closed while a lethal fault current continues to flow through a person or through damaged insulation. This is, in standards language, a “loss of protection” — a class of failure that is never acceptable in any safety-critical device.
⚠️ The Golden Rule Every Electrician Must Burn Into Memory
The neutral conductor of every final circuit must be connected to the specific RCBO that protects that circuit, and to no other point. The neutral must flow into the RCBO from the supply-side neutral busbar (or from the supply neutral terminal block) and flow out from the RCBO to the load — following exactly the same physical path through the same RCBO as the line conductor does. You must never connect the outgoing neutrals from multiple RCBOs to a common neutral terminal block — that is the standard wiring convention for an MCB-only consumer unit and it is categorically wrong in an RCBO installation. The simplest way to remember it: whatever RCBO the line wire goes through, the neutral wire goes through the same one. Line and neutral are married to each other and to their RCBO. They never separate.
The neutral conductor of every final circuit must be connected to the specific RCBO that protects that circuit, and to no other point. The neutral must flow into the RCBO from the supply-side neutral busbar (or from the supply neutral terminal block) and flow out from the RCBO to the load — following exactly the same physical path through the same RCBO as the line conductor does. You must never connect the outgoing neutrals from multiple RCBOs to a common neutral terminal block — that is the standard wiring convention for an MCB-only consumer unit and it is categorically wrong in an RCBO installation. The simplest way to remember it: whatever RCBO the line wire goes through, the neutral wire goes through the same one. Line and neutral are married to each other and to their RCBO. They never separate.
2.2 Correct Wiring vs. Common Errors: A Visual Comparison
The table below contrasts the correct wiring method with the two most prevalent field errors and their consequences:
| Wiring Method | Line (L) Connection | Neutral (N) Connection | Earth Leakage Protection | Overcurrent Protection | Outcome |
|---|---|---|---|---|---|
| ✅ Correct | Supply L busbar → RCBO L-in → RCBO L-out → Load L | Supply N busbar → RCBO N-in → RCBO N-out → Load N | ✅ Functions as designed | ✅ Functions as designed | Full protection |
| ❌ Error A: Shared neutral busbar | Each RCBO L-in correctly fed from L busbar | All circuit neutrals connected to a common neutral busbar; each RCBO N-out is left unconnected or looped incorrectly | ❌ Completely defeated — the CBCT sees line current with no corresponding neutral return | ✅ Overcurrent still works | Instant or intermittent tripping at any load; if RCBO does not trip, the RCD channel is defective |
| ❌ Error B: Cross-neutral (swapped neutrals) | RCBO1 feeds circuit 1’s line | Circuit 1’s neutral is connected to RCBO2’s N terminal | ❌ Both RCBOs may nuisance-trip or fail to trip under fault conditions | ✅ Overcurrent still works | Intermittent tripping; impossible to trace fault circuit; both RCD functions unreliable |
✅ Best Practice — Pre-Wired RCBO Busbar Assemblies
To eliminate neutral miswiring at the root, modern consumer unit design strongly favours pre-wired distribution assemblies or RCBO-specific T-shaped busbar combs that integrate the line and neutral supply busbars into a single physical unit. The RCBO plugs directly into the assembly via its bottom terminals, with the physical keying of the busbar ensuring the line and neutral cannot be swapped. The outgoing load wires (line and neutral) then exit from the RCBO’s adjacent top terminals, making the line-neutral pairing visually unmistakable. For retrofit installations where pre-wired assemblies are impractical, use colour-coded ferrule markers and a permanently affixed circuit schedule inside the consumer unit lid that maps each RCBO’s neutral terminal to its designated circuit.
To eliminate neutral miswiring at the root, modern consumer unit design strongly favours pre-wired distribution assemblies or RCBO-specific T-shaped busbar combs that integrate the line and neutral supply busbars into a single physical unit. The RCBO plugs directly into the assembly via its bottom terminals, with the physical keying of the busbar ensuring the line and neutral cannot be swapped. The outgoing load wires (line and neutral) then exit from the RCBO’s adjacent top terminals, making the line-neutral pairing visually unmistakable. For retrofit installations where pre-wired assemblies are impractical, use colour-coded ferrule markers and a permanently affixed circuit schedule inside the consumer unit lid that maps each RCBO’s neutral terminal to its designated circuit.
🔌 3. Selection Parameters, Tripping Characteristics, and Selectivity
3.1 Residual Current Types — Not All RCBOs Are Equal
IEC 61009 inherits the residual current classification system from IEC 60755, dividing RCBOs into three fundamental types. Selecting the wrong type for the connected load creates a protection blind spot:
| RCBO Type | Residual Current Waveforms Detected | Typical Loads | Unsuitable For |
|---|---|---|---|
| Type AC | Sinusoidal AC residual currents only | Incandescent lighting, resistive heaters (immersion heaters, electric underfloor heating), purely resistive loads | Any load containing electronic components or rectifiers |
| Type A | Sinusoidal AC + pulsating DC (including up to 6 mA smooth DC superimposed) | Washing machines, inverter air conditioners, LED drivers, switch-mode power supplies, appliances with half-wave rectification or phase-angle control | Three-phase VFDs, large UPS systems, EV chargers (which can produce smooth DC beyond the 6 mA threshold) |
| Type B | Sinusoidal AC + pulsating DC + smooth DC + high-frequency residual currents (≤ 150 kHz) | EV Mode 3 AC charging stations, PV inverters, industrial variable-speed drives, UPS output circuits | — (highest protection level available) |
💡 Selection Guidance — Type AC Is Being Phased Out
Given the extraordinary penetration of electronic loads in modern homes (a typical household now contains fifteen or more switch-mode power supplies — phone chargers, LED drivers, inverter-driven fridge compressors, heat pump inverters, induction hobs, TV standby circuits), IEC 60364-5-53:2020 now mandates that RCDs and RCBOs for residential use be rated no lower than Type A. A Type AC RCBO can be blinded by pulsating DC residual currents: the DC component pushes the CBCT core into magnetic saturation, rendering it insensitive to the very AC leakage it is supposed to detect. The breaker stays closed while the leakage persists. For all new installations, specify Type A as the absolute minimum. Circuits serving EV chargers or battery storage inverters should be upgraded to Type B, as these devices can inject smooth DC fault currents that will similarly saturate a Type A core.
Given the extraordinary penetration of electronic loads in modern homes (a typical household now contains fifteen or more switch-mode power supplies — phone chargers, LED drivers, inverter-driven fridge compressors, heat pump inverters, induction hobs, TV standby circuits), IEC 60364-5-53:2020 now mandates that RCDs and RCBOs for residential use be rated no lower than Type A. A Type AC RCBO can be blinded by pulsating DC residual currents: the DC component pushes the CBCT core into magnetic saturation, rendering it insensitive to the very AC leakage it is supposed to detect. The breaker stays closed while the leakage persists. For all new installations, specify Type A as the absolute minimum. Circuits serving EV chargers or battery storage inverters should be upgraded to Type B, as these devices can inject smooth DC fault currents that will similarly saturate a Type A core.
3.2 Overcurrent Trip Curves — The B/C/D Decision
The overcurrent protection side of an RCBO follows the same instantaneous trip characteristic classification as standalone MCBs. For 1P+N RCBOs, this applies to the line pole only:
- B-curve (3–5 In): The workhorse for lighting circuits and general-purpose socket outlets. The magnetic trip operates between 3 and 5 times the rated current, comfortably above the cold-filament inrush of incandescent lamps and the start-up surge of small inductive loads. Approximately 80% of residential RCBO circuits should be B-curve.
- C-curve (5–10 In): Designed for circuits with moderate inrush currents — air-conditioning compressor motors, washing machine motors, and power tools in a workshop or garage. The higher magnetic threshold prevents nuisance tripping during normal motor starts. Kitchen socket circuits and dedicated air-conditioner circuits are typical candidates for C-curve RCBOs.
- D-curve (10–20 In): Reserved for high-inrush loads — large transformers, xenon lamp ballasts, or substantial motors. Rarely needed in residential settings (unless the home contains an X-ray machine or a heavy machine tool). The high magnetic threshold means lower short-circuit sensitivity; the loop impedance of the final circuit must be verified against IEC 60364-4-41 maximum values to guarantee disconnection within the required time (0.4 s or 5 s depending on the earthing system).
3.3 Selectivity and Coordination — Who Trips First Is an Engineering Decision
Selectivity (discrimination) between upstream and downstream protective devices ensures that a fault on a final circuit trips only that circuit’s RCBO, leaving upstream devices and other circuits undisturbed. This is especially important in hospitals, data centres, and premium residential installations where unnecessary loss of supply carries a high penalty. While IEC 61009 itself primarily defines the product-level performance, the selectivity behaviour of an RCBO in a cascaded system is governed by two distinct dimensions:
- Overcurrent selectivity: Achieved through deliberate staggering of rated currents and trip curves. For example, a 63 A C-curve main breaker upstream of a 16 A B-curve RCBO provides natural selectivity: the B-curve element trips at 48–80 A (3–5 × 16 A) while the upstream C-curve requires at least 315 A (5 × 63 A) to trip instantaneously. In the window between 80 A and 315 A, only the downstream device operates. For bolted faults exceeding both thresholds, selectivity depends on cascading (backup protection) coordinated tables published by the manufacturer.
- Residual current selectivity: This is unique to RCD/RCBO coordination. If the installation includes a Type S (selective, time-delayed) 300 mA RCD at the main incomer for fire protection, and 30 mA instantaneous RCBOs on each final circuit, vertical selectivity is achieved through both a current threshold ratio (300 mA vs. 30 mA) and a time delay (S-type with intentional 130–500 ms delay vs. < 300 ms instantaneous). The downstream 30 mA device opens on a domestic earth fault long before the upstream 300 mA device has accumulated enough delay to act.
⚠️ Selectivity Trap — Two 30 mA Devices in Series Do Not Create Redundancy
A recurring mistake: installing a 30 mA RCD (or RCBO) at the main incomer and then fitting 30 mA RCBOs on each final circuit, believing this creates “double-layer” earth leakage protection. It does not. Both devices share the same 30 mA threshold and will both detect the same earth fault simultaneously. Both have sub-300 ms tripping times — which one opens first is a matter of manufacturing tolerance and instantaneous supply conditions, not design. The result is that a single socket-outlet fault can black out the entire installation, erasing the very benefit the per-circuit RCBOs were intended to provide. The correct topology is either: (a) a non-RCD main switch feeding per-circuit 30 mA RCBOs (“pure per-circuit protection”), or (b) a Type S 100 mA or 300 mA incomer RCD feeding per-circuit 30 mA RCBOs, with the selectivity established on both time and current thresholds.
A recurring mistake: installing a 30 mA RCD (or RCBO) at the main incomer and then fitting 30 mA RCBOs on each final circuit, believing this creates “double-layer” earth leakage protection. It does not. Both devices share the same 30 mA threshold and will both detect the same earth fault simultaneously. Both have sub-300 ms tripping times — which one opens first is a matter of manufacturing tolerance and instantaneous supply conditions, not design. The result is that a single socket-outlet fault can black out the entire installation, erasing the very benefit the per-circuit RCBOs were intended to provide. The correct topology is either: (a) a non-RCD main switch feeding per-circuit 30 mA RCBOs (“pure per-circuit protection”), or (b) a Type S 100 mA or 300 mA incomer RCD feeding per-circuit 30 mA RCBOs, with the selectivity established on both time and current thresholds.
❓ Frequently Asked Questions
- Q1: Besides saving space, what fundamental advantages does an RCBO have over a separate RCD + MCB combination?
- A: The space saving (3–4 modules down to 1–2) is the visible advantage but far from the only one. Three deeper advantages stand out. First, the internal RCD and MCB channels share a single trip mechanism that has been type-tested as a coordinated system — the short-circuit withstand capability of the RCD contacts and the MCB contacts are verified together. In a separate RCD+MCB cascade, neither the RCD manufacturer nor the MCB manufacturer has tested the pair for mutual thermal and dynamic stress during a short-circuit, and in the worst case the RCD contacts can weld before the MCB clears the fault. Second, an RCBO has half the number of screw-terminal connections for the same protective function, and since terminal reliability follows a multiplicative failure law (the probability of any terminal in the chain being loose is roughly proportional to the number of terminals), the RCBO’s mean time between failures is inherently higher. Third, fault diagnosis: when an RCBO trips, the tripped circuit and the tripped device are one and the same, giving a single unambiguous fault location. With a shared RCD + 6 MCBs, the tripped RCD tells you nothing about which of the six circuits has the earth fault.
- Q2: Can I mix RCBOs and ordinary MCBs in the same consumer unit?
- A: Yes, but with a non-negotiable precondition: any circuit protected by a plain MCB must still be covered by an upstream RCD or RCBO for earth-leakage protection. If your consumer unit uses the “all-RCBO” topology (no main RCD at the incomer), then any circuit with a plain MCB has no earth-leakage protection at all — this is a code violation in essentially every jurisdiction. The most common mixed architecture keeps RCBOs for socket-outlet, bathroom, and outdoor circuits (where personal shock risk is high and per-circuit isolation is desirable), and uses plain MCBs for lighting circuits protected by a shared upstream RCD (on the reasoning that lighting circuits present lower shock risk and the cost of fitting an RCBO in every lighting circuit adds up). The essential rule: whenever you add an RCBO to the consumer unit, you must rethink the entire neutral busbar arrangement. You cannot keep the traditional “all neutrals to a common bar” layout. Every RCBO demands its own dedicated neutral path.
- Q3: In a home with solar PV and battery storage, where should RCBOs be installed?
- A: In a hybrid PV + battery + grid system, current direction is no longer simply from the grid to the loads. The critical principle is that the RCD channel of an RCBO relies on vector-sum (zero-sequence) detection, which is inherently direction-agnostic — bidirectional power flow does not impair earth-leakage detection. The overcurrent element, however, requires thought: when the battery inverter exports to the grid, currents can flow in the reverse direction through an RCBO whose overcurrent element was sized for unidirectional load flow. Specifically: (1) every final circuit should still use at least a Type A RCBO (upgrade to Type B for circuits fed by transformerless inverters that may inject smooth DC); (2) the inverter AC output and the grid-tie point should each have their own RCBO or RCD-equipped MCCB; (3) if a circuit can be fed alternately from the grid or from the inverter (via an automatic transfer switch or EPS mode), the RCBO protecting that circuit must have an uninterrupted neutral path through the transfer switch arrangement — otherwise the CBCT loses its zero-sequence reference and the RCD function is compromised when operating in islanded mode.
- Q4: Do RCBOs have a service life? When should they be replaced?
- A: IEC 61009 requires RCBOs to pass an endurance test of typically no fewer than 4,000 operating cycles (with at least 2,000 electrical operations at rated current), after which the residual current tripping characteristic must remain within specification. In practical residential service, the dominant degradation mechanism is not contact wear from frequent tripping (which is rare in a healthy installation) but rather mechanical stiction of the trip mechanism from years of inactivity. The most important maintenance action is to press the Test (T) button every 6 months — a requirement printed on the front of every IEC 61009-compliant RCBO. The test button injects a fixed residual current (typically 2.5 IΔn) through a dedicated resistor inside the device, exercising the entire detection, amplification, and tripping chain. An RCBO whose test button has not been exercised for years is a device whose functional status is unknown — it may look healthy but be mechanically frozen. If the test button does not cause immediate tripping, the RCBO must be replaced. Additionally, after a severe short-circuit event, consider replacing the RCBO — the arc erosion on the contacts from a high-current interruption degrades both breaking capacity and contact resistance. For a typical residential installation, a service life of 10–15 years is a reasonable planning assumption, with a full RCD functional test (using a dedicated RCD tester to measure actual trip time and trip current) recommended at the point of any major electrical renovation or at the time of a pre-purchase building inspection.
