RCD Wiring Diagram
This is a free printable rcd wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
An RCD monitors the residual current between line and neutral conductors and trips within 40 ms when leakage to earth exceeds the rated sensitivity, providing earth-fault shock protection for multiple downstream circuits.
A Residual Current Device — also designated RCD or, in older terminology, Earth Leakage Circuit Breaker (ELCB) — is a protective device that continuously measures the vector sum of current flowing in the line conductor and current returning in the neutral conductor. Under normal conditions, with no current path to earth, these two currents are equal and opposite and their sum is zero. If a live conductor is touched, or cable insulation breaks down, current flows to earth through a path outside the device's monitoring toroid. The returning neutral current becomes smaller than the outgoing line current. The toroid detects the imbalance, its secondary winding drives the trip coil, and the contacts open in under 40 ms at the rated residual current.
An RCD does not provide overload or short-circuit protection — that is a fundamental distinction from an RCBO. When an RCD is used, separate overcurrent protection (MCBs) must be installed downstream for each circuit.
RCDs are manufactured in three principal sensitivity ratings. A 30 mA RCD is the standard for personal protection on socket circuits, because currents above approximately 30 mA can induce ventricular fibrillation in healthy adults. A 100 mA RCD is used for fire protection on circuits where the earth-leakage level sufficient to cause fire is lower than the shock risk threshold. A 300 mA RCD protects equipment by tripping on catastrophic insulation failure without tripping on routine equipment capacitive leakage.
Two device types govern what leakage waveforms are detected. A Type AC RCD detects only sinusoidal alternating current leakage — appropriate for traditional resistive and inductive loads. A Type A RCD additionally detects pulsating DC leakage, which is produced by modern electronic equipment, variable-speed drives, EV chargers, and switch-mode power supplies. IEC 60364 and BS 7671 (18th Edition) require Type A RCDs on circuits feeding electronic equipment.
In a consumer unit, the RCD is typically wired ahead of a group of MCBs. The RCD's line-in terminal receives the busbar feed and line-out feeds the downstream MCB busbar. The neutral-in and neutral-out pass both conductors through the sensing toroid. The protected circuits' earth conductors connect to the common earth bar and do not pass through the RCD.
How to wire rcd wiring diagram
- Isolate, lock off, and verify dead Disconnect the main supply to the consumer unit or distribution board. Apply a lock-off hasp and tag. Verify zero voltage on all live conductors using a calibrated voltage indicator (approved to GS38 or equivalent). Do not proceed until dead is confirmed.
- Identify the RCD installation position in the consumer unit The RCD is installed upstream of the MCB group it will protect. In a split-load consumer unit, the RCD feeds the protected busbar section. Verify the neutral bar arrangement — protected and unprotected circuits must have separate neutral bars, with only the protected circuits' neutrals passing through the RCD's neutral terminal.
- Connect the supply-side (incoming) terminals Connect the incoming line conductor to the RCD Line-in terminal. Connect the incoming neutral conductor to the RCD Neutral-in terminal. These conductors come from the main isolator or the unprotected busbar.
- Connect the load-side (outgoing) terminals The RCD Line-out terminal feeds the protected MCB busbar. The RCD Neutral-out terminal feeds the protected neutral bar. Ensure the neutral conductors of all protected circuits connect only to the protected neutral bar, not to the main neutral bar.
- Verify earth continuity is not passing through the RCD All circuit earth conductors connect to the main earth bar — they must not pass through the RCD. A common error is connecting the earth bar to the neutral bar; these must be separate in TN-S and TT earthing systems.
- Energise and test using the TEST button Restore supply. Switch the RCD to ON. Press the TEST button — the RCD must trip immediately. Reset the RCD and confirm downstream MCBs are re-energised. For full compliance testing, use a calibrated RCD tester to measure trip current and trip time and record results in the installation test schedule.
Specifications
| Standard sensitivity ratings | 30 mA (personnel protection), 100 mA (fire protection), 300 mA (equipment protection) |
|---|---|
| Trip time at rated residual current | < 300 ms (IEC 61008) |
| Trip time at 5× rated residual current | < 40 ms (IEC 61008) |
| Device type — leakage waveform | Type AC (AC sinusoidal only), Type A (AC + pulsating DC), Type F (AC + pulsating DC + high frequency) |
| Rated current range | 25 A, 40 A, 63 A, 80 A, 100 A (common sizes) |
| Operating voltage | 230 V AC (single phase); 400 V AC (three phase, 4-pole) |
| Standard | IEC 61008-1 (RCD without integral overcurrent protection) |
Safety warnings
- An RCD provides earth-leakage protection only. It does not protect against overload or short-circuit currents. Always install appropriate MCBs downstream for overcurrent protection of every circuit.
- Incorrect separation of protected and unprotected neutrals — connecting an unprotected circuit's neutral to the protected neutral bar — causes continuous imbalance in the RCD sensing toroid and persistent nuisance tripping or, in extreme cases, failure to trip on a real fault.
- Pressing the TEST button confirms the trip mechanism operates but does not verify the correct trip current or trip time. For certification and compliance, use a calibrated RCD test instrument and record results as part of the installation test schedule.
- All consumer unit work must be carried out by a licensed electrician and must comply with IEC 60364, BS 7671, AS/NZS 3000, NEC/NFPA 70, or the applicable national wiring standard. Consumer unit busbars remain live even with individual MCBs in the OFF position — always isolate and verify at the main isolator.
- This page provides illustrative reference information only. Do not rely on this content alone for installation or design decisions.
Tools needed
- Calibrated voltage indicator (GS38 approved or equivalent)
- Lock-off hasp and safety tags
- Calibrated RCD test instrument (measures trip current and trip time)
- Insulated screwdrivers (flat blade for terminal screws)
- Torque screwdriver (for terminal torque compliance)
- Digital multimeter
- Insulation resistance tester (500 V DC range)
- Conductor ferrule crimping tool
Common mistakes
- Connecting a circuit earth to the protected neutral bar instead of the main earth bar — this routes earth-fault current through the RCD sensing toroid and causes nuisance tripping or masking of real earth faults.
- Using a Type AC RCD on circuits feeding electronic equipment, EV chargers, or variable-speed drives — these produce pulsating DC leakage that Type AC devices cannot detect reliably. Use Type A.
- Failing to test with a calibrated RCD tester after installation — the TEST button check verifies mechanical function, not calibrated trip threshold. Record measured trip current and trip time in the installation schedule.
- Installing the RCD without separate protected and unprotected neutral bars in a split-load consumer unit — sharing a neutral bar between protected and unprotected circuits defeats RCD protection on the protected group.
- Over-loading one RCD with too many circuits — accumulated capacitive leakage from many circuits can approach the 30 mA threshold, causing nuisance tripping. Split circuits across two RCDs or use individual RCBOs.
Troubleshooting
- RCD trips immediately when switched on
- Cause: Active earth-leakage fault on a connected circuit or appliance Fix: Disconnect all downstream MCBs. Reset the RCD. Switch on MCBs one at a time until the RCD trips — this identifies the faulty circuit. Disconnect all loads on that circuit and repeat to identify the faulty appliance or check cable insulation resistance.
- RCD does not trip on TEST button press
- Cause: Faulty RCD tripping mechanism — the internal test circuit simulates a leakage current at the rated sensitivity Fix: The RCD must be replaced. Do not leave a device in service that fails the TEST button check. Record the failure in the installation fault log.
- RCD trips intermittently, no identifiable fault
- Cause: Accumulated capacitive leakage from multiple appliances approaching 30 mA threshold; insulation resistance declining with temperature or moisture; Type AC device detecting DC leakage from electronic equipment Fix: Test insulation resistance on all protected circuits (500 V DC, minimum 1 MΩ). Use a power quality analyser to measure earth-leakage current over time. Consider splitting circuits across two RCDs, upgrading to Type A, or replacing with individual RCBOs to isolate the fault.
Frequently asked questions
What is the key difference between an RCD and an RCBO?
An RCD detects earth-leakage current only and protects multiple downstream circuits simultaneously. It provides no overload or short-circuit protection — MCBs must be installed downstream. An RCBO combines both functions in one device, protecting a single circuit independently. A trip of one RCBO does not affect other circuits.
Why does my RCD trip randomly at night?
Nocturnal RCD tripping is often caused by temperature-driven insulation degradation — cable insulation resistance is lower when cold and damp, increasing earth-leakage current. Appliances left on standby (refrigerators, freezers, televisions) can also produce accumulated capacitive leakage. Use an RCD tester or insulation resistance meter to identify the source circuit.
Can an RCD protect against all types of electric shock?
An RCD protects against earth-fault shocks — where current flows through a person to earth. It does not protect against line-to-neutral contact (where current flows through the person between the two supply conductors, balanced so the RCD sees no residual current). This is why RCDs do not replace the need for fusing, insulation, and physical guarding.
What trip current sensitivities are available and when is each used?
30 mA: mandatory for socket-outlet circuits and locations with a risk of contact, per IEC 60364 and BS 7671. 100 mA: fire protection on fixed equipment circuits where lower fault currents could ignite materials. 300 mA: equipment protection on industrial circuits where capacitive leakage from equipment makes 30 mA too sensitive for practical use.
How often should an RCD be tested and how?
IEC 60364, BS 7671, and most national standards recommend testing the RCD by pressing the integral TEST button at least every six months (quarterly in commercial premises). For full verification, a calibrated RCD tester measures actual trip current and trip time at rated sensitivity, half sensitivity, and five times rated sensitivity.
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