What does an RCD do

What Does an RCD Do? A Complete Guide

An RCD (Residual-Current Device) is a crucial safety device that protects against electric shock and electrical fires. This guide explains what it is, how it works, and how to choose the right one.
1. What is an RCD?
2. How Does an RCD Work?
3. RCD Sensitivity: 30mA, 100mA, 300mA - Differences and Selection
4. RCD Types: AC, A, F, B, B+, S, EV - Differences and Selection
5. Key Considerations for RCD Selection
6. RCD Installation and Maintenance
7.RCD vs. RCBO, MCB, AFDD - The Difference
 

1. What is an RCD?

An RCD (Residual-Current Device) is a life-saving device designed to prevent fatal electric shocks and reduce the risk of electrical fires. The term "RCD" is a broad category that includes products like RCCBs (per standard IEC 61008) and RCBOs (per standard IEC 61009). In some countries, like the UK, it specifically refers to electromagnetic RCCBs. Other common names include RCCB, ELCB, and IDR.

2. How Does an RCD Work?

An RCD continuously monitors the balance of current flowing in the live (phase) and neutral wires. Under normal conditions, these currents are equal. If an imbalance is detected—meaning some current is leaking away, possibly through a person—the RCD will rapidly disconnect the circuit.

There are two main types of current-based RCDs:

• Electromagnetic RCDs: These are purely mechanical. They use a magnetic core to sense the current imbalance. If a leakage current reaches the device's rated sensitivity, it triggers a mechanical trip mechanism without needing an external power supply. They are highly reliable.

• Electronic RCDs: These contain electronic circuitry and require an auxiliary power supply (usually from the load side) to function. If the line voltage drops too low, they may not operate, even if a leakage current is present.

3. RCD Sensitivity: 30mA, 100mA, 300mA - Differences and Selection

Differences:

• 30mA (High Sensitivity):

  • Protection: Protects against fatal electric shock. 30mA is the internationally recognized threshold for preventing ventricular fibrillation in humans.

  • Speed: Very fast operation (typically ≤0.1 seconds).

  • Use: Essential for all final circuits with sockets, portable equipment, and locations with increased shock risk (e.g., bathrooms, kitchens).

• 100mA & 300mA (Medium Sensitivity):

  • Protection: Primarily for fire protection, as even small, persistent leakage currents can generate enough heat to start a fire. Also used for backup protection.

  • Speed: Often used as time-delayed (Type S) devices (0.1-0.5 seconds) to allow downstream RCDs to trip first, preventing total installation shutdown.

Selection Guide:

• Final Circuits (Sockets, Lighting): Always use 30mA RCDs for personal protection.

• Main Switchboard (Upstream Protection): Use a 100mA or 300mA time-delayed (Type S) RCD for fire protection and to provide selective coordination with downstream 30mA RCDs.

• Special Cases: In rural areas with older wiring, a 100mA main RCD may be suitable. For industrial equipment with higher inherent leakage, 300mA RCDs can be used.

4. RCD Types: AC, A, F, B, B+, S, EV - Differences and Selection

5. Key Considerations for RCD Selection

• Residential:

  • Socket circuits: Type A, 30mA.

  • Main switch: Type AC or A, 100mA/300mA (S-type) for fire protection.

  • Inverter ACs/Appliances: Type F, 30mA.

• Commercial/Industrial:

  • EV Chargers, Elevators, Solar: Type B,Type EV.

  • Machinery with VFDs: Type B or A.

  • Noisy electrical environments: Prefer Electromagnetic RCDs.

• Special Cases:

  • Photovoltaic (PV) Systems: Always use Type B or dedicated PV RCDs.

  • Medical, Data Centers: Use high-reliability Electromagnetic Type A or B RCDs.

Always match the RCD type to the load characteristics to ensure proper protection and avoid nuisance tripping.

6. RCD Installation and Maintenance

Installation:

• Install at the origin of the installation, close to the meter.

• Both live and neutral conductors must pass through the current transformer. The neutral must not be shared or connected to earth elsewhere.

• Install in a clean, dry environment. Use enclosures with sealed doors for kitchens or damp locations.

Maintenance:

• Test Monthly: Press the "TEST" button to ensure the mechanical trip mechanism works.

• Increase testing frequency during humid seasons or after electrical storms.

7. RCD vs. RCBO, MCB, AFDD - The Difference

• RCD (RCCB): Provides only earth leakage (shock) protection. Requires a separate MCB for overcurrent protection.

• RCBO: An all-in-one device that combines the functions of an RCD (shock protection) and an MCB (overload & short-circuit protection). Saves space and provides dedicated circuit protection.

• MCB: Provides only overload and short-circuit protection. It does not protect against electric shock.

• AFDD: A more advanced device that detects arc faults (caused by loose connections, damaged cables) to prevent electrical fires. It often includes MCB functionality and may be combined with RCD (RCBO-AFDD).

Brief Definitions

• RCD (Residual-Current Device): This is a generic term for any device that can detect a leakage of current to earth (a "ground fault") and break the circuit. Both RCCBs and RCBOs are types of RCDs.

• RCCB (Residual Current Circuit Breaker): This is a pure earth leakage protection device. It provides no protection against overcurrents or short circuits. It must be used in series with an MCB.

• RCBO (Residual Current Circuit Breaker with Overcurrent Protection): This is a combination device that integrates the functions of an RCCB and an MCB into a single unit. It provides protection against earth leakage, overload, and short circuit.

• MCB (Miniature Circuit Breaker): This device protects against overloads and short circuits but provides no protection against earth leakage.

• AFDD (Arc Fault Detection Device): This is an advanced protection device designed to detect dangerous electrical arcs (caused by damaged cables, loose connections, etc.) to prevent electrical fires.

Comparative Table: RCD vs. RCCB vs. RCBO vs. MCB vs. AFDD