RESIDUAL CURRENT DEVICES: WHAT IS RCD AND WHAT DOES IT DO?

Fig-Residual Current Device Connection Procedure

What are the Residual Current Devices?

By definition, a residual current device is a current-activated circuit breaker used as a safety device for mains-operated electrical tools and appliances.

By means, the Residual Current Device is a sensitive safety device that switches off the electricity within 10 to 50 milliseconds if there is an electrical fault. An RCD is designed to protect against the risks of electrocution and fire caused by earth faults.

A residual current device or RCD is a device is used to save a life. A residual current device or RCD  is designed to avoid a person from getting a deadly electric shock in the event that they touch something live, like an energized conductor or any live device. Residual Current Devices also provided some protection during an electrical fire occurs.

Residual Current Devices?

RCD or Residual Current Device is a generic term that covers both RCCB (Residual Current Circuit Breaker) and RCBO (Residual Current Circuit Breaker with Overload).

RCD is a protective device in an electrical system that automatically disconnects the supply system when an imbalance is detected in the value of phase and neutral current. British Standard BS 7671 defined RCD as below:

A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions.

The protective device RCD actually protect- against direct and indirect contact, fire, and thermal effect.

To clearly understand the difference between RCD and other broadly used Circuit breakers, we should remember the definition of RCCB, RCBO, ELCB, GFCI, etc.

RCCB (Residual Current Circuit Breaker) is an electrical protective device that breaks or disconnects the circuit to become in a safe mood whenever it detects an unbalance current flow between alive and neutral conductors.

Generally in an electrical circuit in which a number of currents flow to the load from the source through a live or phase conductor, the same amount of current must be returned to the source from the load. If not happen, means some current passing to the earth due to a faulty circuit; then RCCB breaks the circuit quick enough to prevent any injury.

RCBO (Residual Current Circuit Breaker with Overload) is actually a combined design of RCD and MCB. RCD alone does not provide protection against overcurrent. Overcurrent protection is provided by a fuse or an MCB (Miniature Circuit Breaker), so RCBO combines overcurrent protection and leakage detection functions.

ELCB (Earth Leakage Circuit Breaker) also known as GFCI (Ground Fault Circuit Interrupter) is designed to detect the current flowing through the ground or earth wire.

MCB/MCCB (Miniature Circuit Breaker/ Molded Case Circuit Breaker) is a protective device for an electrical distribution system designed to protect a circuit from overcurrent which would be caused severe damage to equipment.

Common Types Of RCD and Their Uses 

We already mentioned that RCD is a generic term for protective devices. Common types of RCD and their usages which are incorporated with other devices are as below:


RCCB 
A residual current circuit breaker which not integral with over-current protection. This device operates by comparing the predetermined value of residual current and is mainly used in consumer units and distribution boards.

RCBO

A residual current circuit breaker which integral with over-current protection. This device operates by comparing the predetermined value of residual current and incorporating over-current protection and is mainly used in consumer units and distribution boards.

SRCD
It is a socket-outlet or fused connection unit that is incorporated with RCD. Mainly used in portable outdoor units for protection against direct contact.

SRCBO
It is a socket-outlet or fused connection unit that is incorporated with RCBO. Mainly used in portable outdoor units for protection against direct contact.

PRCD
Portable Residual Current Device that provides RCD protection for any equipment connected with a plug and socket. Mainly used in socket-outlet.

CBR

Circuit Breaker that is incorporated with residual current protection. Mainly used in distribution boards in a larger installation.

According to the number of connecting poles RCD may be two poles for a single-phase supply with phase and neutral; three poles for a three-phase supply and four poles for a three-phase supply with neutral.

How does RCD Prevent Electrical Shock?

RCD can operate by very smaller current, typically 5 to 30 milli-amperes which is several amperes for a conventional CB (Circuit Breaker), it is also possible to operate within a very short time,  like 25 to 40 milliseconds which is faster than the minimum time required for an electrical shock.

Rated and Sensitivity Current of RCD

Don’t confuse about the rated current rating and sensitivity current of RCD. The rated current mentioned in RCD is according to the maximum sustainable load current that can carry. Actually rated current is as same as the conventional circuit breaker.

Unlike another conventional circuit breaker, RCD bears one more current rating which is normally a milli-ampere rating sensitivity current. Sensitivity actually expressed rated residual operating current.

• Rated sensitivity current 10, 30 mA uses for High Sensitivity;
• Rated sensitivity current 100, 300, 500, and 100 mA uses for normal Sensitivity;
• Rated sensitivity current 10, 30 A used for Low Sensitivity
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How Fast Operates a RCD?

RCD operating or breaking time depends on the groups' characteristics of the devices. The basic idea for RCD, how fast it operates-

Minimum break time is immediate, and maximum break time 40, 150, 200 mili-second at 5 times, 2 times, rated sensitivity current respectively.

 

How to Select RCD for Use? 

When the main objective of using RCD is safety, then RCD selection depends on the degree of safety. Also, need to consider the load's characters. The common selection criteria of RCD according to sensitivity are given below:

10 mA RCD 
Uses very sensitive devices like laboratory benches in school.


30 mA RCD
Uses for portable equipment, 230v or similar voltages bathroom/shower-room equipment, socket-outlet in bedroom/ workshop/ school laboratories, caravan & caravan park, underfloor heating system, etc.

100 mA RCD
Uses in TT earthing system where earth loop impedance is too high for shock protection to be provided by an overcurrent device.

300 mA RCD
Uses in busbar trunking system, mainly in TN & TT systems.

500 mA RCD
Uses in agricultural and horticultural premises, any exhibition or shows need to take extra care for cable protection.

2000 mA or Adjustable
Uses for special industrial or distribution systems if design seeks for requirements.

How residual current device is connected?

The residual current device connection or wiring is pretty simple. Just connect the phase (brown) and neutral (blue) wires to the RCD input in a single-phase circuit. The protective conductor is connected with e.g. a terminal strip. The phase wire at the RCD output should be connected to the overcurrent circuit breaker, while the neutral wire can be connected directly to the installation.

Fig: Residual Current Device Connection Diagram

An easy connection diagram of the Residual Current Device or RCD is shown in the schematic drawing below. Anyone can do it following the wiring diagram.