RESIDUAL CURRENT DEVICES: WHAT IS RCD AND WHAT DOES IT DO?
RCD or Residual Current Device is a generic term which covers both RCCB (Residual Current Circuit Breaker) and RCBO (Residual Current Circuit Breaker with Overload).
What Is RCD?
RCD is a protective device in electrical system which automatically disconnect the supply system when imbalance detected in the value of phase and neutral current. British Standard BS 7671 defined RCD is 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- direct and indirect contact, fire and thermal effect.
For clear understand the difference between RCD and other broadly used Circuit Breaker, we should remember the definition of RCCB, RCBO, ELCB, GFCI etc.
RCCB (Residual Current Circuit Breaker) is an electrical protective device that break or disconnects the circuit to become in safe mood whenever it detect an unbalance current flow between live and neutral conductor.
Generally in an electrical circuit which amount of current flow to the load from source through live or phase conductor, the same amount of current must be return to source from load. If not happen this, means some current passing to the earth due to faulty circuit; then RCCB breaks the circuit as quick enough to prevent any injury.
RCBO (Residual Current Circuit Breaker with Overload) is actually combined designed of RCD and MCB. RCD alone does not provide protection against overcurrent. Overcurrent protection is provided by a fuse or a MCB (Miniature Circuit Breaker), so RCBO combines the functions of overcurrent protection and leakage detection.
ELCB (Earth Leakage Circuit Breaker) is also known as GFCI (Ground Fault Circuit Interrupter) is designed to detect the current flowing through ground or earth wire.
MCB/MCCB (Miniature Circuit Breaker/ Molded Case Circuit Breaker) is a protective device for electrical distribution system that designed to protect a circuit from over current which would caused a serious damage equipment's.
Common Types Of RCD and Their UsagesWe already mentioned RCD is a generic term for protective devices. Common types of RCD and their usages which incorporated with other devices is as below:
Residual current circuit breaker which not integral with over current protection. This device operates comparing the predetermined value of residual current; and mainly uses in consumers units and distribution boards.
Residual current circuit breaker which integral with over current protection.This device operates comparing the predetermined value of residual current and incorporate over current protection, and mainly uses in consumers units and distribution boards.
|SRCD||It is a socket-outlet or fused connection unit that is incorporated with RCD. Mainly uses in portable outdoor unit for protection against direct contact.|
|SRCBO||It is a socket-outlet or fused connection unit that is incorporated with RCBO. Mainly uses in portable outdoor unit for protection against direct contact.|
|PRCD||Portable Residual Current Device that provides RCD protection for any equipment connected with plug and socket. Mainly uses in socket-outlet.|
Circuit Breaker that is incorporated with residual current protection. Mainly uses in distribution boards in larger installation.
According to the number of connecting poles RCD may be: two poles for single phase supply with phase and neutral; three poles for three phase supply and four poles for three phase supply with neutral.
How RCD Prevent Electrical Shock?RCD can operate by very smaller current, typically 5 to 30 mili-amperes where it is several amperes for a conventional CB (Circuit Breaker), it also possible to operate within very short time, like as 25 to 40 mili-seconds which is faster than the minimum time required for an electrical shock.
Rated and Sensitivity Current of RCD
Don’t confuse about rated current rating and sensitivity current of RCD. Rated current mentioned in RCD according to the maximum sustainable load current that can carry. Actually rated current is as same as the conventional circuit breaker.
Unlike to others conventional circuit breaker, RCD bears one more current rating which is normally mili-ampere rating is 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, 100 mA uses for Medium Sensitivity;
Rated sensitivity current 10, 30 A uses for Low Sensitivity
How Fast Operate RCD?
RCD operating or breaking time depends on the groups characteristics of the devices. 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 Select RCD to Uses?
When the main objective of uses RCD is safety, then RCD selects depend on the degree of safety. Also need to consider the load characters. The common selection criteria of RCD according to sensitivity are given below:
|10 mA RCD||Uses for 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, under floor heating system etc.|
|100 mA RCD||Uses in TT earthing system where earth loop impedance is too high to for shock protection to be provided by an over current device.|
|300 mA RCD||Uses in busbar trunking system, mainly in TN & TT system.|
|500 mA RCD||Uses in agricultural and horticultural premises, any exhibition or shows where need to take extra care for cable protection.|
|Uses for special industrial or distribution system if design seek for requirements.|