Choosing protective device guide

[tahmed17]

Is there any relevant literature available to understand which protective device need to be used before completing a electrical design ?

[kyriakos]

Hello,
Actually there is not a documentation which determines what protective device to use for a specific distribution or final circuit. What you can do is understand the specific features and the way each protective device operate and through tests and work experience you will be able to choose the suitable one.
The main Protective Devices are:


 
MCB (Miniature Circuit Breaker):
According to BS EN 60898 MCBs are suitable for operation by ordinary persons and have fixed protection settings, generally a two position on/off operating handle and a performance relative to the final circuits in an electrical installation. They would normally be the final overcurrent protection measure in the electrical system, for example before sockets or lighting circuits.
Typical current ratings are from 0.5 A to 125 A. Short-circuit ratings may be up to 25 kA. Performance and testing is in accordance with BS EN 60898 for domestic and similar applications categorised by the trip characteristic types B, C & D. MCBs may also be available with application specific tripping characteristics.
MCBs may also be rated in accordance with BS EN 60947-2 for industrial or similar applications.

MCCB (Molded Case Circuit Breaker):
MCCBs may have fixed or adjustable protection settings, normally a three position toggle operating handle giving on-off-tripped indication plus reset function, and a performance level relative to the incoming supply such that they can be installed at a point close to the supply transformer. Typical current ratings are from 16 A to 1600 A though ratings up to 3,200 A are available. Short-circuit ratings may be up to 100 kA. Performance and testing is in accordance with BS EN 60947-2.

ACB (Air Circuit Breaker):
ACBs are normally used as the main incoming protection and have a spring-operated mechanism to open and close the device often charged by an internal motor. The protection settings will include time delays and the devices will have a short-time withstand value to give full discrimination under fault conditions with downstream protection devices. Typical current ratings are 630 to 6,300 A. Short-circuit ratings may be up to 150 kA. Performance and testing is in accordance with BS EN 60947-2.

The typical Time/Current curve according to the type of the trip is produced from the following graph:
 



 

For more information you can refer to manufacturer’s technical guide.

You can also refer to our help guide:

Design Current and Current Carrying Capacity Calculations
https://electricalom.com/app/help/DesignCurrentandCurrentCarryingC.html

Maximum Phase Fault Current Calculation
https://electricalom.com/app/help/MaximumPhaseFaultCurrentCalculat.html

Protective Devices Discrimination Problems
https://electricalom.com/app/help/ProtectiveDevicesDiscriminationP.html

Motor Starting Check
https://electricalom.com/app/help/MotorStartingCheck.html