Let Through Energy

I²t = Let Through Energy.

I²t – Let through Energy is available thermal energy resulting from Current Flow during circuit opening in Fuse, MCCB, MCB.Unit is A²S (Ampere² Second)

Circuit Breakers with current limiting charestristics due to their operating principle neither have current based selectivity nor time based selectivity. Selectivity = Discrimination.

• Current Limiting Breakers, drastically reduce the prospective short circuit Ipk Peak
• Current.Current Limiting Breakers, drastically reduce the let through energy.

Due reduced Ipk and reduced let through,

• System Eletrodynamic stresses are reduced.
• Thremal Stresses are reduced.Reduction in Cable and Busbar size.

Let through energy and Cables

Major Cable Selection Criteria :

1. Type of Cable Installation
2. Thermal withstand capablity
3. Short Circuit withstand capability

Due to Copper/Aluminium resistance, when cable carryes current, Cable Temperature increases, and the same also related to Cable installation type considering heat dissipation. Hence Cable Installation type and Cable Thermal withstand are inter related.

For Selecting Cable size, Short Circuit withstand capability of cable ( K²S²) also need to be considered. Cable Short Circuit Energy withstand = K²S²

• Cable Short Circuit Energy withstand = K²S²
• K=Factor of cable Insulation and Conducting material.
• S=Cross Section of Cable in mm²
• In case of multiple parallel cables, Cross Section of only one cable Or Cross Section of Smallest cable to be considered.

For Proper Cable Protection, Let through energy I²t of upstreal SCPD device – Fuse, MCCB or MCB, must be lower than Short Circuit withstand capability of cable. I²t < K²S²

I²t (of circuit breaker) < K²S² (of cable)