Overload and Overcurrent - the debate

[Dave47]

Hello,

if there are any engineers in here, especially from Schneider, can some please settle and argument I had with my instructors today: I said that thermal overload and magnetic overcurrent are the protections within a CB, however they said it is thermal overcurrent and magnetic overload.

I said that overload is heat I.e. thermal caused by slower excessive current (motor stalls, high loads) that generates the excessive heat, whereas the magnetic overcurrent is for shortcut and/or high inrush currents caused by faults, shorts etc. My instructors argue that the opposite is in effect what happens.

 What is it?

Kind regards,

Dave

[Romain_Polette · Accepted solution]

Hello @Dave47, 

I will try to make it simple as possible and take the example of Miniature Circuit Breaker (this is a bit different for industrial circuit breakers).

A MCB has 2 kind of protections:

• Thermal (low overcurrent)
• Magnetic (high or very high overcurrent and short circuits)

Let's take a 16 Amps C curve MCB:

• 16 = Nominal Current (In)
• C curve = 5x to 10x In
• Breaking Capacity = 10 kA (Icu) = Max possible current that a breaker can handle during a short circuit. Ensure that installation cannot be damaged after the breaker trip.

Thermal protection is dedicated to small overcurrents (cable heat up):

• When your current flowing through your circuit breaker is more than In.
• If your current is 20 A (1,3x In) => MCB will trip after 1 hour
• If your current is 32 A (2x In) => MCB will trip after 10 sec

Magnetic protection is dedicated to high overcurrents :

• Related to the electromagnet part inside a MCB
• Will depend on the B, C or D curve
• C curve = instant trip when current is 5x to 10x In
• During a short circuit (some kA)

This is more a story of current quantity / variation inside your circuit breaker. For me overcurrent and overload are similar.

Regards,