Overcurrent protection – design and options

Blog post10. July 2025 by Dr. Hendrik Köpf - Reading Time 2 min.

Different applications and loads present different technical requirements for overcurrent protection devices. We will clarify some basic questions on this topic to help the users decide on their design and documentation.

1.1 Overcurrents – Protection through current limitation

An overcurrent exceeds the normal operational load of appliances or equipment. There are two different types of overcurrent:

Overload:         Overcurrent in an electrically faultless circuit

Short circuit:   Overcurrent in an electrically faulty circuit

1.2 Which short circuit parameters must be considered when selecting overcurrent protection?

Key values:

  1. Uninfluenced or prospective short circuit current Ip – In our example with peak value 10 kA
  2. On-state current Id  – Current-limiting overcurrent protection devices, such as a circuit breakers or fuses, limit the current to the on-state current.
  3. Trip time t1 or melting time
  4. Arc burning time or switching time t2  – After the trip time has elapsed, the overcurrent protection device limits the current and forces it to 0 A.
  5. Clearing time t3 is the sum of the trip time t1 and the switching time t2

1.3 Which overcurrent protection devices are available from E-T-A?

In the field of common overcurrent protection devices, E-T-A provides IEC 60934 circuit breakers for equipment protection (CBE), IEC 60947-2 miniature circuit breakers (MCB) or IEC 60269 circuit breaker/switch combinations with fuses.

Symbol number 1 shows a circuit breaker with insulation capability in accordance with IEC 60947 Part 2. Symbol number 2 is assigned to a fuse. Finally, symbol number 3 shows a semiconductor circuit breaker (SCCB) in accordance with the draft of the future IEC 60947-10.

1.4 What is an electronic-based circuit breaker?

According to the "Draft of the IEC 60947-10 Semiconductor Circuit Breaker” standard, there are the following differences between circuit breakers:

1. "Semiconductor circuit breakers - SCCB" - Circuit breakers with semiconductor switches and mechanical disconnectors connected in series

2. "Semiconductor hybrid circuit breakers - SCHCB" - Circuit breakers with semiconductor switches and bypass switches connected in parallel and mechanical disconnectors connected in series.

1.5 What are the response times of E-T-A switching devices in the event of a short circuit?

Depending on the technology used in switching devices, they react differently in the event of a short circuits caused by a fault. As part of a technical evaluation, the following switching times t2 were determined for E-T-A switching concepts:

 

E-T-A-MCB Range less than 10 ms
E-T-A-SCHCB-63 A Range less than 1 ms
E-T-A-SCCB-16 A Range less than 5 µs

(Table: Switching time – Resistive load – Maximum 4 A to 1000 A – Voltage 350 V)

The results of the analysis were presented at the Albert Keil Contact Seminar in Karlsruhe in September 2025.

28. Albert Keil Contact Seminar 2025 - Registration portal for participants

 

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