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Home / Technical Articles / Two Circuit-Breaker Types For Automatic Disconnection Of the Supply

Automatic disconnection

The Standard IEC 60364 prescribes automatic disconnection of the supply for protection against indirect contact.

2 Most Common Devices For the Automatic Disconnection Of the Supply
2 Most Common Devices For the Automatic Disconnection Of the Supply

What does it mean //

The protective device shall automatically disconnect the supply so that, in the event of a fault between a live part and an exposed-conductive-part or a protective conductor, a prospective touch voltage exceeding 50 V a.c. (25 V in special environments) does not persist for a time sufficient to cause a risk of harmful physiological effect in a person in contact with simultaneously accessible conductive parts.

This protective measure requires co-ordination between the connection to earth of the system and the characteristics of the protective conductors and devices.

This technical article deals with two most common devices suitable for the automatic disconnection of the supply and able to detect earth fault currents, and these are presented below. Note that there are few other types of devices, but they are not mentioned in this article.

  1. Automatic circuit-breakers with thermomagnetic release
  2. Automatic circuit-breakers with microprocessor-based electronic relay

Hereunder there is a description of such protective devices.

Automatic circuit-breakers //

1. With thermomagnetic release

The protections ensured by the automatic circuit-breakers equipped with thermomagnetic release are:

  • Protection against overloads;
  • Protection against short-circuits;
  • Protection against indirect contacts.
Tmax Molded Case Circuit Breaker, type T1
Tmax Molded Case Circuit Breaker, type T1

The protection against overload is provided by the thermal release with inverse time-delay curve, i.e. the higher the overload current, the faster the tripping time.

The protection against short-circuit is provided through the magnetic release with an indipendent time trip curve, i.e with disconnecting time independent from the short-circuit current.

The protection against indirect contacts can be carried out both by the thermal release as well as by the magnetic release since the earth fault current involves at least one phase; if this current is high enough, it can cause the tripping of the circuit-breaker.

It is necessary that the protective device is coordinated with the distribution system and the earthing modality of the exposed conductive-parts, so that tripping is guaranteed to occur in such times to limit the persistence of the dangerous touch voltages present in the exposed-conductive-parts further to the fault.

Figure 1 shows an example of the earth fault current path in a system with the neutral is directly earthed and the exposed-conductive-parts are connected to the same earthing arrangement of the neutral (TN system) and the trip curve of a thermal magnetic circuit-breaker type Tmax T1C160 R160.

Earth current path
Figure 1 – Earth current path

Trip curve Tmax T1C160 In160
Figure 2 – Trip curve Tmax T1C160 In160

As the diagram shows, by assuming an earth fault current of 940 A, the circuit-breaker shall trip in maximum 5s (value read on the curve with the higher tolerance).

Go back to Index ↑

2. With microprocessor-based electronic relay

The protections provided by the automatic circuit-breakers with electronic relays are completely analogous to those assured by the circuit-breakers with thermomagnetic release.

ABB Tmax Circuit Breaker T2
ABB Tmax Circuit Breaker T2

The protection functions implemented by microprocessor-based electronic relay allow protection against:

  1. Overload (protection L),
  2. Short-circuit (protection S and I) and
  3. Indirect contact to be realized.
Protection against overload (protection L), short-circuit (protection S and I) and indirect contact to be realized
Figure 3 – Protection against overload (protection L), short-circuit (protection S and I) and indirect contact to be realized

Electronic releases allow to get an accurate settings both as regards the trip times as well as the current thresholds so that the installation requirements are fully satisfied. Figure 3 shows the same example as before, but a circuit-breaker type Tmax T2 S160 PR221DS-LS/I In160 with electronic release is installed as protective device.

Earth current path
Figure 4 – Earth current path

Trip curve T2S160 PR221DS-LS/I In160
Figure 5 – Trip curve T2S160 PR221DS-LS/I In160

The possibility of setting a low magnetic threshold (at about 750 A) allows to achieve a trip time corresponding to the magnetic tripping (some tens of milliseconds), which is remarkably quicker than the time obtainable under the same conditions with a thermal magnetic circuit-breaker of the same size.

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Tmax molded case circuit breaker (VIDEO)

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Reference // Distribution systems and protection against indirect contact and earth fault – ABB

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Henry Akyea
    Jul 22, 2015

    Easy to read articles with deeper insight into electrical engineering, bless you.

  2. Abayneh L
    Jul 21, 2015

    Read & thank you for posting.

  3. Vicente Martinez
    Jul 20, 2015

    Very good article we appreciate it

  4. Vicente Martinez
    Jul 20, 2015

    Very good article

  5. Hugh Longacre
    Jul 20, 2015

    Will this work with a high resistance ground system?

    • Rolando Monreal - ABCONTROL
      Jul 22, 2015

      there’s an option called LSI-G for the electronic relay on ABB equipments, that “G” parameter is to detect any fault current to ground via a current transformer directly connected to the Power Circuit Braker.

      • Arisa Iheanyi
        Jul 22, 2015

        Thanks for posting this article

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