Rockwell Automation T80008 User Manual

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AN-T80008

Application Note

MCB Discrimination

In a radial feeder layout, as shown in Figure 1 overleaf, the purpose of discrimination is to disconnect
only the faulty load or feeder from the network and no others, thus ensuring maximum continuity of
service. If discrimination studies are not or are incorrectly carried out, an electrical fault may cause
several protective devices to trip, thus provoking an interruption in the supply of power to a large part of
the network. That constitutes an abnormal loss in the availability of electrical power for those parts of
the network where no fault occurred.

Issue Record

Issue
Number

1 Jun 06 Julia Bourn Initial Issue

2 Feb 07 Nick Owens Ray Brown Pete Stock Format

Issue 2 Feb 07 AN-T80008 1

Date Revised by Technical

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Authorised by Modification

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AN-T80008 MCB Discrimination

Figure 1 Several circuit-breakers are concerned by the fault I

Several types of over currents may be encountered in an installation:

• overloads, short-circuits, inrush currents, as well as:

• earth faults, transient currents due to voltage dips or momentary loss of supply.

The MCB characteristic curves must be checked against the power supply overload graph to ensure
that the MCB trips before the power supply in an overload situation.

f

Issue 2 Feb 07 AN-T80008 2