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ManualsBrandsRockwell Automation ManualsEquipmentT80008 Application Note MCB Discrimination

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AN-T80008

Issue 2 Feb 07 AN-T80008 1

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

Date Revised by Technical

Check

Authorised by Modification

1 Jun 06 Julia Bourn Initial Issue

2 Feb 07 Nick Owens Ray Brown Pete Stock Format

Summary of content (6 pages)

PAGE 1
TrustedTM 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.
PAGE 2
TrustedTM AN-T80008 MCB Discrimination Figure 1 Several circuit-breakers are concerned by the fault If 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.
PAGE 3
TrustedTM AN-T80008 MCB Discrimination Trusted Power supplies T824X The trend for increasing power in an ever decreasing package requires a higher level of sophistication of the internal electronics and self protection circuitry. Improved self protection however, means that an MCB discrimination study becomes an important part of the power distribution design. Tests carried out on the power units under severe overload situations produced the following results. (Maximum rating is 31 amps per unit at 24V, i.
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TrustedTM AN-T80008 MCB Discrimination Example analysis 10A 10A Power unit 10A If the circuit breakers used were ETA 2210-S2 (fast acting) 10A (see Figure 3) then at two times the MCB rating i.e. 20A, it would take 10 seconds to trip the MCB. If one of the MCBs in the example above were subjected to an overload, resulting in a load of 20A in one branch, then the power pack total load is increased to 40A.
PAGE 5
TrustedTM AN-T80008 MCB Discrimination Conclusion The internal self-protection of T824x power units may cause them to trip before the MCBs. Ensure that MCBs and other circuit protection devices are rated to ensure that any one short will not overload the supply. Low current ratings are therefore recommended, to ensure that the protection device trips first, due to its time/current curve.
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