discrimination curve

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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 disconnectonly the faulty load or feeder from the network and no others, thus ensuring maximum continuity ofservice. If discrimination studies are not or are incorrectly carried out, an electrical fault may causeseveral protective devices to trip, thus provoking an interruption in the supply of power to a large part ofthe 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


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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 ensurethat the MCB trips before the power supply in an overload situation.


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Trusted Power supplies T824X

The trend for increasing power in an ever decreasing package requires a higher level of sophisticationof the internal electronics and self protection circuitry. Improved self protection however, means that anMCB 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.e. 750W.)

Figure 2 Power unit(s) severe overload Current v Time

The graph above shows the response of a single power unit to severe overloads. As the load is

increased, the power supply protection mechanism operates and shuts down the output. It can beseen that for a period of just in excess of 200ms the power unit can provide currents up to 115 amps or380% of its stated maximum(115/31 = 3.8). This 200ms/380% can be extrapolated for additionalpower units in order to perform MCB discrimination studies.

Power Unit(s) Overload Current Vs Time

0

50

100

150

200

250

300

0 20 40 60 80 100 120 140

Current / Amps

Time/ms

1 Power Unit

2 Power Units


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Example analysis

If the circuit breakers used were ETA 2210-S2 (fast acting) 10A (see Figure 3)then at two times theMCB 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 inone branch, then the power pack total load is increased to 40A. At 40A the power pack goes intoprotective shutdown after 700mS and hence will shutdown before the branch MCB has opened. Alarger overload will result in protective shutdown after 200ms (see Figure 2)

Figure 3 Characteristics of ETA Thermal-magnetic circuit breaker 2210-S2 (fast acting)

Power unit

10A

10A

10A


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Conclusion

The internal self-protection of T824x power units may cause them to trip before the MCBs. Ensure thatMCBs and other circuit protection devices are rated to ensure that any one short will not overload thesupply. Low current ratings are therefore recommended, to ensure that the protection device trips first,due to its time/current curve.


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