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TM
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
Check
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
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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.e. 750W.)
P
ower Unit(s) Overload Current Vs Time
300
250
200
1
50
Time / ms
100
50
0
0 20 40 60 80 100 120 140
Current / Amps
1 Power Unit
2
Power Units
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 be
seen that for a period of just in excess of 200ms the power unit can provide currents up to 115 amps or
380% of its stated maximum(115/31 = 3.8). This 200ms/380% can be extrapolated for additional
power units in order to perform MCB discrimination studies.
Issue 2 Feb 07 AN-T80008 3
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AN-T80008 MCB Discrimination
Example analysis
10A
10A
ower unit
P
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. At 40A the power pack goes into
protective shutdown after 700mS and hence will shutdown before the branch MCB has opened. A
larger overload will result in protective shutdown after 200ms (see Figure 2)
Figure 3 Characteristics of ETA Thermal-magnetic circuit breaker 2210-S2 (fast acting)
Issue 2 Feb 07 AN-T80008 4
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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.
Issue 2 Feb 07 AN-T80008 5
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