Rockwell Automation T80008 Application Note MCB Discrimination - Manual (Page 3)

Trusted TM
AN-T80008 MCB Discrimination
Issue 2 Feb 07 AN-T80008 3
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.)
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.
Power Unit(s) Overload Current Vs Time
0
50
100
150
200
250
300
0 20 40 60 80 100 120 140
Current / Amps
T
im
e
/
m
s
1 Power Unit
2 Power Units