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MCBs technical details
Particular supply sources and loads

Fluorescent lamps. 230 VAC three-phase – Delta connection
Type of lamp Tube diss. pwr. [W] Number of lamps per phase
Single without 18 2 5 8 16 28 45 56 70 90 113 141 178 226 283
capacitors 36 1 2 4 8 14 22 28 35 45 56 70 89 113 141
58 0 1 2 5 8 14 17 21 28 35 43 55 70 87
Single with 18 4 8 12 24 40 64 81 101 127 162 203 255 324 406
capacitors 36 2 4 6 12 20 32 40 50 64 81 101 127 162 203
58 1 2 3 7 12 20 25 31 40 50 63 79 100 126
Double with 2x18=36 2 4 6 12 20 32 40 50 64 81 101 127 162 203
capacitors 2x36=72 1 2 3 6 10 16 20 25 32 40 50 63 81 101
2x58=116 0 1 1 3 6 10 12 15 20 25 31 39 50 63
In [A] - 3P circuit-break. 1 2 3 6 10 16 20 25 32 40 50 63 80 100

Transformer protection Protection on the secondary side
Insertion current Due to the transformer’s high insertion current, the
When the LV/LV transformers are powered up, very strong circuit-breaker on the primary winding may not guarantee
currents occur, which must be considered when selecting thermal protection for the transformer and its feeder line
the protective device. The peak value of the first current on the primary side.
wave often reaches a value between 10 and 15 times the
transformer’s effective rated current. This is typical of modular circuit-breakers which must have
a higher rated current than the transformers. In such cases,
For power ratings below 50 kVA, it may reach between in the event of a single-phase short-circuit at the
20 and 25 times the rated current. This transient current transformer’s primary terminals (minimum Icc at end of line),
decreases very rapidly with a time constant T varying check that the circuit-breaker’s magnetic releaser is tripped.
from several ms to 10, 20 ms. In the normal application in distribution panels,
this condition is always satisfied provided that the length
Main protection on the primary side of the feeder lines is reduced.
The tables below are the result of a set of tests on
co-ordination between circuit-breakers and BT/BT The transformer can be provided with thermal protection
transformers. The transformers used in the tests are by installing a circuit-breaker with a rated current less than
normalized. The table, referring to a primary supply or equal to that of the transformer secondary winding
voltage of 230 or 400 V and to single-phase and immediately downstream of the LV/LV transformer.
three-phase transformers, indicate which circuit-breaker
should be used according to the transformer power rating. In lighting systems protection against overloads is not
The transformers considered have the primary winding necessary if the number of light points is clearly defined
outside the secondary winding. (no overloads).
The circuit-breakers suggested allow:
• transformer protection in the event of maximum Moreover, the Standard in force for these systems
short-circuit; recommends the omission of protection against overloads
• prevention of unwanted tripping when the primary in circuits in which unwanted tripping may prove hazardous,
winding is powered up using e.g.: circuits which supply fire-fighting equipment.
1. modular circuit-breakers with a high magnetic threshold,
curve D or K
• 2. circuit-breakers with magnetic only releaser;
• guaranteed circuit-breaker electrical life.