Chapter N

Characteristics of particular sources and loads

Choice of protection schemes with UPS

From Electrical Installation Guide

The circuit-breakers have a major role in an installation but their importance often appears at the time of accidental events which are not frequent. The best sizing of UPS and the best choice of configuration can be compromised by a wrong choice of only one circuit-breaker.

Circuit-breaker selection

Figure N25 shows how to select the circuit-breakers.

Fig. N25 – Circuit-breakers are submitted to a variety of situations


The selected rating (rated current) for the circuit-breaker must be the one just above the rated current of the protected downstream cable.

Breaking capacity

The breaking capacity must be selected just above the short-circuit current that can occur at the point of installation.

Ir and Im thresholds

The table below indicates how to determine the Ir (overload; thermal or longtime) and Im (short-circuit; magnetic or short time) thresholds to ensure selectivity, depending on the upstream and downstream trip units.


(see Fig. N26)

  • Time selectivity must be implemented by qualified personnel because time delays before tripping increase the thermal stress (I2t) downstream (cables, semi-conductors, etc.). Caution is required if tripping of CB2 is delayed using the Im threshold time delay
  • Energy selectivity does not depend on the trip unit, only on the circuit-breaker
Fig. N26 – Ir and Im thresholds depending on the upstream and downstream trip units
Type of downstream circuit Ir upstream /

Ir downstream ratio

Im upstream /

Im downstream ratio

Im upstream /

Im downstream ratio

Downstream trip unit All types Magnetic Electronic
Distribution > 1.6 > 2 > 1.5
Asynchronous motor > 3 > 2 > 1.5

Special case of generator short-circuits

Figure N27 shows the reaction of a generator to a short-circuit.

To avoid any uncertainty concerning the type of excitation, we will trip at the first peak (3 to 5 In as per X”d) using the Im protection setting without a time delay.

Fig. N27 – Generator during short-circuit