Choice of protection schemes with UPS: Difference between revisions
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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. | 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. | ||
| Line 21: | Line 18: | ||
== Ir and Im thresholds == | == 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 | 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. | ||
=== Remark === | === Remark === | ||
(see {{FigRef|N26}}) | (see {{FigRef|N26}}) | ||
*Time | *Time selectivity must be implemented by qualified personnel because time delays before tripping increase the thermal stress (I<sup>2</sup>t) downstream (cables, semi-conductors, etc.). Caution is required if tripping of CB2 is delayed using the Im threshold time delay | ||
*Energy | *Energy selectivity does not depend on the trip unit, only on the circuit-breaker | ||
{{ | {{tb-start|id=Tab1364|num=N26|title=Ir and Im thresholds depending on the upstream and downstream trip units|cols=3}} | ||
{| class="wikitable" | |||
|- | |- | ||
! Type of downstream circuit | ! Type of downstream circuit | ||
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| > 2 | | > 2 | ||
| > 1.5 | | > 1.5 | ||
| | |} | ||
== Special case of generator short-circuits == | == Special case of generator short-circuits == | ||
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{{FigImage|DB422661_EN|svg|N27|Generator during short-circuit}} | {{FigImage|DB422661_EN|svg|N27|Generator during short-circuit}} | ||
Latest revision as of 09:48, 22 June 2022
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.
Rating
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.
Remark
(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
| 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.
