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3.1.2 Selectivity
The system where the UPS is inserted must be set up in such a way that in the event of a shortcircuit on one of the
lines downstream of the UPS, the fuse on the output opens while the upstream fuse normally works. This is known
as selectivity, and ensures that the
remaining feeders maintain the
power supply.
In order to select the correct
protection devices to be inserted
downstream of the UPS, the
following two operating modes have
to be taken into consideration:
mains power supply and battery
power supply.
With mains power supply the output fuse must be selective with the input fuse; the condition is verified for the
following values:
At least two feeders are necessary in order to use the UPS at rated load with fuses of type gG.
With battery power supply ( first fault) in the event of a shortcircuit on one of the outputs (second fault) it must be
possible for the fuse to be open before the inverter shuts down.
If the shortcircuit is of the three-phase type, the inverter can supply a current for 1 second that is 1.8 times the value
of the rated current of the UPS output (with a single phase shortcircuit the current is around 3 times the value).
In the worst-case scenario, that is a three-phase shortcircuit then a smaller current, the condition occurs if:
At least three or four feeders are necessary in order to use the UPS at rated load with fuses of type gG.
To sum up, when there is a shortcircuit on the output, there are two alternatives if only the line affected by the
shortcircuit is to be isolated; let us consider the example of the 20kVA:
selectivity with both mains power supply and battery power supply;
the load has to be shared between at least three feeders, each sized at 33% of the rated power.
selectivity, with mains power supply only:
since it is considered unlikely that after a first fault a second fault will occur in the limited time of
operation from the battery, it is sufficient to share the output between two lines, each sized for 50% of the
rated power.
H
+/-
LOAD
UPS
SWBY
SWIN
SWOUT
+/-
BATTERY CABINET
kVA 10 15 20 30 40 60 80
rated output current: [A] 14,5 22 29 43 59 87 116
type gG fuse used at the bypass input
(as shown in the table of connections) [A]
20 25 32 50 80 100 125
maximum fuse at the UPS output for selectivity:
fuse size for type gG fuse [A] 10 16 20 32 50 63 80
fuse size for type aM fuse [A] 6 8 10 16 25 36 40
kVA 10 15 20 30 40 60 80
rated output current: [A] 14,5 22 29 43 59 87 116
shortcircuit current (three-phase) 1.8 times the rated output current for 1 second
maximum fuse at the UPS output for selectivity:
fuse size for type gG fuse [A] 6 10 12 16 20 25 32
fuse size for type aM fuse [A] 2 4 4 6 10 12 20
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