METREL MI 3399, Инструкция с упражнениями

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Electrical Installation Safety Trainer AD 1 - MI 3399
50
Figure 2-5: Short circuit current between Line and Neutral
2.2.2.2 Short circuit line
– line impedance and prospective short
circuit current
The line-line short circuit loop consists of:
Z
T
– power transformer secondary impedance,
Z
Lx
– first phase wiring from source to fault,
Z
LY
– second phase wiring from source to fault.
The line-line impedance is the sum of impedances and resistances that forms the line-
line loop. In a three-phase system there are three line-line impedances (Z
L1-L2
, Z
L1-L3
,
and Z
L2-L3
).
Z
LINE
= Z
Lx-Ly
= Z
Lx
+ Z
Ly
+ Z
TLL
The prospective short circuit current I
PSC
is defined as:
𝑰
𝑷𝑺𝑪
=
𝑼
𝑳𝒙𝑳𝒚
𝒁
𝑳𝒙𝑳𝒚
=
𝑼
𝑳𝑵
∙ √𝟑
𝒁
𝑳𝒙𝑳𝒚
> 𝑰
𝒂
I
PSC
must be higher than I
a
(current for rated disconnection time) of the over current
disconnection device.
The line
– line impedance should be low enough e.g. prospective short circuit current
high enough that installed protection device will disconnect the short circuit loop within
the prescribed time interval. Limit current and impedance depend on selected fuse
type, size and required trip out time.