METREL MI 3399, Инструкция с упражнениями

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Electrical Installation Safety Trainer AD 1 - MI 3399
65
In case of a short circuit or high leakage current between phase and PE conductor a
dangerous contact voltage occurs on accessible metal parts. The voltage must stay
below 50 V (25 V for aggravating conditions) otherwise the RCD must trip out:
𝒁
𝑳−𝑷𝑬
<
𝑼
𝑪
𝑰
𝜟𝑵
=
𝟓𝟎 𝑽
𝑰
𝜟𝑵
where
U
C
– contact voltage,
Z
L-PE
– loop impedance,
I

N
– nominal trip out current of RCD.
Figure 2-15: Fault loop in TT system
2.3.1.3 Fault loop, fault impedance and prospective fault current in
RCD protected TN and TT systems
In TT and TN systems where RCD is used a different function for measuring the fault
loop impedance is used -
Z
S RCD
.
If the installation is protected with over current devices, the following condition must be
fulfilled:
I
PFC
> I
a
where:
I
PFC
– Actual prospective fault current.
I
a
– Current for rated disconnection time of the over current disconnection
device.
In this case the measured impedances are low (typically < 1.5Ω). The test current must
be small to avoid tripping out the RCD. The function introduces a small test current (<