METREL MI 3399, Инструкция с упражнениями

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Electrical Installation Safety Trainer AD 1 - MI 3399
41
- Within allowed limits and all outlets produce the same result , then we may
conclude that TN earthing system is being tested.

Loop impedance is much higher than Line impedance (Z
LOOP
>> Z
LINE
):
- If such impedances are present on all measured outlets, then we may conclude
that TT earthing system is being tested.
- If TT earthing system is not present, then there is an error on the PE conductor.

Loop impedance is much lower than Line impedance (Z
LOOP
<< Z
LINE
):
- There is an error on the Neutral conductor.
Maximum allowed Line and Fault loop impedances to ensure the trip-out ability with built-
in fuses type B, C or gL-gG in the installation with nominal mains voltage
U
L-N
= 230 V are
presented in the example table below.
Nominal current of
over-current
protection device (A) Type of automatic
Fuse B Type of automatic
Fuse C Type of
Fuse gG
Ia=5·In
(A)
Zs (
Ω
)
(0,2s)
Ia=5·In
(A)
Zs (
Ω
)
(0,2s)
Ia (A) Zs (
Ω
)
(0,4s)
2 10 22 20 11 16 13,7
4 20 11 40 5,5 32 6,8
6 30 7,3 60 3,65 47 4,6
10 50 4,4 100 2,2 82 2,6
16 80 2,8 160 1,4 110 2,0
20 100 2,2 200 1,1 147 1,4
Table 1-1: Maximum allowed fault loop impedances in case of used melting fuses
type B, C and gG
3
Fuse trip-out times depend on the characteristics of a built-in fuse or RCD.
Circuit type Trip-out time
Explosive 100 ms
3-phase 200 ms
1-phase 400 ms
Fixed connection 5 s
Table 1-2: Fuse trip-out times
Where an installation is protected with a RCD, special measuring techniques are used
to avoid tripping out the RCD during the test.
1.4.4 RCD
Installation of RCDs is:

Mandatory in TT systems.

Optional in TN systems, though European countries today typically install RDCs
also on these types of systems.
3
See Metrel’s Handbook
Guide for testing and verification of low voltage installations for more
parameters/limits.