Electrical Installation Safety Trainer
AD 1 - MI 3399
112
𝑅
𝐿𝑂𝑂𝑃
= 𝑅
𝐿𝐻
+ 𝑅
𝐸𝐻
+ 𝑅
𝐸𝐷
+ 𝑅
𝑇
[Ω]
If the total earth resistance of R
EH
is higher than the resistance R
ED
and the return path
(resistance of L conductors, secondary of power transformer) then the result can be
considered as
R
EH
.
This test is applicable:
Applicable for TT systems, where the measured installation’s earthing resistance is
higher than the (well grounded) auxiliary one.
Applicable in urban areas if there is no appropriate place for test probes.
Applicable in areas where different local earthings are connected together, making
the local earthing system very large.
Figure 2-51: Two-wire test (only for TT systems), no probes
2.8.1.4 3-wire earthing resistance test, internal generator, two probes
The three-wire test is the standard earthing resistance test method. It is the only choice if
there is no well earthed auxiliary terminal available. The measurement is performed with
two earthing probes.
The drawback if using three wires is that the contact resistance of E terminal is added to
the result.
Содержание MI 3399
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