Electrical Installation Safety Trainer
AD 1 - MI 3399
77
It must be assured that the fault and leakage currents do not cause touch voltages
higher than conventional safety limit of 50 (25)V.
Z
LOOP
∙I
ΔN
U
C
Allowed touch voltages vary depending on the presence of water in a room/building.
Typically, 3 contact voltages are allowed:
12 V
– for hospitals, saunas.
25 V
– for rooms where there is presence of water (e.g. bathrooms).
50 V
– for rooms where there is no presence of water.
Impedances in TN systems are usually much lower than allowed (for example:
impedance of fault loop protected by RCD protection device with rated differential
current of 30 mA could be as high as 1666
, while actual values are lower than 2
).
In TT systems, fault currents supplied by line voltage flow through the fault to protection
earth conductor (PE) and then to ground via the system’s earthing resistance. The
current is driven to the grounding system of the power transformer and thus to the
neutral terminal of the transformer. Total impedance of the fault loop consists of more
serial impedances, where the major part presents global resistance of earthing system;
other impedances are negligible in comparison with this resistance. The following
condition must be fulfilled:
𝑹
𝑬
∙ 𝑰
𝜟𝑵
𝑼
𝑪
where:
R
E
– global earthing resistance in
I
N
– nominal differential current of RCD,
U
C
– limit contact voltage (50 V or 25 V).
RCD test parameters (test current shape, size) must be set correctly before the test.
Disconnection time t
N
and actual disconnection currents I
are measured. A complete
analysis of the proper operation of installed RCD includes:
disconnection times t
N
at ½, 1 and 5 I
N
contact voltage test.
Limit values are defined in appropriate standards and are usually inbuilt in measuring
equipment.
Summary of Contents for MI 3399
Page 27: ...Electrical Installation Safety Trainer AD 1 MI 3399 27...
Page 46: ...Electrical Installation Safety Trainer AD 1 MI 3399 46 3 Exit settings...
Page 74: ...Electrical Installation Safety Trainer AD 1 MI 3399 74...
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