Siemens SENTRON - 5 Notes on Installation and Use; General Notes; Selection of Protective Devices

38
5. Notes on installation and use
5.1 General notes
5.1.1
Selection of protective devices
When selecting a suitable protective device for the protective measure „automatic
disconnection of the power supply“ in accordance with DIN VDE 0100-410 for fault
protection the conditions of disconnection depending on the supply system must
be taken into account. Table 4 summarizes the relevant characteristic variables for
the conditions of disconnection from the supply.
Characteristic Values in the TN system Values in the TT system
Impedance of the fault
loop Z
S
(measured values)
several 10mΩ to
approx. 2 Ω
up to 100 Ω
Residual
current
Approx. 115 A to
several 1,000 A
At least 2.3 A
Maximum permissible
break time t
a
according
to table 41.1 in IEC
60364-4-41 *
0.4 s 0.2 s
Touch voltage U
T
(empirical values)
80 V up to 115 V 160 V up to 230 V
Touch
current
Impedance of the human
body in case of hand-to-
foot flow of current
80 mA up to 115 mA 160 mA up to 230 mA
* For final circuits with a rated current not greater than 32 A in a 230/400V
system (50Hz)
Table 4 shows the clear differences in respect of touch voltages and the resulting
touch currents in TN and TT systems. These differences explain why the maximum
permissible break times in the TT system must be shorter than in the TN system in
order to provide the same protection.
Notes on installation and use
Table 4: Characteristic variables for the conditions of disconnection in TN
and TT systems with rated voltages of 230/400 V AC
I
F
=
230 V
Z
S
U
T
1.000 Ω
I
T
=