Notes On Installation And Use; General Notes; Selection Of Protective Devices - Siemens SENTRON Manual

Notes on installation and use

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
Fault loop impedance Z
(measured values)
Residual 230 V
I =
F
current Z
Maximum permissible
break time t according
a
to table 41.1 in
DIN VDE 0100-410 *
Touch voltage U
T
(empirical values)
Touch
I =
T
current 1,000 Ω
Impedance of the human
body for hand-to-foot flow
of current (guide value)
* For final circuits with a rated current not greater than 32 A in a 230/400 V
system (50 Hz)
Table 4: Characteristic variables for the conditions of disconnection in TN and
TT systems with rated voltages of 230/400 V AC
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.
44
Values in the TN system Values in TT system
Several 10 mΩ to
S
approx. 2 Ω
Approx. 115 A to
several 1,000 A
S
0.4 s
80 V up to 115 V
U
T
80 mA up to 115 mA
Up to 100 Ω
At least 2.3 A
0.2 s
160 V up to 230 V
160 mA up to 230 mA