Siemens SINAMICS S120 Combi Equipment Manual page 31

As a general rule, the higher loop impedance of TT line supplies means they are not
suitable for tripping the installed overcurrent protection devices within the prescribed period
should an insulation fault occur. If TT line supplies are used, residual-current-operated
circuit breakers (refer to the chapter titled "Residual-current-operated circuit breakers
(RCD)") should ideally be used in addition to the overcurrent protection devices.
Selecting the overcurrent protection devices
In order to avoid the risk of fire or electric shock, overcurrent protection devices should be
dimensioned so that in the case of a fault, the equipment is switched off sufficiently quickly.
At the place of installation, measurements should be performed to check whether a suitably
high short-circuit current flows to trip the protective devices quickly enough.
To do this, the loop impedance (faulted-circuit impedance) should be measured and the
prospective short-circuit current calculated as well as the time-current characteristics of the
overcurrent protection devices calibrated using measuring instruments according to
EN 61557-3.
If the necessary break times are not maintained, then the next smaller overcurrent
protection device must be used.
It is not permissible to overdimension fuses as this can result in significant levels of danger
and also faults.
Note
The devices can be connected to line supplies up to 480 V
of 65 kA symmetrical ("prospective current" according to EN 60269-1).
SINAMICS S120 Combi
Manual, Edition 05.2010, 6SL3097-4AV00-0BP0
2.3 Overcurrent protection by means of line fuses and circuit breakers
DANGER
WARNING
WARNING
Line-side power components
, which can supply a maximum
AC
31