NORD Drivesystems BU0530 - Manual (Page 28)

Functional safety with NORD SK 500E frequency inverters
28 BU 0530 GB-3712
2.3.5 Ruling out wiring faults
In the examples above, a separate shielded cable, whose shield is connected at both ends is used each of the
inputs used for the implementation of the safety function. This measures serves to rule out faults as per DIN
EN ISO 13849-1 in case of a short circuit between any of the conductors. This exclusion of faults is necessary
in order to fulfil the requirements of Safety Category 4 as per DIN EN ISO 13849-1. According to this, a single
detected fault or an accumulation of undetected faults must not lead to the loss of the safety function. A short
circuit from an external voltage, e.g. from a 24V control cable, to the 24V input of a safe shut-down method
could lead to the loss of a safety function. I.e. this fault must be prevented by means of suitable measures.
The use of a separate shielded cable for each input is not mandatory. For example, the cables for the digital
inputs and the safe shut-down method may be jointly laid in a shielded cable, if the monitored safety outputs of
the switching device is equipped with short-circuit detection (See Fig.). If necessary, the effectiveness of the
short-circuit detection must be demonstrated.
Other measures (separate cable duct, installation in armoured conduit) are possible. More precise details
result from the risk assessment and the FMEA for the specific application.
For the "safe pulse block" it is also conceivable that this is triggered via two safety outputs, one switching the
24V output and the other switching the GND output.
In this case a shielded cable is not strictly necessary if both safety outputs are monitored. If, for example, other
24V control cables are laid in the same cable duct and an error in the form of a short-circuit from 24V_SH to a
control cable (=24V) is assumed, this fault would be detected by the output monitoring of the switching device
and the "safe pulse block" would be triggered by the second safety output. More precise details result from the
FMEA for the specific application.
If a shielded cable is not used for the wiring of the safety function, the effects of electromagnetic fields may
need to be taken into account. Hence the use of a 1 m long cable (in a separate cable duct) in an environment
without strong electromagnetic fields is relatively safe, while the installation of a long cable in the direct vicinity
of a powerful transmitter or a medium voltage distributor may cause the failure of the safety function. Because
of this, the use of shielded cables is generally recommended.
GND SH
24V SH
88
89
24V
Frequency inverter
Safety output 1
Safety output 2
Monitored outputs
DIN1
GND
21
40
24V
optional
Safety output 1
Filter
for
OSSD
24V SH
GND SH
89
88 24V
Safety output 2
Monitored outputs
without short-circuit detection
Frequency inverter
DIN1
GND
21
40
24V
optional
Safety output 1
Filter
for
OSSD
24V SH
GND SH
89
88 24V
Safety output 2
Monitored outputs
without short-circuit
detection
Frequency inverter