Siemens SIMATIC S7-400 Reference Manual page 19

The battery/power supply that powers the non-incendive connection must comply with the
following ratings:
Cc = Cable capacity
Lc = Cable inductance
Note
The concept of non-incendive field wiring allows the interconnection of devices with non-
incendive field wiring using any wiring method approved for unclassified sites.
If you do not know the capacity and inductance of the cable, you can use the following values:
Cc = 197 pF/m (60 pF/ft.), Lc = 0.66 mH/m (0.2 mH/ft)
Marine Approvals
Classification societies:
● ABS (American Bureau of Shipping)
● BV (Bureau Veritas)
● DNV (Det Norske Veritas)
● GL (Germanischer Lloyd)
● LRS (Lloyds Register of Shipping)
● Class NK (Nippon Kaiji Kyokai)
Safety requirements for installation
The S7-400 automation system is "open-type equipment" in accordance with standard
IEC 61131-2 and therefore EU Directive 2014/35/EU (Low Voltage Directive), and "open type"
in accordance with UL/CSA certification.
To fulfill requirements for safe operation with regard to mechanical stability, flame retardation,
stability, and shock-hazard protection, the following alternative types of installation are
specified:
● Installation in a suitable cabinet
● Installation in a suitable housing
● Installation in a suitably equipped, enclosed operating area.
S7-400 Automation System Module Data
Reference Manual, Ausgabe 11/2016, A5E00850736-08
Battery/power supply
Voc
Isc
Ca
La
1.1 Standards, certificates and approvals
CPU input "Ext. Batt." with cable
≤ Vmax (15 V)
≤ Imax (50 mA)
≥ Ci + Cc (25 nF + Cc)
≥ Li + Lc (2 mH + Lc)
General specifications
19