Fail-Safe CPU and signal module (SM) installation
4.2 Fail-Safe system electrical design rules
S7-1200 Functional Safety Manual
90 Manual, 02/2015, A5E03470344-AA
Fail-Safe system electrical design rules
4.2.1
Safe functional extra low voltage requirement (power supplies and other system
components)
Fail-safe modules must be operated with safe functional extra-low voltage (SELV, PELV)
power sources.
To maintain safe S7-1200 low voltage circuits, external connections to communications
ports, analog circuits, 24 VDC nominal power supplies, and I/O circuits must be powered
from approved sources that meet the requirements of SELV, PELV, Class 2, Limited
Voltage, or Limited Power according to various standards.
The external power supply should limit the maximum voltage to 35 VDC even under fault
conditions. Please consult the manufacturer's data for the power supplies that you use.
Maximum applied voltage to S7-1200 Fail-Safe devices
● Operational voltage: The fail-safe CPU and fail-safe signal modules operational voltage is
20.4 VDC - 28.8 VDC, 35 VDC surge for 0.5 second. Operation of the unit to
specifications is assured by design and testing. Defined transients from defined source
impedances per EN 61000-4-2, 61000-4-4, 61000-4-6, as specified in the data sheet for
each product, may be imposed on this voltage without disrupting operation or causing
damage. Sustained operation in the range of 28.8 - 35 VDC can result in unacceptable
temperature rise and thermal damage, causing the product to become inoperable.
● Absolute maximum rating regarding supply voltage: The absolute maximum rating to
prevent module damage and to ensure the functional safety of the modules is 35 VDC.
Only use SELV / PELV power supplies, connected directly or through field devices, with
the fail-safe CPU and fail-safe signal modules. These power supplies must be specified
by the manufacturer to limit the output voltage to 35 VDC or less under fault conditions.
Otherwise, external protection must be supplied that will reliably open the circuit or limit
the output voltage to less than 35 VDC to the CPU and signal modules.
● Surge immunity: Wiring systems subject to surges from lightning strike coupling must be
equipped with external protection. This protection must be sufficient to clamp surge
voltages and/or open the supply circuit to assure the PLC system is not exposed to
voltages greater than 35 VDC. One specification for evaluation of protection from
lightning type surges is found in EN 61000-4-5, with operational limits established by
EN 61000-6-2. S7-1200 DC fail-safe CPUs and fail-safe signal modules require external
protection to maintain safe operation when subject to surge voltages defined by this
standard. Refer to Appendix A.1: "General technical specifications (Page 132)", "Surge
immunity" for further information.
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