Profisafe Safety Function Response Time (Sfrt) - Danfoss iC7 Series Operating Manual

Operating Guide | iC7-Automation Functional Safety
Sometimes, it is difficult to determine the bus transfer time that is used to calculate the watchdog time. For more information
on the cycle times, see the user guides of the specific fieldbus. The F_WD_Time can be calculated via the following
formula:
Table 27: Parts of Watchdog Time
Symbol
DAT
HAT
BT
The F-parameter F_WD_Time must have a value that is slightly greater than the sum of DAT, HAT, and 2 times the bus transfer time. It
is recommended not to exceed the calculated value by more than 30%. Setting a shorter watchdog time does not affect the safety of a
system, but it can cause a fault and make the AC drive trip.
For example, if HAT is 4 ms and the PROFINET cycle time is 4 ms, F_WD_Time should be set
to:
If there is extreme electromagnetic interference, the communication systems use retry mechanisms to increase
NOTE:
the robustness of the system. Before setting the F_WD_Time, it is recommended to find the number of retries of each
connection and adjust the minimum watchdog time if necessary.

PROFIsafe Safety Function Response Time (SFRT)

4.5.6
PROFIsafe specifies a safety function response time (SFRT), during which the safety system must react to a fault in the system. The SFRT
includes all individual delays, including the bus transfer times. All of these elements have minimum and maximum delays, and the actual
delay is likely to be somewhere in between these values. For safety reasons, every communication cycle has its own watchdog time
WDTime after which the safe state is activated if a fault occurs in that communication cycle.
i
The safety function response time is calculated using the following formula:
Table 28: Components in the Safety Function Response Time Calculation
Abbreviation
SFRT
WCDT
i
WDTime
i
Adding the worst case delay times to the components of the safety system gives the total worst case delay time, as stated in
Table 29: Time Parameters
Device
The complete AC drive system
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Name
Device Acknowledgment Time
Host Acknowledgment Time
Bus Cycle Time
Definition
Safety Function Response Time
Worst Case Delay Time of entity i
Watchdog Time of entity i. See
Worst case delay time
120 ms
Functional Safety for Frequency Converters, Upgradable (+BEF2)
Description
60 ms for the complete AC drive system.
Application-specific.
The bus cycle time.
4.5.5 PROFIsafe Watchdog
Watchdog time
Recommended 250 ms or more
Time.
Table
AQ319741840653en-000401 / 136R0268
29.