Beijer Electronics BFI-P2 V1.21 - Manual (Page 23)

User Guide Revision 1.121
www.beijerelectronics.com
23
4.10. Safe Torque Off
Safe Torque OFF will be referred to as “STO” through the remainder of this section.
4.10.1. Responsibilities
The overall system designer is responsible for defining the requirements of the overall “Safety Control System” within which the drive will be
incorporated; furthermore the system designer is responsible for ensuring that the complete system is risk assessed and that the “Safety control
System” requirements have been entirely met and that the function is fully verified, this must include confirmation testing of the “STO” function
before drive commissioning.
The system designer shall determine the possible risks and hazards within the system by carrying out a thorough risk and hazard analysis, the
outcome of the analysis should provide an estimate of the possible hazards, furthermore determine the risk levels and identify any needs for risk
reduction. The “STO” function should be evaluated to ensure it can sufficiently meet the risk level required.
4.10.2. What STO Provides
The purpose of the “STO“ function is to provide a method of preventing the drive from creating torque in the motor in the absence of the “STO“
input signals (Terminal 12 with respect to Terminal 13), this allows the drive to be incorporated into a complete safety control system where
“STO“ requirements need to be fulfilled.
1
The “STO“ function can typically eliminate the need for electro‐mechanical contactors with cross‐checking auxiliary contacts as per normally
required to provide safety functions.
2
The drive has the “STO“ Function built‐in as standard and complies with the definition of “Safe torque off“ as defined by IEC 61800‐5‐2:2007.
The “STO“ Function also corresponds to an uncontrolled stop in accordance with category 0 (Emergency Off), of IEC 60204‐1. This means that
the motor will coast to a stop when the “STO” function is activated, this method of stopping should be confirmed as being acceptable to the
system the motor is driving.
The “STO“ function is recognised as a fail safe method even in the case where the “STO“ signal is absent and a single fault within the drive has
occurred, the drive has been proven in respect of this by meeting the following safety standards :
SIL
(Safety Integrity Level)
PFH
D
(Probability of dangerous Failures per Hour)
SFF
(Safe failure fraction %)
Lifetime assumed
EN 61800‐5‐2 2 1.23E‐09 1/h (0.12 % of SIL 2) 50 20 Yrs
PL
(Performance level)
CCF (%)
(Common Cause Failure)
EN ISO 13849‐1 PL d 1
SILCL
EN 62061 SILCL 2
Note : The values achieved above maybe jeopardise if the drive is installed outside of the Environmental limits detailed in section 10.1
“Environmental“.
4.10.3. What STO does not provide
Disconnect and ISOLATE the drive before attempting any work on it. The “STO“ function does not prevent high voltages from being
present at the drive power terminals.
1 Note : The “STO“ function does not prevent the drive from an unexpected re‐start. As soon as the “STO“ inputs receive the relevant
signal it is possible (subject to parameter settings) to restart automatically, Based on this, the function should not be used for
carrying out short‐term non‐electrical machinery operations (such as cleaning or maintenance work).
2
Note : In some applications additional measures may be required to fulfil the systems safety function needs : the “STO“ function
does not provide motor braking. In the case where motor braking is required a time delay safety relay and/or a mechanical brake
arrangement or similar method should be adopted, consideration should be made over the required safety function when braking as
the drive braking circuit alone cannot be relied upon as a fail‐safe method.
When using permanent magnet motors and in the unlikely event of a multiple output power devices failing then the motor could
effectively rotate the motor shaft by 180/p degrees (Where p denotes number of motor pole pairs).