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Diagnostics Optimization CT MODBUS RTU Technical Data
Installation and System Design Guide 123
Issue Number: 1
The Safe Torque Off (STO) function can be used to eliminate electro-
mechanical contactors, including special safety contactors, which would
otherwise be required for safety applications.
UL Approval
The Safe Torque Off function has been independently assessed by
Underwriters Laboratories (UL). The on-line certification (yellow card)
reference is: FSPC.E171230.
Safety Parameters as verified by UL:
According to IEC 61508-1 to 7
According to EN ISO 13849-1
Safe Torque Off has been designed to have a response time of greater
than 1 ms so that it is compatible with safety controllers whose outputs
are subject to a dynamic test with a pulse width not exceeding 1 ms.
When the drive is disabled through Safe Torque Off, a possible (although
highly unlikely) failure mode is for two power devices in the inverter
circuit to conduct incorrectly.
This fault cannot produce a steady rotating torque in any AC motor.
It produces no torque in a conventional induction motor with a cage
rotor. If the rotor has permanent magnets and/or saliency, then a
transient alignment torque may occur. The motor may briefly try to rotate
by up to 180° electrical, for a permanent magnet motor, or 90° electrical,
for a salient pole induction motor or reluctance motor. This possible
failure mode must be allowed for in the machine design.
With Safe Torque Off there are no single faults in the drive which can
permit the motor to be driven. Therefore it is not necessary to have a
second channel to interrupt the power connection, nor a fault detection
circuit.
It is important to note that a single short-circuit from the Safe Torque Off
input to a DC supply of > 5 V could cause the drive to be enabled. This
can be excluded under EN ISO 13849-2 by the use of protected wiring.
The wiring can be protected by either of the following methods:
• By placing the wiring in a segregated cable duct or other enclosure.
Or
• By providing the wiring with a grounded shield in a positive-logic
grounded control circuit. The shield is provided to avoid a hazard
from an electrical fault. It may be grounded by any convenient
method; no special EMC precautions are required.
Safe Torque Off over-ride
The drive does not provide any facility to over-ride the Safe Torque Off
function, for example for maintenance purposes.
Certificate of Conformity
number
Date of issue Model
44 799 13196202 2015-04-08 E300
Value
Safety Rating SIL 3
SFF > 99 %
PFH (1/h)
4.21 x 10
-11
1/h
(< 1 % of SIL 3 allowance)
HFT 1
Beta Factor 2 %
CCF Not applicable
Value
Category 4
Performance Level (PL) e
MTTF
D
2574 years
Diagnostic coverage HIGH
CCF 65
The design of safety-related control systems must only be
done by personnel with the required training and experience.
The Safe Torque Off function will only ensure the safety of a
machine if it is correctly incorporated into a complete safety
system. The system must be subject to a risk assessment to
confirm that the residual risk of an unsafe event is at an
acceptable level for the application.
Safe Torque Off inhibits the operation of the drive, this
includes inhibiting braking. If the drive is required to provide
both braking and Safe Torque Off in the same operation (e.g.
for emergency stop) then a safety timer relay or similar device
must be used to ensure that the drive is disabled a suitable
time after braking. The braking function in the drive is
provided by an electronic circuit which is not fail-safe. If
braking is a safety requirement, it must be supplemented by
an independent fail-safe braking mechanism.
Safe Torque Off does not provide electrical isolation.
The supply to the drive must be disconnected by an approved
isolation device before gaining access to power connections.
It is essential to observe the maximum permitted voltage of
5 V for a safe low (disabled) state of Safe Torque Off. The
connections to the drive must be arranged so that voltage
drops in the 0 V wiring cannot exceed this value under any
loading condition. It is strongly recommended that the Safe
Torque Off circuit be provided with a dedicated 0 V conductor
which should be connected to terminal 30 at the drive.
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