CB-110-426
The system designer shall determine the possible risks
and hazards within the system by carrying out a thor-
ough risk and hazard analysis, the outcome of the ana-
lysis should provide an estimate of the possible haz-
ards, 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.
What STO provides
The purpose of the STO function is to provide a method
of preventing the FI from creating torque in the motor in
the absence of the STO input signals (Terminal 12 with
respect to Terminal 13), this allows the FI to be incor-
porated into a complete safety control system where
STO requirements need to be fulfilled.
WARNING
The STO function does not prevent the FI from
an unexpected re-start. As soon as the STO in-
put receives the relevant signal it is possible 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).
The STO function can typically eliminate the need for
electro-mechanical contactors with cross-checking aux-
iliary contacts as per normally required to provide
safety functions.
The FI 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 failsafe method
even in the case where the STO signal is absent and a
single fault within the FI has occurred, the FI has been
proven in respect of this by meeting the following safety
standards:
EN 61800-5-2
SIL (Safety Integrity Level) 2
PFHD (Probability of danger-
ous Failures per Hour)
1.23E-09 1/h
(0.12% of SIL 2)
SFF (Safe failure fraction %) 50
Lifetime assumed 20 years
EN ISO 13849-1
PL (Performance Level) PL d
CCF (%) (Common Cause
Failure)
1
MTTFd 4525a
Category 3
EN 62061
SILCL SILCL 2
The values achieved above maybe jeopardised if the FI
is installed outside of the environmental limits.
WARNING
Disconnect and isolate the FI before attempting
any work on it. The STO function does not pre-
vent high voltages from being present at the FI
power terminals.
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).
STO operation
When the STO input is energised, the STO function is
in a standby state, if the FI is then given a “Start signal/
command” then the FI will start and operate.
When the STO input is de-energised then the STO
function is activated and stops the FI (Motor will coast),
the FI is now in “Safe Torque Off” mode.
STO status and monitoring
FI display:
In Normal FI operation (Mains AC power applied, com-
pressor not running), when the FI's STO input is de-en-
ergised (STO Function activated) the FI will highlight
this by displaying “INHIBIT". If the FI is in a tripped con-
dition then the relevant trip will be displayed and not
“INHIBIT”. If the STO input is energised and the display
still shows "INHIBIT", there is a fault at the STO input.
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