Rockwell Automation T8094 8000 Series TMR System Safety Manual - Manual (Page 58)

SAFETY MANUAL
D oc N umber T8094
I ssue 27 – June 2013
Page 37 of 103
3.9 MAINTENANCE OVERRIDES
Maintenance Overrides set inputs or outputs to a defined state that can be different
from the real state during safety operation. It is used during maintenance, usually to
override input or output conditions in order to perform a periodic test, calibration, or
repair of a module, sensor or actuator.
To correctly implement a maintenance override scheme within the TMR Controller the
override, or ‘bypass’ logic shall be programmed within the Application Program, with a
separate set of safety-related input points or variables enabling the bypass logic.
In order to accommodate maintenance overrides safely, TÜV has documented a
set of principles that shall be followed. These principles are published in the
document "Maintenance Override" by TÜV Süddeutschland / TÜV Product
Service GmbH and TÜV Rheinland.
There are two basic methods now used to check safety-related peripherals connected
to the TMR system
:
1. Special switches connected to conventional system inputs. These inputs are
used to deactivate sensors and actuators during maintenance. The
maintenance condition is handled as part of the system’s application program.
2. Sensors and actuators are electrically switched off during maintenance and are
checked manually.
In some installations, the maintenance console may be integrated with the operator
display, or maintenance may be covered by other strategies. In such installations, the
guidance given in para. 3.11.5 is to be followed. A checklist for the application of
overrides is given in para. 4.2.3.