Basic safety instructions
TR-Electronic GmbH 2019, All Rights Reserved
Printed in the Federal Republic of Germany
Page 14 of 29
TR - ECE - BA - GB - 0142 - 01
06/12/2019
Device type 3 refers to devices with a failure behavior which is dependent on the switching frequency
(cycle) and in terms of the draw-wire corresponds to complete extension and retraction of the wire.
This fact is expressed by the
B10d-value
and represents the average number of cycles by which
10 % of the components have failed dangerously. The draw-wire box has not been developed in
accordance with any specific safety standard, but this does not in principle exclude its use in
accordance with DIN EN 61508, EN ISO 13849-1 or IEC 62061.
Generally, however, the use of such devices, if they are used as a safety-relevant part of a control
system, must be independently assessed by the user in relation to safety.
As the combination of measuring system and draw-wire box is a series connection, this "unified
structure" must be reassessed in relation to safety. The component with the lowest reliability in the
series connection is determining for the highest possible level of safety that can be achieved. For
mechanical reasons draw-wires only have a limited number of cycles, which in turn is strongly
dependent on the type used.
In practice this means that the draw-wire is the limiting component in the series connection
and the safety requirement level of the measuring system can never be achieved for the unified
structure. For this reason there is also no TÜV certification for the combination of measuring
system and draw-wire box!
This fact means that the unified structure may only be used as the subsystem of a safety
function if the safety requirement level of the unified structure corresponds to the required
safety requirement level for the subsystem.
To enable the user to assess the safety function, TR-Electronic provides the relevant safety indicators
in the product data sheets valid for the respective measuring system,
see
www.tr-electronic.com/s/S020955
TR-Electronic can provide the relevant B10d value for the draw-wire box on request.
d
value for the draw-wire box can be calculated as follows:
MTTF
d
=
B10
d
0.1 * n
op
The total MTTF
d
value for the draw-wire box + measuring system can be calculated from this:
MTTF
d
(total)
=
( MTTF
d (draw-wire box)
* MTTF
d (measuring system)
)
= value in years [a]
( MTTF
d (draw-wire box)
+ MTTF
d (Mess-System)
)
1)
Abbreviations, also see page 8
