TC · Edition 08.17 53
Glossary
10 Glossary
10.1 Tightness control
The term “tightness control” is the product name of
product group TC from Elster GmbH. Tightness control
TC is a valve proving system (VPS).
10.2 Valve proving system VPS
System to check that automatic shut-off valves have
closed effectively by detecting leakage. This system is
frequently comprised of a program unit, a measuring
device, valves and other functional equipment. Valve
proving systems for gas burners and gas appliances
pursuant to DIN EN 1643 determine whether an auto-
matic shut-off valve has closed on the basis of the de-
tected leakage rate.
see EN 1643
10.3 Safety interlocks
Linking of all the relevant safety control and switching
equipment for the use of the application. The burner
start enable signal is issued via the safety interlock out-
put (terminal 6).
10.4 Diagnostic coverage DC
Measure of the effectiveness of diagnostics, which may
be determined as the ratio between the failure rate of
detected dangerous failures and the failure rate of total
dangerous failures
NOTE: Diagnostic coverage can exist for the whole or
parts of a safety-related system. For example, diagnos-
tic coverage could exist for sensors and/or logic system
and/or final elements. Unit: %
see EN ISO 13849-1
10.5 Operating mode
High demand mode or continuous mode
Operating mode, where the frequency of demands for
operation made on a safety-related system is greater
than one per year or greater than twice the proof-test
frequency
see EN 61508-4
10.6 Hardware fault tolerance HFT
A hardware fault tolerance of N means that N + 1 is the
minimum number of faults that could cause a loss of
the safety function
see IEC 61508-2
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