Directed Electronics 3100 Betriebsanleitung PDF herunterladen (Seite 28)

l The communication via the IIC bus interface is only used

for default scaling and for service purposes.

l The repair time (exchange of the measuring system) after a

fail-safe error is eight hours (MTTR = 8 h)

l In the mode with the lowest demand rate, the reaction time

of a connected control and processing unit to dangerous

detectable errors is max. 1 hour.

If the demand rate is only once a year, then the measuring

system can be used as safety-relevant subsystem in "low

demand mode" (IEC 61508-4, 3.5.12).

If the ratio of the internal diagnostics test rate of the measuring

system to the demand rate exceeds the value 100, the

measuring system can be treated in the way it is executing a

safety function in the mode with low demand rate (IEC 61508-2,

7.4.3.2.5).

Corresponding characteristics is the value PFD

avg

(average

Probability of dangerous Failure on Demand). It is dependent

on the test interval T

Proof

between the function tests of the

protective function.

For number values see paragraph "Safety-technical charac-

teristics".

If the "low demand rate" does not apply, the measuring system

as safety-relevant part system in "high demand mode" should

be used (IEC 61508-4, 3.5.12).

The fault tolerance time of the complete system must be higher

than the sum of the reaction times or the diagnostics test

periods of all components in the safety chain.

Corresponding characteristics is the value PFH (failure rate).

For number values see paragraph "Safety-technical charac-

teristics".

A safe failure exists if the measuring system goes to the

deﬁned safe condition without being required by the process or

the interference current outputs "Fail low" or "Fail high".

If the internal diagnostics system detects an error, a fault

current of < 2.3 mA will be outputted.

A dangerous undetected failure exists if the measuring system

switches neither to the deﬁned safe condition, nor to failure

mode when the process requires it.

Low demand mode

High demand mode

Safe condition and fault de -

scription

Functional safety

28 OPTISWITCH 310 0 C - with two-wire output

29955-EN-060112

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