fail safe - dark test

Hi

I'm curious about the effects of the dark test on fail safe outputs.

The document "44452714_bit_pattern_test_en" states - "If you use actuators whose reaction time is fast enough to react to the test pulses, you must switch relays in between so that your actuators are not briefly disabled during the dark test."

However I'm a little unsure about real world implementations of this. Does anyone have experience with this that could provide an example of how they solved the problem with actual (or a description of suitable) products that can be used?

I'm not aware of products that advertise themselves as suitable for use with Siemens fail safe dark test. Maybe Siemens make something but I'm not aware of it. I guess Sirius 3rt might be fine but would like to be sure and have more options. 

Thanks

edit** maybe some advice on how these implementation details affect sil ratings too?

Hello newbiplc;

First point: the "dark" test is not a Siemens development or Safety applications. As you can see from the various links below, especially the last, it is a requirement from IEC 60204-1 Safety of machinery standard on the availability of F-DOs during process control:

VersaSafe - Test Pulses of Safe Output Modules - GE Customer Center

Safety Programmable Controller/Safety Controller ... - Mitsubishi Electric

Integrated Safety - Rockwell Automation

Standard EN 60204-1 - Safety of machinery - User Interface

Basically, and as the document you point to explains quite well, the dark (or light) tests consist of regular, (Siemens uses a period of 15 minutes initially) very short pulses sent to the F-DOs during operation. The response of the F_DO is monitored and evaluated; if the "0" signal (for dark test) is detected, then the F-DO is considered functional, and no further step is taken. If it is not detected within a specified interval, then the test is repeated at 1 minute intervals, and then the module may be passivated, or the output relay de-activated to preserve the Safety function.

Now understand that you want these test pulses to affect the monitoring circuit only, not the actual output of the F-DO. If your actuator is too sensitive and is affected by the test pulse, then your program might not function as you wish, and therefore put the personnel at risk..

In our Safety applications, I do not remember having that sort of problem affect our actuators, their response time was generally very tolerant of the short test pulses sent to the F-DO. 

Actually you will find that the Sirius 3SK2 relays (for one) are designed to handle the F-DO dark test, and the 3RT2 relay must connect through a resistor to a 3ST2 to preserve the dark test function;:

- Manual - SIRIUS 3SK2 Safety Relays

https://support.industry.siemens.com/cs/ww/en/view/109444336

- Confirmation regarding connection of 3RT2 contactors and contactor assemblies to 3RK3 MSS, 3SK1 and 3SK2 safety relays.

https://support.industry.siemens.com/cs/ww/en/view/109481229

When connecting a 3RT203.-. NB3.-.... and 3RT203.-.KB4.-.... to a 3SK1 Advanced 3SK1122-.CB4. relay with adjustable OFF-delay time, an additional 1.2 kΩresistor must be connected in parallel to A1-A2 on the 3RT20.

Reason:
A capacitor is built into the contactor coils of 3RT203.-. NB3.-.... and 3RT203.-.KB4.-.... units.
The capacitor shapes the pulses to the contactor coil and hampers the dark test for electronic enabling circuits.
For this reason, an external resistor is necessary in some cases.

Hope this helps,

Daniel Chartier

Thanks. That was definitely a helpful reply. The solution in the first link is very useful. 

Still this is a topic I should investigate more. You say that the dark test isn't specifically a siemens thing but it seems to me that each vendors implemention may vary slightly which may be important.

I've just started thinking about this topic so I'm sure a generic process of managing it will become apparent soon.

The iec 60204-1 link you posted doesn't seem to include information relating to dark tests. I'll investigate that too. 

Thanks