Line integrity monitoring - SIS Design

[massunpcao]

I would like to know some manufacturers of line monitoring equipment for instrumentation signal lines. For instance, I would like to know when the line that carries the 4-20A signal from a field transmitter has broken.
Best,
Marc Assunpcao

 

[jsummerfield]

All of the TMR systems such as HIMA, ICS Triplex, Triconex, ABB August Systems, Siemens Quad Log as well as Trip Alarm out of Florida and various fire and gas detection system manufacturers. Search for "supervised I/O". This also applies to discrete devices but you would need some resistive device at the field instrument to differentiate a broken wire from an open contact.


John

 

[sjohnr]

I thought the whole idea of a 4 to 20 mA loop is that an open circuit gives you 0 mA so you have built in open cct detection. If the broken wire grounds out the circuit will overrange. Either way the fault gets picked up. I must be missing something.

 

[jsummerfield]

sjohnr is correct.

Whether 3-15 psig or 4-20 mAdc, the below scale reading of an elevated zero helps to identify problems. The 0 mA signal can indicate a transmitter failure, a wiring problem anywhere in the loop or a failure within the monitoring electronics such as the I/O module or other processor, etc. The below scale reading is easily detected and alarmed. This is among the reasons to use an analog instrument for shutdown measurement where digital devices were previously most common.

Supervisory techniques exist to monitor the wiring for the discrete signals that differentiate the wiring malfunction from the contact opening. This still does not identify a diaphragm failure that could prevent a pressure switch from functioning when demanded.

With a sigle transmitter, the TMR or quad systems differentiate the field side failure from the I/O or logic system failure. Other techniques would be required to differentiate the transmitter failure from the open wiring. I think that some TMR systems may have diagnostics that can make this differentiation too.


John