It could do just that if there is an internal calibration mode that happens to get activated. But otherwise, I can imagine 0 mA 20+ mA or some intermediate value. But not exactly 20.00 mA.
It was a generic question really. I was looking at failure modes of a system which is using a PLC and was considering analogue inputs. If a wire drops off the cable to the remote 4-20mA transducer then you get 4mA so this case is covered. But I wondered whether a transducer could fail in some way to give a maximum deflection on the analogue input when this may not be the case. i.e. a load cell, or a pressure transucer may only be at half scale but the transducer fails to full output giving an indication at the PLC of full scale.
1> A wire fallen off will give you 0 ma, not 4 ma. Some controllers use a 0 ma condition as a broken wire indication and alarm on 0 ma.
2> Some transmitters (mostly temperature) will ramp upscale if a connection to a thermo couple or resistance temperature detector is broken. A 100% temperature indication will close the heat input and set the high limit alarms. This is a safe course of action when reliable instrumentation is lost.
This is not meant in any way to contradict Gunnar. His comments are accurate for an internal failure in an instrument.
3> Some controllers are programmable. You may select upscale or down scale in the event of a wire break.
eg: 0 ma in may = 4 ma out, OR 0 ma in may = 20 ma out.
Some controllers do not take special action on 0 ma in.
Bill
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"Why not the best?"
Jimmy Carter
Be aware that pressure transmitters in particular can have a failure mode where the transmitter's internal diagnostics drive the transmitter's output to a range outside the normal 4-20mA range, when the diagnostics consider the process value to be invalid.
The European user's group NAMUR actually has a standard for failure mode, typically configurable as 'high' or 'low', to something around 21mA or 21.5mA, I don't recall exactly, or about 3.5mA on the low side.
A number of other pressure transmitters use the same scheme but don't use NAMUR values/numbers. I seem to recall one brand that drove high to 22mA. I don't have access to those notes at the moment, but using 'outside' the normal range' fault modes might be worth considering if you're in the planning stage.
