What is Dead Zero Problem in Analog Signals?
An unusual situation in analog current signal systems, namely in typical 4-20 mA loops, when the signal suddenly drops to 0 mA is known as the “Dead Zero Problem.” Since 4 mA is the live zero the lowest valid value of a process variable, such as 0°C or 0 psi and anything below about 3.6 mA usually denotes a failure, this is a major issue.
The 4-20 mA signal normally transmits process information in a linear fashion:
0% of the measured value is equal to 4 mA.
100% of the measured value is equal to 20 mA.
A loop fault is indicated when the current reads 0 mA, not that the process value is zero. Typical reasons include:
Signal cables that are broken or unplugged
loss of the transmitter’s power
blown circuit breaker or fuse
An internal issue with the sensor or transmitter
Systems or controllers that mistakenly consider 0 mA to be a legitimate process reading of zero pose a risk. For example, a pump may run dry while a level transmitter indicates 0 mA because it believes the tank is empty when in fact it is not.
The majority of PLCs, DCS, and safety systems are set up to consider signals below 3.6 mA as a failure state in order to lessen this. To identify and notify operators of this problem, certain systems employ loop integrity checks, redundant transmitters, or HART diagnostics.
In order to provide safe and precise process management, the Dead Zero Problem, a crucial failure mode in analog signal loops, must be appropriately identified and managed.