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Sensor interconnection is a simple 2-wire industry standard 4-20mA signal. The nominal power supply is 24 volt dc., although this can be increased to a maximum of 30 volt dc.
Sensors with integral electronics require a "lift-off" supply of 9 volt dc. Those with remote electronics require 11 volts dc.
To determine the maximum permitted impedance of any receiving instruments, ie., indicators, plc inputs etc. use the following calculation.
| Maximum impedance = | supply voltage - sensor lift off voltage |
0.02 |
Use of a screened signal cable is essential in electrically noisy environments where the 4-20mA signal could otherwise be effected.
Units with integral electronics
These transmitters are either fitted with a DIN standard 3-pin plug(+, -, & screen) for clients wiring, or if a submersible version , prefitted with a 2-core screened cable.
The electronics housing for these transmitters is manufactured from GRP and has an internal screen coating which complies with international standards for EMC/RFI protection. The sensor cable has an overall screen and the 4-20mA signal output cable should be likewise screened. These screens are terminated in the housing’s metal cable glands to ensure circuit continuity. This overall screen should be tied to earth at the power supply.
Note that the actual sensor body is not connected to this screen. This is necessary to comply with the requirements of intrinsically safe installations where there would otherwise be a danger of contamination of the IS earth.
Since it is normal that the sensor body is not connected to earth via the cable screen, this gives rise to potential problems on marine/shipboard applications especially where cathodic protection is used as a means to inhibit corrosion. In these instances it must be ensured that the sensor body is bonded to the ship’s hull.
Depending on the sensor type, this connection may be automatic, i.e. flanged or threaded connections. However, where for example, the sensor is installed via a stilling pipe (which are often of non-metallic construction), the sensor should be specified with an earth tag to enable bonding to the hull at a convenient location.
