Thermocouple category definition
It  exists  two main categories of thermocouple sensors

1) “Isolated “   or  “ungrounded” thermocouple : the 2  wires junction is insulated  from the Metalic part  of the sensor and so from the shield  (present all around and along the sensor’s cable )
2) “grounded” or “non isolated”  thermocouple  : the 2 wires junction is connected to the metallic part , and so indirectly connected to the shield all around and along the thermocouple cable

Within the TMC2TI2 cartridge and TM3TI4x, TM3TI8Tx, TM3TM3x modules it’s advised to use exclusively “isolated” thermocouples ( and not a “grounded thermocouple)
else when more than one sensor is connected onto different channel of one such module, the result is wrong temperature measurements for all the channels

Deeper explanation :inside  the TMC2TI2/TM3xx  all their channel inputs pins named Ix- (with x =0,1,2,3,4  to n,  n number of analog channel supported by the module)  are all internally connected together , as soon as 2 “grounded” thermocouple sensors are connected to the same module/cartridge,  (same way all sensors would have been mounted in parallel onto the same channel input) , this would provide wrong temperature measurements, by giving nearly same temperature measurements values for all the sensors inputs what ever real temperature on each sensor !

Only with TMC4TI2 cartridge , it’s possible to used both “isolated” or “grounded” kind of thermocouple sensors .
There is no restriction because TMC4TI2  channel inputs pins I0- and  I1- are non connected internally, they are isolated .

Udgivet til: Schneider Electric Danmark