Temperature Sensors; Measurement Signal Transfer - TECSYSTEM NT538 WS Instruction Manual

TEMPERATURE SENSORS

Each Pt100 temperature sensor has a white wire and two red ones (CEI 75.8). The figure on page 11 shows the
configuration on the terminal board of the connection cables to the control unit. Each channel can be
independently programmed with two alarm thresholds (alarm and trip).

MEASUREMENT SIGNAL TRANSFER

All the Pt100 measurement signal transfer cables must strictly comply with the following rules:

1. Every Pt100 must be connected with a three-wire cable with a minimum section of 0.35mm² and a maximum of 1 mm².
2. The extension cable must be screened with tinned copper braid with an 80% cover
3. Conductors must be twisted, maximum recommended step 60mm
4. The cable screening must only be earthed with a termination, preferably on the control unit side.
5. The sensors' signal transfer cable must not be near electrical cables, either low or medium-high voltage.
6. The Pt100 cable and the signal transfer cable must be laid in a straight line, without any winding.
7. Any caps used to butt conductors must be crimped properly to avoid false contacts.
NOTE: to install the sensors and signal transferring cable correctly, read the SCS/SENSORS installation rules
manual.
What may happen when installation rules are not complied with.
1) The electrical field propagating from the power line of another circuit couples capacitively with the conductors (in
particular with unscreened cables). The effect of this coupling creates a signal that overlaps the signal transmitted by the
nearby conductors, causing incorrect readings.
2) The variations in magnetic flux in the power lines may induce an electromotive force on the signal transferring cables (in
particular non-twisted cables), that, being a closed circuit, generates a current. This interference current, multiplied by the
circuit resistance, gives a voltage value that overlaps the signal to be transmitted, distorting the sensor measurement.
3) False contacts can alter the signal with the consequent variation in the temperature detected.
In specific cases, when the rules for connecting the Pt100 sensors are not complied with, the following anomalies can
occur between the SCS box and the temperature control unit:
a) incorrect temperature readings, alarms or anomalous tripping
b) mechanical/electrical fault of the Pt100 sensors
c) damage to the Pt100 inputs of the control unit.
TECSYSTEM S.r.l. has designed its own special cable to transfer the measurement signals, CEI-compliant, with all the
protection requirements provided for mod. CT-ES
NOTE: the use of cables not complying with the above might cause possible reading anomalies. It is always
important to take into account that any interference on the signal lines might cause anomalies on the Pt100
inputs (CH1-CH2-CH3-CH4...) or on the sensors themselves.
All "NT" series control units have linearity of the sensor signal, with a maximum error of 1% of full scale value.
NT538 WS
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