Practical Wiring Reference

Thermocouple wire colour codes

*Type R/S must use copper–copper-nickel compensating wire (not thermocouple wire) for extension, and is accurate only near ambient temperature.

Polarity mnemonics

Remember:IEC negative is White; ANSI/JIS negative is Red— this is the most common source of polarity errors.

Sensorwiring configuration

Single thermocouple wired directly to the controller (2-wire)

  TC probe                 extension cord                    controller
  --------                ----------------           ----------
  [hot junction]  ----(+)=========(+)===============(+) TC input +
          ----(-)=========(-)===============(-)  TC input -

  Key points:
  - Use matching thermocouple-grade or extension-grade wire throughout
  - No joints other than terminal blocks / connectors
  - Ground the shield at the controller end only

Thermocouple + transmitter(4-20mA output)

  TC probe        TC wire         transmitter         standard cable        controller
  --------       ---------      -----------     ----------      ----------
  [hot junction] --(+)===(+) TC In                  4-20mA + --------- AI +
          --(-)===(minus)  TC In                  4-20mA minus --------- AI minus

  Transmitter power supply: 2-wire (loop-powered) or 4-wire (separate power supply)
  Advantages: immune to line resistance, suitable for long-distance transmission
  Range: typical 0-1200 C corresponds to 4-20mA

Dual thermocouple input (measurement + independent overtemperature protection)

  TC1 (control)                        PID controller
  ---------------                    --------------
  [process TC] --(+)-------------- (+) TC input 1
            --(minus)-------------- (minus) TC input 1    - control heating output (PID loop)

  TC2 (safety)                        over-temperature protection instrument / trip
  ---------------                    --------------------------------
  [safety TC] --(+)-------------- (+) TC input      - trips above setpoint
            --(minus)-------------- (minus) TC input         opens safety relay
                                                   (independent of PID)

  Important:
  - The safety TC must be independent of the control TC
  - The safety instrument must be a separate device
  - The safety relay and main contactor coil are in series
  - Over-temperature trip must be manually reset

Terminal head (JB) wiring rules

• Use only thermocouple-grade terminal blocks or matching compensating terminal blocks; ordinary copper terminal blocks are forbidden
• Clearly mark polarity at every wiring point
• Keep cold and hot junctions apart to avoid temperature gradients across the terminal block
• One shield ground braid per cable, grounded at the controller end only
• Thermocouple extension wire must never share conduit/tray with power cable
• Maximum recommended extension-wire length 100 m; beyond that, switch to a transmitter
• Label every cable with the thermocouple type, loop ID and position at both ends

Thermocouple junction types: grounded / insulated / exposed

controlleroutputType and powerwiring

Eurotherm 3216 terminallayout

  Eurotherm 3216 terminal layout (rear view, from left to right)
  ==============================================

  upper row:    1    2    3    4    5    6    7
  lower row:    V+   V-   HA   HB   HC   LA   LB

  terminal 1-2:     power (100-240V AC)
  terminal V+ V-:   TC/RTD input (+ and minus)
  terminal HA-HC:   output 1 (relay: HA=COM, HB=NO, HC=NC)
                  (or SSR drive: HA=output, HC=common)
                  (or 4-20mA: HA=output+, HC=output minus)
  terminal LA-LB:   output 2 (relay: LA=COM, LB=NO)

  Communications (optional, module dependent):
  terminal EIA-485: Modbus RTU (A, B, COM)

  Description: Eurotherm 3216 is a 1/16 DIN single-loop controller.
  Output type depends on the fitted hardware module, check the module code on the label before wiring.

SSR wiring:controller → heating element

  Controller              SSR                Fuse      Heating Element
  ----------        --------             ------     --------
  SSR Drive + ----> Input (+)
  SSR COM    ----> Input (−)
                    Output (L) <---- L (fused) ----> Element L
                    Output (N) <---- N  -----------> element N
                                          |
                                         MOV (275V)
                                          |
                                          N

  Protection:
  - Fuse: HRC type, sized to the element full-load current
  - MOV (varistor): 275V AC across the SSR output, suppresses voltage spikes
  - Heatsink: the SSR must be fitted with an adequate heatsink, rule of thumb 1 C/W per amp of load
  - Add air cooling when ambient >40 C or load >25A

contactorwiring:controller → three-phaseheating element

  Controller            Contactor                     3-phase Heating Element
  ----------       -----------                 ------------------
  Relay NO -----> A1(coil +)
  Relay COM ----> A2(coil −)  [via 240V supply]

                   L1 -------> Fuse -------> Element L1
                   L2 -------> Fuse -------> Element L2
                   L3 -------> Fuse ------->  element L3

  Safety chain (in series with the coil):
  A1 <-- emergency stop NC <-- over-temperature NC <-- furnace-door interlock NC <-- 240V

  Key points:
  - Contactor coil voltage must match the power supply (usually 240V AC)
  - Auxiliary NO contacts for heating indication
  - For long cable runs, parallel an RC snubber across the coil at both ends
  - Size to AC-3 load where an SSR/SCR is upstream

Overtemperature protection chain

  Over-temperature Protection Chain
  ============

  Safety TC(independent) --> Over-temp Protector
                      (e.g. Eurotherm 3216 alarm mode)
                        |
                      Relay output (NC)
                        |
                      Series into safety chain: 
                      ... --> [Overtemp NC] --> [Next device NC] --> ...

  Setpoint:
  - Setpoint = process temperature + 25-50 C (or per specification)
  - AMS 2750: over-temperature setpoint at the TUS uniformity upper limit
  - Sensor-break direction: over-range (TC fault reads high = trip)
  - Alarm type: latching (requires manual confirm + reset)

  Requirements:
  - Use a separate TC independent of the control TC
  - Use a separate instrument independent of the PID controller
  - Prefer an independent power supply
  - Annual calibration check

heating elementwiring

three-phasestar / deltaconnection method

  Star (Y) connection method                       Delta (triangle) connection method

  L1 ---[element 1]---+                  L1 ---+---[element A]---+--- L2
  L2 ---[element 2]---+-- star point (N)          |                 |
  L3 ---[element 3]---+                  +---[element C]---+
                                            |
  Uelement = Uline / sqrt(3)               L3 ---+---[element B]---+
  (415V supply -> element 240V)            Uelement = Uline
  Iline = Iphase                            (415V supply -> element 415V)
  Ptotal = sqrt(3) x Uline x Iline          Iline = sqrt(3) x Iphase
  Element voltage is lower, power is        Ptotal = sqrt(3) x Uline x Iline
  1/3 of the equivalent delta connection   Full line voltage, power is 3x the star
  Suitable when the element is rated for    Element must be rated for line voltage
  phase voltage or lower power is required

single-phaseelementwiring

• Via SSR:L → fuse → SSR output → element → N
• Via contactor:L → fuse → contactor NO → element → N
• Multiple elements in parallel (same voltage):L → fuse → switch → elements in parallel → N
• Multiple elements in series (voltage divider):L → fuse → element 1 → element 2 → N, each element sees U-total/N

SiC element special requirements

MoSi₂ element (Kanthal Super) special requirements

Element resistance calculation

Element ageing behaviour

• NiCr (Nikrothal 80): resistance rises 5-10% over service life
• FeCrAl (Kanthal A1/AF): resistance rises 3-8% over service life
• SiC: resistance can rise 200-400% (a major consideration)
• MoSi₂: resistance stays essentially stable when used correctly
• Rule: replace metal elements when measured resistance exceeds nameplate by 20%

Safety circuits

Emergency-stop circuit (safety-critical)

  Emergency-stop Circuit (NC series circuit, Safety-critical)
  =============================================

  240V L --> [E-stop 1 NC] --> [E-stop 2 NC] --> ... --> Safety-relay coil A1
  240V N ------------------------------------------------> Safety-relay coil A2

  Safety-relay outputs: 
    NO1 (13-14) --> Heating-contactor coil supply
    NO2 (23-24) --> Gas-valve supply
    NO3 (33-34) -->  Indication / PLC input

  Critical requirements:
  - All emergency stops are NC (normally closed) in series
  - Pressing any emergency stop opens the circuit
  - Wire break equals emergency-stop press (fail-safe)
  - Safety relay provides a monitoring output
  - Safety relay must be manually reset
  - Use a Cat 3 or Cat 4 safety relay (e.g. Pilz PNOZ, Allen-Bradley MSR)
  - Emergency stop button: red mushroom head, yellow collar (ISO 13850)

Furnace-door interlock

Blower proving circuit

  Fan-proving Circuit (Fan Proving)
  ===========================

  Circulation Fan / Combustion-air Fan: 
  Fan motor --> Run --> Differential-pressure Switch (across fan)
                          NC contact closes when: dP > setpoint
                          = Fan running and producing airflow
                              |
                            Series into safety chain: 
                            ... --> [Fan proved NC] --> [Next device NC] --> ...

  Alternative: motor power-circuit current-sensing relay
  (less reliable than a differential-pressure switch, a broken belt may still let the motor run)

  Applications:
  - Gas firing: combustion air blower must be verified before the gas valve opens
  - Circulation: blower must be verified before elements are energised (prevents local overheating)
  - Quenching: quench blower must be verified before the quench cycle starts

gasSafety circuits

  Gas Safety Interlock Chain (simplified)
  ========================

  All conditions met (NC contacts in series): 

  240V --> [E-stop] --> [Overtemp] --> [Fan proved] --> [Gas low-pressure]
       --> [Gas high-pressure] --> [Flame monitor] --> [Door closed]
       --> [Purge complete] -->  BMS / gas valve power supply

  Gas-valve sequence (typical):
  1. Pre-purge: air blower runs 5x the furnace-chamber volume in air exchange (no gas)
  2. Ignition: ignition gas valve opens, igniter sparks
  3. Low-fire confirm: UV/ionisation detects the ignition flame (3-5 seconds)
  4. Main gas: main gas valve opens (low fire maintained)
  5. Main-flame confirm: UV/ionisation detects the main flame
  6. Operate: proportional or on/off control
  7. Shutdown: gas valve closes, then purge (blower keeps running)

  Valve leak-detection system (VPS):
  - Test valve tightness before each start
  - Pressurise between the two valves and monitor leakage
  - IGEM/UP/2 requires mandatory fitting of valves DN50 and above

Overtemperature trip cascade

  Over-temperature trip cascade
  ==================

  1. Over-temperature alarm triggered
     |
  2. Gas valve closes immediately (via safety relay)
     Elements de-energised (contactor trips out)
     |
  3. Circulation fan keeps running
     (prevents thermal shock, evens out residual heat)
     |
  4. Exhaust blower keeps running (expels residual atmosphere)
     |
  5. Audible/visual alarm activated
     |
  6. PLC/SCADA logs the event with a timestamp
     |
  7. Safety state held until:
     - Temperature falls below trip value minus hysteresis
     - Operator confirms the alarm
     - Safety relay is manually reset
     - Complete post-purge cycle finished (gas system)

  Critical: the blower must never stop during an over-temperature trip.
  Suddenly losing air circulation in a hot furnace causes local overheating,
  element damage, and a potential ignition risk in the atmosphere system.

commonwiringfaultdiagnosis