Diagnosing S7-300 FB41/FB58 Temperature Runaway on Extruder Heater Zones

On an extrusion line using a Siemens S7-300 CPU 315-2 DP with FB58 (TCONT_CP) controlling a bank of heater zones through solid-state relays, Zone 1 climbs to 268 °C against an 180 °C setpoint even though the FB58 output has commanded the heater off and the cooling fan on the zone is running. Autotune through the connected MP377 12" Key panel does not stabilise the loop, and replacing the thermocouple has not changed the symptom. This article walks through the field-proven diagnostic tree used to find the root cause when a closed PID loop appears to be "stuck on" while the HMI shows the SSR output as OFF.

The dominant fault in this exact symptom pattern is almost never the PID block — it is the field side. The PID controller is reporting that it has commanded the output to zero. The SSR, the heater, the thermocouple, or the thermal path back to adjacent zones is failing to honour that command. The rest of this guide walks through every link in that chain, in the order in which they should be ruled out, with the parameter-level checks and field tests that convert a "PID is broken" suspicion into a verifiable root cause. The FB41 / FB58 block reference is documented in the Siemens Standard PID Control manual available at Siemens Industry Online Support.

1. Symptom Decomposition

The reported symptom has three separable sub-observations. They must each be confirmed independently before the diagnostic tree is entered, because a misread of any one of them pushes the engineer toward the wrong branch.

2. Root-Cause Matrix

3. Diagnostic Flowchart

The flowchart above is the actual decision tree to walk on site. Every box is a measurement or test that takes less than two minutes to perform. The most common failure path is the first YES branch — SSR short — and the article walks through how to confirm and fix it.

4. Step-by-Step Diagnostic Procedure

4.1 Force FB58 to 0 % and observe the heater feeder

Open the FB58 instance DB in STEP 7 (or the equivalent on the MP377), navigate to the SP_INT input and write 0.0. If the operator interface exposes a manual output, set MAN_ON = TRUE and MAN = 0.0 instead — this is cleaner because it forces the output to zero without the controller re-asserting it. While doing this, have a second engineer with a Fluke 376 or similar true-RMS clamp meter on each phase feeding the zone-1 heater bank.

Observe the clamp meter reading. With the heater cold, a clamp meter on a resistive element carrying current will show a steady RMS current equal to nameplate FLA. If the meter shows any current greater than 50 mA (typical SSR off-state leakage is 5–15 mA), the SSR is leaking or shorted. A hard short will show full line current.

4.2 Confirm SSR integrity

With the FB58 output still forced to zero and QPULSE = FALSE, de-energise the heater feeder upstream and isolate the SSR. Use a true-RMS multimeter on the load side of the SSR (heater side), with the control side de-energised. A healthy SSR shows open-circuit (> 1 MΩ) between its two main terminals. A shorted SSR reads below 50 Ω or even dead short because the triac is destroyed. Replace the SSR if the resistance is below 1 kΩ with the control input de-energised.

If the SSR is a panel-mount unit with a built-in heatsink and thermal pad, also check that the thermal pad is intact and that the heatsink is not running hotter than 70 °C under load — an overheated SSR derates rapidly and eventually fails short.

4.3 If the SSR checks good - verify the thermocouple chain

If the SSR is intact but PV still climbs, the fault is on the measurement side. Cross-check the PV reading with a calibrated handheld reference probe (Fluke 52-II, WIKA CTH7000, or similar) taped to the same heater band. A deviation of more than 5 °C between the FB58 PV and the reference probe at steady state points at the SM 331 channel, not at the controller.

The SM 331 AI8x12bit module referenced (6ES7331-7KF02-0AB0) supports thermocouple types J, K, T, N, E, R, S, B only when configured for TC mode in HW Config. The 12-bit resolution gives roughly 8 mV per LSB, which over a type-K span of 0–1370 °C is about 7 °C per LSB at low temperatures and 0.5 °C per LSB at high temperatures. For tight PID control on extruder zones (typically +/- 2 °C), this resolution is the absolute minimum and is one of the reasons an FB58 loop on a 7KF module can behave noisily. Where the application permits, replace it with an SM 331 AI8x16bit thermocouple module (6ES7331-7PF11-0AB0) — the 16-bit resolution drops the LSB to roughly 0.1 °C over the type-K span.

For the exact HW Config menu path and parameter ranges on the 6ES7331-7KF02-0AB0, refer to the SM 331 manual in the Siemens Industry Online Support portal, search under "SM 331".

4.4 If thermocouple checks good - look at FB58 wiring

The FB58 (TCONT_CP) instance has three outputs that are easy to confuse on a first commissioning:

• LMN — the analogue manipulated variable, 0.0 to 100.0 %
• LMN_PER — the same variable scaled to the peripheral range of an analogue output card
• QPULSE — the pulse-width modulated Boolean output intended for a digital output driving an SSR

If QPULSE is wired to the SSR but the FB58 instance has been inadvertently reconfigured to output through LMN_PER to a non-existent analogue card, the QPULSE line will never assert. Equally, if LMN_PER is wired to the wrong word and QPULSE never gets updated, the SSR will not fire even though the controller thinks it is commanding heat.

Open the FB58 instance in STEP 7 and confirm that:

1. The instance DB is the one referenced by OB35 (or whichever cyclic interrupt the project uses for the controller).
2. The output QPULSE is mapped to the correct output byte/bit of the digital output module driving the SSR.
3. CYCLE matches the OB35 period (typically 1000 ms for thermal loops).
4. PFREQ is set to a sensible value (1–10 s for thermal masses of 1–10 kW).

The full FB58 interface is documented in the Siemens Standard PID Control manual, available under the documentation section of the Siemens Industry Online Support.

4.5 FB58 parameter audit for a zone in runaway

If the loop itself is misbehaving (PV oscillates, or PV holds high while output is well below 100 %), the following parameter checks apply. Most are caught at autotune, but autotune assumes the plant is in a representative state — if zone 1 was at 268 °C during autotune, the autotuner was probably trying to identify cooling rather than heating dynamics and the resulting GAIN / TI / TD are wrong for the normal operating point.

For the original FB41 CONT_C block (sometimes referenced in older S7-300 projects), the input list is shorter but the scaling rules are identical. The Standard PID Control documentation covers both blocks side-by-side, including the migration notes for projects that started life on FB41 and have since been reworked onto FB58.

5. Wiring and Field Verification

After the controller-side checks, the field-side wiring is the next place to look. Three patterns account for the majority of field-side faults:

5.1 SSR control polarity

Industrial SSRs have a DC control input that is polarity-sensitive on about 70 % of models. The standard pinout is + on terminal 3 and - on terminal 4 (for a Crydom / Sensata / Carlo Gavazzi panel-mount unit). Reversing the polarity on a polarity-sensitive SSR leaves the input LED forward-biased the wrong way and the SSR will not switch — but in a marginal case where the LED still partially conducts, the SSR may latch on intermittently with no controller command. Verify the SSR data sheet and confirm with a multimeter on the control terminals.

5.2 SSR dv/dt or back-EMF latch-up

On highly inductive heater loads (e.g. mica-insulated band heaters with long cable runs), the line-side dv/dt at SSR turn-off can re-trigger the triac. Symptom is a heater that comes on by itself a fraction of a second after the controller turns it off. The cure is an RC snubber (typically 100 Ω + 0.1 µF across the SSR main terminals) or a zero-cross SSR with built-in snubber. A snubberless / back-to-back SCR SSR should not be used on inductive loads without external snubbing.

5.3 Heater-to-ground leakage

On ceramic-insulated band heaters, ageing can allow the element to contact the body through degraded insulation. With the heater feeder open, meg the heater element to ground at 500 V. A healthy heater shows > 1 MΩ to ground. A failing heater shows tens to hundreds of kΩ to ground, which can produce enough leakage current through a poor ground reference to keep the band warm.

6. Adjacent-Zone Thermal Leakage

Extruder barrels are long steel cylinders with multiple band heaters clamped around them. Even with each zone's band electrically isolated, heat conducts axially along the barrel wall. On a 30:1 L/D extruder running 5 zones, zone 1 at 180 °C and zone 5 at 220 °C can leak 30–50 W from zone 5 into the steel mass around zone 1, requiring the zone 1 controller to command a small negative bias to hold setpoint.

If a zone that was supposed to be running at 50 % duty is being driven by leakage alone, the cooling fan will have negligible effect (forced air over a band heater sitting on a barrel at 230 °C cannot move enough heat), and the PV will sit above setpoint regardless of FB58 output. The diagnostic is to power off the neighbouring zones, wait for the barrel to equilibrate, and observe whether zone 1 PV approaches ambient or at least approaches a lower steady state. If PV falls significantly, the answer is more aggressive insulation between zones (ceramic fibre spacers) or independent heater zoning rather than a control fix.

7. SSR Substitution and Re-Commissioning

Assuming the dominant fault is the SSR short-circuit, the replacement procedure is:

1. Lock out / tag out the heater feeder and the SSR control circuit.
2. Confirm zero energy on both sides of the SSR with a multimeter.
3. Remove control wiring (note polarity on a label).
4. Remove load wiring (line side and load side — many SSRs are reversible but verify from the data sheet).
5. Unbolt the SSR from the heatsink, clean the mounting surface with isopropyl alcohol, apply fresh thermal compound (Wakefield Type 120 or equivalent, 0.05 mm thick).
6. Mount replacement SSR — same rating or higher; confirm voltage and current rating exceed the heater nameplate by at least 25 %.
7. Reconnect control wiring with correct polarity; reconnect load wiring.
8. Remove lockout; bring controller back to auto; observe QPULSE in the VAT to confirm activity.
9. Run zone 1 from cold to setpoint and confirm PV tracks SP within +/- 2 °C.

Thermal compound and mounting torque matter: a poorly mounted SSR runs 20–30 °C hotter at the junction, which is exactly the operating region where SSR lifetime halves for every 10 °C rise.

8. Verification Checklist

9. Preventive Maintenance

SSR failures are the dominant field-side fault in PID-controlled extruders. A scheduled replacement based on operating hours is dramatically cheaper than an unscheduled outage:

10. FB41 vs FB58 - When Each Block Applies

Both FB41 (CONT_C) and FB58 (TCONT_CP) sit in the Standard PID Control library that ships with STEP 7 V5.x. FB58 supersedes FB41 for thermal loops because it integrates the pulse generator that an SSR needs. FB41 still has its place when the final control element is continuous (a thyristor power controller with 0–10 V or 4–20 mA setpoint) because it outputs LMN as an analogue value rather than as QPULSE.

If the project is on FB41 but the load is an SSR, the typical workaround is to write LMN_PER to a PID_PulseGen-equivalent logic and then to a digital output. This works but is more code to maintain and more failure modes to debug. Migrating to FB58 when the controller output side is SSR-based removes the workaround entirely.

11. Frequently Asked Questions