The relay’s job

The PTC’s job is to sharply change resistance at the trip temperature Tk. The relay’s job is to detect that change reliably and to disconnect the motor before insulation damage occurs. Three conflicting requirements have to be balanced:

  • Reliable trip on a real over-temperature event — no missed trips.
  • No nuisance trip during normal heavy loading, motor starting, or supply transients.
  • Wire-break detection — an open PTC cable must also trip, because an absent sensor is a worse fail-safe than a hot motor.

The trip and reset thresholds

The de facto standard, set by KRIWAN’s INT69 family and widely copied:

  • Trip threshold — ~ 4 500 Ω (4.5 kΩ) ± 20 %. When the PTC chain resistance rises above this value the relay output drops out (de-energises the contactor).
  • Reset threshold — ~ 2 750 Ω (2.75 kΩ) ± 20 %. Below this resistance the relay can re-energise, either automatically or after a manual reset depending on configuration.
  • Open-circuit detection — > 20 kΩ. A broken cable looks like infinite resistance and is treated as a fault.
  • Short-circuit detection — < 20 Ω. A shorted cable is also treated as a fault — it would mask a real trip.

Why those specific values? Look at the DIN 44081/44082 R(T) curve breakpoints. A single-element PTC has R ≤ 550 Ω at Tk − 5 K and R ≥ 1 330 Ω at Tk + 5 K. A triple-element PTC has R ≤ 1 650 Ω at Tk − 5 K and R ≥ 3 990 Ω at Tk + 5 K. The 4.5 kΩ trip threshold sits comfortably above both +5 K values and well below the ≥ 12 kΩ at Tk + 15 K breakpoint — so the relay trips squarely inside the ±5 K window around Tk, with a guaranteed margin on both sides.

Why three sensors are wired in series

The standard topology for three-phase motor protection puts three single-element PTC sensors, one per phase, all wired in series — that is the DIN 44082 triple-element variant by another name. The series chain resistance is the sum of the three bead resistances. As long as all three are in the cold region, the chain looks like ~300 Ω or less. As soon as one bead crosses its trip-region knee, that bead’s resistance shoots up into kiloohms and dominates the chain.

The relay sees the same threshold crossing regardless of which phase overheated. That is the whole point: single-phase overheating (caused by a lost phase, an unbalanced supply, a blocked cooling passage on one stator slot) is the most common motor-failure scenario, and a series PTC chain catches it long before a bulk-average temperature sensor would.

Up to nine sensors per channel

The standard INT69 input can drive up to nine PTC sensors in series. This lets you protect three independent points on a large machine (for example, three slots per phase × three phases = nine beads) from one relay input. The 4.5 kΩ trip threshold is sufficient even with the cold-chain resistance of nine series PTCs (typically about 1 kΩ total) because the trip-region resistance of any single bead far exceeds the entire cold chain.

Above nine sensors the cold-chain resistance starts to encroach on the trip threshold margin, and you need a different protection-relay model with a higher trip threshold — or split the protection across multiple relay channels.

Operating voltage and self-heating

An INT69-class evaluation relay applies a measuring voltage of ≤ 2.5 V across the PTC chain in the cold region. This is well below the Umax = 30 V the PTC can survive without damage, and well below the 7.5 V at which self-heating starts to bias the R(T) curve. The 2.5 V upper bound is part of why the DIN standard defines the trip-region breakpoints at U ≤ 2.5 V.

When the PTC trips, the chain resistance jumps to kiloohms, so even at the measuring voltage the current drops to microamps and self-heating becomes irrelevant. The relay can hold the trip state indefinitely without overheating the sensors.

Auto-reset vs manual-reset

Most INT69 variants support both. Auto-reset is appropriate when the over-temperature is expected to be transient and the motor cool-down does not present a safety hazard — HVAC fan motors, pumps, general industrial drives. Manual reset is appropriate when the over-temperature condition might recur (a blocked cooling passage, a process fault) and operator inspection is required before re-energising — cranes, hoists, compressors, conveyors.

The choice has nothing to do with the PTC itself — the PTC will cool down and reset its resistance either way. It is a relay configuration / system safety decision.

DIN EN 60947-8

The relay-side behaviour is governed by DIN EN 60947-8 (formerly EN 60947-8:2003), which specifies control units for built-in thermal protection of rotating electrical machines. The standard covers trip / reset threshold tolerances, isolation requirements, test methods, terminal markings and environmental qualification. Any modern PTC evaluation relay sold in Europe carries a CE mark backed by EN 60947-8 compliance.

What this means for sensor specification

When you specify a Jianlu MZ6 series PTC for a motor protected by an INT69 (or any EN 60947-8 compliant relay), the only thing you need to confirm is:

  1. The PTC is built to DIN VDE V 0898-1-401 (formerly DIN 44081 or DIN 44082) — this guarantees the R(T) curve matches what the relay expects.
  2. The trip temperature Tk matches the motor insulation class (see our insulation class article).
  3. The sensor count fits within the relay’s supported chain length (1-9 for standard INT69).

Everything else — lead length, sleeve material, dielectric withstand voltage, mechanical envelope — is a motor-mechanical decision that does not affect the protection circuit.