Frequently asked questions answered by our experts.

Cable length: what is the maximum length that a connection cable can have?
As PTC thermistors have extremely high impedance and the resistance in the nominal response temperature range changes from 250 Ω to more than 4 kΩ, the ohmic resistance plays a very minor role, meaning lengths of 500 m or longer are generally not a problem. The measuring circuit lines must be laid as separate control lines. Using wires from the motor supply line or other main power lines is not permitted. If high inductive or capacitive interference from parallel power lines can be expected, shielded control lines must be laid.

Testing PTC thermistors: how can I test PTC thermistors to ensure that they are working correctly?
Generally, you cannot do this while they are installed. The cold resistance of a thermistor chain of three is approx. 60 to max. 750 Ohm (typically 150–300 Ohm), meaning in the case of PTC thermistors from one production batch, at best an inference can be drawn of a short circuit in one sensor in the chain. Up to 20 K before the nominal response temperature is reached, the resistance may even decrease as the temperature increases. The trigger point in the event of excessive temperature can only be determined through practical tests (heating in an oil bath). The nominal response temperature can be determined based on the colours of the connecting wires (see ZIEHL catalogue). CAUTION: The measured voltage is max. 2.5 V.

How long is the service life of PTC thermistors? (Technical reliability/MTBF)
We do not have any figures from the PTC thermistor pellet manufacturer regarding the failure rate. Based on information gained through ZIEHL’s experience, a failure rate of ≤ 10 fit can be assumed. This corresponds to comparable values for components made from ceramic (e.g. ceramic capacitators). From this, an MTBF of ≥ 100 years can be calculated. This information depends on the failure criteria, the stress and the operating time. Additional information to estimate the lifecycle of the MINIKA® PTC thermistor under operating conditions can be gained by looking at PTC thermistors with a similar structure (e.g. for level monitoring). Here, information from the manufacturer claims > 5000 switching cycles.

Can PTC thermistors/PTC sensors be connected to inputs for Pt 100?
When using Pt 100 temperature relays with an adjustable hysteresis, e.g. ZIEHL Temperature relays type TR, PTC thermistors can also be connected to the sensor inputs. For this, the input is programmed for the connection of a two-wire sensor and a resistance of 150 Ohm is connected parallel to the PTC thermistor circuit. When the switching point is set to 115 °C and the hysteresis is set to 20 °C, the device switches in the same way as a PTC thermistor relay. The temperature displays and analogue outputs are unusable. We recommend that you use PTC thermistor relays of the type MS ... .

MSF 220 V and MSF 220 VU: during testing, only an advance warning occursg
MSF 220 V (VU) has short circuit and disruption monitoring in the sensor circuit. Every fault in the sensor circuit leads to an advance warning. Here, the “sensor” LED lights up and the LED for the sensor circuit with a fault flashes. To test the PTC thermistor input, a potentiometer (e.g. 0-10 kΩ) can be connected to the input to be tested. Do not increase the resistance too quickly (this leads to evaluation as an interruption). It is easier to test only three points: short circuit (bridge in sensor input), normal operation (PTC thermistor connected) and interruption (terminals disconnected). If these are evaluated and displayed correctly, you can assume that the sensor inputs are evaluated correctly. Test the outputs by pressing and holding the Test button. In the case of devices delivered as of around the beginning of 2004, recognisable by their housing width (only 70 mm), the short circuit and interruption monitoring can be switched off for ten minutes by pressing and holding the button (until the flashing frequency of the “ON” LED changes).

MSF 220 V/MSF 220 VU and MSF 220 K: false tripping. Why is the device tripped immediately or a few seconds after switching on?
The RESET/TEST button was pressed too hard or using an unsuitable tool and has jammed, with the result that a test cycle is run as soon as the system is switched on. Solution: “unjam” the button. (The button has now been changed). Delete this part of the text.

Switching point: how can I set the temperature at which the device switches?
PTC thermistor relays have a fixed switching point of approx. 3 bis 4 kΩ. The actual switching point depends on the PTC sensor, which becomes highly impedant at a specific nominal response temperature. Here, up to six PTC thermistor sensors can usually be switched in a series. Even PTC thermistors with different nominal response temperatures can be switched in series (e.g. to monitor winding and core or storage temperatures for different values).

Measured voltage at sensor input too high?
PTC thermistors have multiple notes indicating that the measured voltage may not be more than 2.5 V. However, measurement inputs for PTC thermistor trip devices generally have much higher voltages when idling (up to 20 V). This is not critical, as the voltage immediately breaks down when it is stressed with a sensor. In the case of cold (low-resistance) sensors, the measured voltage must not be more than 2.5 V. In the case of hot (high-resistance) sensors, it must not be more than 7.5 V.

Click here for further information.


Address for procuring suitable seals for STR100,
e.g. Pullfly type: CHEMTEC Sicherungssysteme
Lindenweg 2, D-60552 Röthenbach
Phone: +49-9120-18 34 22
Fax.: +49-9120-18 34 24

Can reset inputs Y1/Y2 on multiple devices be switched in parallel?
The inputs can be switched in parallel in order to use a shared reset button. This function has been tested with up to four devices. In order to exclude malfunctions (for example due to contact resistance or contact problems), we recommend using a potential-free contact for each input.


Cable length: what is the maximum length that a connection cable can have?
These electronic current converters are switches, to all intents and purposes. The possible length of the connection cable depends entirely on the external wiring.

Cross-section: what is the maximum cable cross-section that can be inserted?
The electronic current converters have an internal diameter of 11 mm. Insulated jumper wires and stranded control wires with a nominal cross-section of up to of 35 ø and UL-approved stranded wires of up to AWG 4 can be inserted.

Calibrating the scale
For three-phase voltage relays without a neutral conductor connection, the voltage is measured at an artificial neutral point. The scale is calibrated to a single-phase voltage change. With a single-phase voltage change, the neutral point moves towards the phases with a higher voltage. If the voltage change is symmetrical, higher voltage changes occur compared with the (unchanged) neutral point. The consequence is that the device switches sooner.

When the “Simulation” menu item is selected, both relays switch off. Simulation is not possible.
With UFR 1000 relays delivered up to April 2009 (up to software version 0-05), the devices immediately switch off both output relays when the “Simulation” menu item is selected. As of software version 0-06 (to display the software version, press the “Set” button for ten seconds), the voltage and frequency values can be simulated, allowing the settings to be tested. For details, see the operating manual.

The device switches off and does not switch back on itself. Display A1/A2/voltage level
In devices with software versions up to and including 0-07, strong electro-magnetic interference due to lightning or switching operations in the switch cabinet may lead to the device not switching itself back on. A remedy is possible by measuring against N and switching to programme 1. Alternatively, the device can be sent to the factory for a firmware update.


Cable length: what is the maximum length that a connection cable can have?
50 m is not usually a problem. Significantly longer connection cables are possible. When laying in parallel to power lines, shielding may be necessary.

Make the input less sensitive to ignore small currents
The response threshold of the current relay can be increased so that a smaller current is ignored and current flow is detected only when the main motor is switched on. To do this, a resistance (0.25 W/200 V) must be activated in front of the current relay input in question, parallel to the STWA 1 current converter. To change the response threshold by the factor x, we recommend the following resistance values: 2x/750 Ohm, 4x/330 Ohm, 10x/120 Ohm. Due to the high tolerances that need to be taken into account, we recommend that you determine the best values by experimenting.