There is a foreign voltage with too large an amplitude at the input of a temperature sensor – in the device’s own words: “Connection error, mains voltage at a temperature sensor input!” This is detected at the common reference measurement of the controller board, and because that measurement is the same for all six channels, the fault cannot be attributed to any single channel. That is why all six report it, and why the device drops the main contactor and leaves it off until the fault has been remedied.

There are two causes, and they differ in severity:

1. A wiring error in the tool connector. A heater conductor is on a sensor contact. Typical after rewiring or with a newly connected tool.
2. A defect in the tool. A shunt between a heater and the sensor circuit – moisture, damaged insulation, a chafed sensor lead.

To distinguish them, unplug the tool connector: if the fault disappears, it is in the tool or its connector; if it remains, it is in the device.

Together with the residual current this can be detected early. A heater whose insulation is giving way first shows up as a rising residual current on its controller board – long before mains amplitude reaches the sensor input. If you look at this value per board after every tool change, you find the shunt before it drops the main contactor. Where the value is shown is described in Section 9.2.9, “Controller #”.