BECKHOFF FM33xx - Manual (Page 17)

Functional description
12 FM33xx
Back voltage protection
The back voltage protection guards against external voltages applied
through the thermocouple inputs. Voltages of up to 230 V
AC
are tolerated,
or are withstood without damage to the module. Those thermocouple
inputs that are not affected remain functionally operative, or are only
affected for a short time. The module switches measurements from the
affected channel off and disconnects the voltage, so that the unwanted
voltage can remain connected to the thermocouple input for a long period.
If it is possible to manage without that particular thermocouple channel, it is
not necessary to rectify the fault immediately.
How the back voltage protection operates
The thermocouple inputs are protected within the module against voltages
that are outside the permitted signal range. The permitted range is about
–10V...+10V. If voltages outside this range appear, the current is limited by
an internal resistance and is diverted to a protective circuit. The current-
limiting function of the input circuit protects the thermocouple wires from
excessive short-circuit currents, so that they are not destroyed. A
protective circuit is responsible for a group of at most eight thermocouple
inputs. The central protective circuit detects the excess voltage and
switches off all the thermocouple inputs in the group. This process affects
the current measurements being taken by the channels in this group, and
those measurements will have to be made again. Monitoring and switching
groups off means that the channel that is directly affected cannot be
identified. The faulty thermocouple input is identified by switching on the
channels that have been switched off again one by one. As soon as it is
seen that the protective circuit for these thermocouple inputs is triggered
again after a channel is switched on, this channel is recognized as faulty,
and remains switched off. When all the channels in the group have been
tested, and are either operating again or have been switched off because
they are faulty channels, it is possible to continue with the normal
measuring process. The fault in the channels that have been switched off
is checked by switching on again after a configurable time, so that the
faulty channels can be automatically brought back into operation.