Thermcraft XST-3-0-36-3V - Manual (Page 142)

Safety Information Installation and Operation Handbook
B-4 2408 and 2404 Controller
Over-temperature protection
When designing any control system it is essential to consider what will happen if any part of
the system should fail. In temperature control applications the primary danger is that the
heating will remain constantly on. Apart from spoiling the product, this could damage any
process machinery being controlled, or even cause a fire.
Reasons why the heating might remain constantly on include:
• the temperature sensor becoming detached from the process;
• thermocouple wiring becoming short circuit;
• the controller failing with its heating output constantly on;
• an external valve or contactor sticking in the heating condition;
• the controller setpoint set too high.
Where damage or injury is possible, we recommend fitting a separate over-temperature
protection unit, with an independent temperature sensor, which will isolate the heating circuit.
Please note that the alarm relays within the controller will not give protection under all failure
conditions.
Grounding of the temperature sensor shield
In some installations it is common practice to replace the temperature sensor while the
controller is still powered up. Under these conditions, as additional protection against electric
shock, we recommend that the shield of the temperature sensor is grounded. Do not rely on
grounding through the framework of the machine.
INSTALLATION REQUIREMENTS FOR EMC
To ensure compliance with the European EMC directive certain installation precautions are
necessary as follows:
• For general guidance refer to Eurotherm EMC Installation Guide, HA025464.
• When using relay or triac outputs it may be necessary to fit a filter suitable for
suppressing the emissions. The filter requirements will depend on the type of load. For
typical applications we recommend Schaffner FN321 or FN612.
• If the unit is used in table top equipment which is plugged into a standard power socket,
then it is likely that compliance to the commercial and light industrial emissions standard
is required. In this case to meet the conducted emissions requirement, a suitable mains
filter should be installed. We recommend Schaffner types FN321 and FN612.
Routing of wires
To minimise the pick-up of electrical noise, the wiring for low voltage dc and particularly the
sensor input should be routed away from high-current power cables. Where it is impractical
to do this, use shielded cables with the shield grounded at both ends. In general keep cable
lengths to a minimum.