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What is the reason for the Mg_2 error message in an XUSLA application when the output is fed to a PLC input?

XUSLA is an obsolete range of relay output light curtains. The outputs were not designed to feed high impedance PLC iputs but to directly control relay or contactor coils.

Mechanical contacts require a minimum current flow through them to overcome any surface contamination/corrosion and ensure constant and reliable conductivity. This minimum current is know as the "wetting current". The value of this current depends on the construction and materials of the contact, but is typically between 10 and 50mA, dependant on contact.

Hi impedance inputs, such as PLCs, electronic counters etc will often not sink enough current to meet this minimum and in the case of the XUSLA relay a PLC input is likely to create this issue and therefore generate this error, which is "insuficient load on the contact".

In non-critical applications this problem can be overcome by fitting a dummy load resistor in parallel with the PLC input to increase the current to within the operating criteria of the contact set. However, this type of "fix" is unlikely to be acceptable in a safety application.

The solution in a safety application is to either use the XUSLA in accordance with the approved schemes, ie operating electromechanical devices,  or to use light curtains with solid state outputs (as in our current range of light curtains) which do not suffer the same problem and are designed to be used in approved solid state safety systems/PLCs etc.

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