Try this one...
http://www.plcs.net/dcforum/DCForumID1/2645.html
In Figure-15, you can see the NPN and PNP field devices providing an input signal to the PLC.
If the field device shorts
internally (from Emitter to Collector, either NPN or PNP), any fuse installed on the source or load side of the device will
NOT blow!
This is because, in normal operation, when a solid-state switch turns ON, for all practical purposes, it
IS a
SHORT ! ("Short" does not necessarily mean Short-to-Ground!). In that case, the PLC input circuit provides the appropriate load to limit the current and that prevents the fuse from blowing - the PLC will always see the input as ON.
In a
grounded system...
If the
input to, or the
output from, the field device is shorted to
ground, then the fuse
will blow and the PLC will never see an input signal - whether the device is NPN or PNP.
In an
ungrounded system...
If the
input to the field device is shorted to
ground, then the fuse
might blow. If it doesn't, then the PLC
might see an input signal. It depends on the nature of the "short-to-ground".
In an
ungrounded system...
If the
output from the field device is shorted to
ground, then the fuse
might blow. If it doesn't, then the PLC
might see an input signal. It depends on the nature of the "short-to-ground".
Statistically, I have to say that the ungrounded method does increase the reliability of the signal.
However, you will not know that you have a problem. Not to mention the grief involved in troubleshooting an ungrounded system.