Reading about emergency stop relays/contactors use to have an extra
normally closed contactor. If I got right this contactor may be used to
monitor relay is not welded together and to fulfill highest requirements
they should be monitored individually since otherwise a single failure
may be missed. Idea must be failure is very unlikely to happen exactly
then emergency stop is needed.
Thinking a little about it normal stop is a common operation and have
slightly different requirements if machines is enclosed with an
electrical operated lock. Stop is the same, use safety relay to
disconnect supply voltage operating the relays and other power supply
voltages which may be used to move machine parts. Then idea is to use
extra normally closed contactor to open door lock. This circuit should
be rather simple and if I think correct very safe provided
relays/contactors are built so that extra normally closed connector
never will switch on if main contactor(s) are welded together or other
failure.
There may of course be a problem with problem with internally stored
energy in for example DC bus capacitors. Nice thing is normal
relays/contactors may be used even though they mail fail, would even
dare to use bi-stable but it is very important normally closed contactor
never close unless main contactors have been separated. Door lock will
only open electrically but must be closed mechanically otherwise the
normally closed circuit to stop machine will be broken.
Anyone with experience?
Knowledge?
Or other with any comments in particular about the extra normally closed
contactor?
Regards Nicklas SB Karlsson
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