Hardwired Two handed controls for presses have been out there years before PLCs or Twin Button Safety relays existed.
This type of control did
NOT cause the loss of limbs or life.
1. the buttons are/were normally snap action
2. they have both a N/O and a N/C contact
Normally the buttons are used to either close a guard OR
run the press for the first 160 degrees (past trapping/closing point)
the Press then returns to TDC via its rotary cams (usually two or three)
the N/C contacts are checked to prevent the next cycle as previously mentioned.
with a fully mechanically guarded Press the human risk is considerably reduced.
the major issue is with Light Curtains.
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As far as the twin button controllers
I understand the concern for a maximum delay time to press the both buttons - might be a good idea
BUT
this is impossible to use if there is more than one operator starting station.
Even with Cat.4 (SIL 3 I think) the aim has always been to prevent the mechanical jamming of one button then cycling with ONLY One button.
It is all in the timing
http://www.plctalk.net/qanda/showthread.php?t=334