I'm off home now and while I don't have near enough time to give to this thread as I'd like; still, having read the mention of using contactors after drives, while also using Safe Torque-Off (STO) on the same drives, I just want to throw this into the mix...
This is straight from my copy of Rockwell's "MACHINERY SAFEBOOK 5" here on my desk...
Rockwell said:
Drives and Servos
Safety rated drives and servos can be used to prevent rotational energy from being delivered to achieve a safety stop as well as an emergency stop.
AC drives achieve the safety rating with redundant channels to remove power to the gate control circuitry. The redundant channels are monitored by either external or integral logic depending on the type of drive. This redundant approach allows the safety rated drive to be applied in emergency stop circuits without the need for a contactor.
The Servo achieves a result in a manner similar to the AC drives by using redundant safety signals which are used to achieve the safety function “safe torque-off”.
Also, the traditional Safety contactors are classed as "Safety actuators", of which, drives and servos that support embedded or add-on STO are also included i.e. either/or may provide the necessary Safety actuation for a Safety Function...
Rockwell said:
Safety actuators
• Safety contactors and Starters
• PowerFlex® AC drives
PowerFlex drives are available with safety features. The PowerFlex 525 AC drives include embedded Safe Torque-Off as a standard feature. Safe Torque-Off is an optional feature for the PowerFlex 40P, 70, 700H, 700S, and 750-Series AC drives, which also support Safe Speed Monitor functionality.
• Kinetix® integrated motion
Kinetix 300, 6000, 6200, 6500 and 7000 servo drives all feature built-in safety functionality. With Safe Torque-Off, a drive output is disabled to remove motor torque without removing power from the entire machine. Safe Speed Monitoring permits users to reduce and monitor the speed of the application to help an operator safely perform some types of work without completely stopping the machine.
On your mention of also wiring the drive STO signals through the contactors - this should not be necessary as these signals would be interlocked through the Safety relay so you should not have to use contactor auxiliaries to double up here. If the Safety relay has de-energized then its output contacts will have opened, which will turn off the STO channels.
I haven't time to get into the Performance Level (PL) side of things too much, but essentially, and in keeping with Ken's quoted information, the PowerFlex 525, using Safe Torque-Off, can achieve SIL 2 Category 3 /PL
d alone. To achieve SIL 3 Category 3 / PL
e, STO would have to be used in conjunction with other Safety measures to achieve this higher Performance Level.
You need to know which Performance Level (PL) you require here before deciding whether or not it is permissible to use STO only on these drives.
That is, you may or may not need Safety contactors here. PL
d, for instance, may permit the use of no contactors here.
You also have to consider the cascading arrangement for the STO on this many drives (17). Can you load one Safety relay with this many STO connections in parallel or must you split them over several Safety relay contacts, or must you use more than one Safety Function, etc.? Is there Zoned Safety implemented or now required?
Using STO you will achieve a Category 0 uncontrolled stop for each driven motor. Is this acceptable from a risk or hazard point of view?
These, and other questions, are really only best answered by a thorough Risk Assessment. Just because the current installation is Category 3 / PL(?), does not mean your changes do not warrant a re-assessment. It may still end up being that Category 3 / PL(?) is sufficient. But you must determine this once more and not just assume that the current PL will still be what is required after the change. It "probably" will be, but you should not just assume it.
Cydog said:
...Safety evaluations are more complicated than I thought...
Unfortunately, this is an all too often response in these situations.
Regards,
George