Correct positioning of safety circuits

Whilst you can add interlock or emergency stop functions in many ways in a machine control system, you need to make sure the level of reliability is appropriate for the risks.

Think of reliability in the way that car braking systems have developed. When I first started driving, cars had a single master cylinder with the outlet split 4 ways to the wheels. Properly maintained, this was safe but it was lacking in reliability; a single leak would mean total loss of braking. These days, cars have dual circuits with cross-checking. Should a single leak occur, the brakes will continue to function, and the cross-check will illuminate a warning on the dashboard.

The level of reliability in machine control systems is determined using EN 13849. The following is taken from SSS White Paper 4http://www.strategicsafety.co.uk/pdf/WhitePapers/WhitePaper4-EN13849.pdf explaining this.

Interlock switches and "Pilz-type" safety relays are high integrity devices.

However, PLC controls and their programs do not have high integrity unless specifically intended for safety circuits.

Therefore, having a high integrity safety circuit solely as a feed into a PLC negates its reliability.  Typically, this may take a Cat.2 system and turn it into a Cat.B one.
By all means, use an output from your safety circuit as an input to the PLC so a message can be displayed, but use the output from the safety circuit to interrupt the power output side of the PLC.  SSS White Paper 7 shows some ways of how this may be done.

Although many companies do not design control systems, ensure that your purchase orders for machinery include a statement that the following must not be part of the design: