Creating a Control Reliable Safety System

Where required by the appropriate ANSI standard (example clause 4.5.4 of RIA 15.06.1999), the importance of using safety relays to achieve control reliable circuits can be explained. Control Reliable Systems must be designed "such that a single component failure within the system does not prevent the stopping action from taking place but will prevent successive system cycle until that failure has been corrected."

Ladder Diagram of a Common Emergency Stop Circuit

In this typical emergency stop circuit the weakest link is relay CR1. The contacts of CR1 can weld closed or, since this relay is spring applied, it can fail mechanically. If this failure occurred, energy to the load would continue resulting in an UNSAFE CONDITION that would cause machine damage and/or personnel injury. ANSI standards and OSHA regulations demand prevention of such a condition.

Ladder Diagram using two Force-Guided Relays to Achieve Redundancy

According to the definition of control reliability we need to guard against failure of CR1. It is the one source for a single component failure. Redundancy is not sufficient. If one of the two relays fail you are back to square one - with redundancy lost, the second relay could fail on a subsequent machine cycle.

We must monitor the condition of the redundant relays. Force or positive guided relays provide the best solution to accomplish monitoring.

Ladder Diagram of a Circuit using three Force-Guided Relays

This circuit is approaching control reliable. Using positive guided relays offers redundancy and cross-monitoring, but does not monitor for short circuits or reset problems.

Ladder Diagram using three Force-Guided Relays

Advantages:

• Control reliable
• Has redundancy and cross-monitoring

Disadvantages (in comparison to the Jokab Safety Relay):

• No safety approvals
• 38 wiring points
• High chance of wiring errors
• Installation is labor intensive
• More costly
• Larger in overall size (panel space)
• Easy to tamper with and bypass connections
• No short circuit protection on the inputs
• Reset is not monitored
• Difficult to troubleshoot
• The more contacts needed, the more complicated the circuit.

Wiring Diagram using a RT6 Jokab Safety Relay Connected in Input Configuration Mode 4 to Achieve Control Reliability

Advantages (in comparison to the 3 Force-Guided Relays Circuit):

• Control reliable
• Has redundancy and cross-monitoring
• 4 input configuration modes which are hardwire selectable (selectable category of safety)
• 2 reset configuration modes which are hardwire selectable
• Manual supervised reset mode monitors the button and wiring against failure
• Input configuration modes 3 and 4 monitor all input devices and wiring against failure
• Monitors external positive or force-guided contactors/relays
• Universal (multi-purpose)
• Retrofits easily into existing systems
• 5 LED indicators: Power On, Input 1, Input 2, Output K1 and Output K2
• 3 NO safety outputs, 1 NC monitoring output
• 2 transistor outputs for input status and output status
• Available in a variety of source voltages
• Terminal strips are removable for easy change
• 17 wiring points
• Cost effective
• Compact in size (45 mm in width)
• Several safety approvals