Safety stop circuits and Master control relay circuits:
There is a lot of controversy in this area right now, and rightfully so, Fear and Reluctance to change rule the rest of our lives, why not industrial circuits?
As with many things in our world today, Responsibility becomes Liability, it is no longer enough to be "Engineered safe" unless you are using a Category 1, 2, 3 or 4 (this will most likely be expanded soon) "Safety circuit", utilizing redundant circuits, relays/contactors with Force Guided Contacts, Safety controllers, Safety relays, etc. Where the E-stop and Run/Stop circuits used to be up to the designer, this will migrate to a number of standard safety circuits to be used based on the application.
There is much talk about the fact that a "Safe" circuit can be designed without the use of "Safety Rated" components, Regulation and liability may render this method as not worth the risk. As we all know, when bureaucracy gets involved, logic goes out the window.
What could happen? In the event of an accident, it will be investigated as to whether you "Did all that you could" to make your machinery safe. My guess would be that UL will be the first one to require these types of circuits That being said, on new construction, you should consider investing in a safety circuit for your emergency stop, guard switches, and any other Operator protection circuits on your equipment.
The other thing that could happen is that your plant workman's comp or other insurance may give you a premium reduction. Yea Right! more likely is a rebate from the standard increase.
So what is a Safety Circuit these days? - A safety circuit that can monitor its own validity, and not allow the machine to run if there is a problem with the integrity of the circuit.
SAFETY RELAY - A true Safety Relay a relay with Force Guided Contacts (less likely to stick closed, but more likely to fail early) and a contact and mechanical arrangement that guarantees that the auxiliary contacts will remain in the state of the failed contact. So when the contact is welded, a normally closed auxiliary contact on that device that is in your safety circuit will remain open when power is removed from the device. This is generally used in your safety controller, not allowing a reset in this condition, rendering the machine permanently stopped until the part is replaced.
SAFETY CONTROLLER - These come in various sizes and circuit capacities, some are expandable, some are modules that plug into a PLC IO rack, But the ones worth buying have the following features:
A. At least one set of connections for E-stops with redundant contacts or guard switches with redundant contacts.
B. Each of these circuits operates an internal relay contact. These contacts are wired in series. Through these contacts is where you connect your Safety Rated master control relay.
C. Resetting means - a reset circuit that is wired through a normally closed contact on your Safety rated master control relay so that reset can only occur when the Safety rated master control relay is de-energized.
D. The safety controller should also have built in monitoring of all circuits for fault conditions such as short circuit, increased current draw, etc. that will also shut down the circuit.
Force Guided Contacts - Force guided relays, also known as directly driven relays or positively driven relays, have contacts that are mechanically linked. This mechanical link for the force guided contacts provides "monitoring" functionality that's not available in standard relays (non-force guided) in regards to the ability of the normally closed contact providing status for the normally open contacts.
There are variations for different applications, but that is the basic principal. A safe circuit that is even safe when it is faulted.
If you have a need for a safety circuit, contact: firstname.lastname@example.org