Conventional interlock systems rely heavily on hard wired electromagnetic relays. If larger systems have to be implemented in relay logic, the complexity limit is soon reached; the systems become too bulky, and wiring expenses sky-rocket; moreover, the intelligent of those designs is limited such a way to be left out. Relay interlocks are inherent inflexible, if the configuration of the system they protect has to change, a disproportional amount of time, work and money has to be invested in order to adapt the hard wiring of the interlock system to the new requirement.
More powerful interlock systems can be built by using computer based approaches. In this realm, Programmable Logic Controllers (PLCs) represent the most cost efficient approach. The interlock logic is no longer hard wired, but coded in software. The PLC approach offers several advantages:
• Flexible system configuration due to modular hardware and software.
• Regularly scheduled background tests of PLC system and sensitive I/O.
• Comprehensive system self tests.
• Intelligent fault diagnostics simplify troubleshooting.
• Easy reconfiguration of the interlock logic.
• No mechanical wear and tear.
• Improved security due to logic encapsulation in firmware.
Since PLCs, associated I/O modules and software are available off the shelf major vendors, this solution doesn’t require any major developments and becomes very attractive in price. This paper describes a PLC based electrical hazard interlock system which is currently under development at SLAC. In addition to its obvious design goals, it is also intended as being a prototype PLC application in sight of a larger project.