DISADVANTAGES OF RELAY BASED INTERLOCK SYSTEMS

Disadvantages of relay based interlock systems

Commissioning is held up while wiring changes, contact by-passing, coil replacement and timer settings are carried out.

You have to ask for plant shut down to carry out changes in control schemes.

Dust and humidity impair performance of electromechanical devices over a period.

Regular maintenance schedules and indenting for variey of spares is called for.

Change in operations or process involves extensive re-wiring / additional panel space.

Limitations of relay logic

Large space requirements

Very little or no flexibility for carrying out field alteration of system logic.

Frequent maintenance requirement.

Programmable logic controller

With the advent of reliable and low cost microprocessor, the answer to the above was a microprocessor based programmable logic controller.

Using plcs, the desired interlocking system need not be hard-wired, but can be programmed and stored in its memory.

The proogram sequence can be altered and modified easily in order to cater to any field changes required in the interlocking system.

The plc, based on its program stored in memory, checks the status of each such individual input contact, performs the necessary logic and sets each of the outputs to the desired status by energising or de-energising the contactor or solenoid valve or by giving supply to an indicating lamp etc..

Each interlock function is cyclically scanned at a fast rate and the outputs are set / reset depending on current status of inputs / outputs involved.

The basic principle of operation in a stored programmable logic is that every logic involving inputs and outputs is performed at a high speed under the directin of program statements stored in the memory.

Statements stored in the memory are fetched one by one in a serial order and based on the instruction in the statement, the necessary task is carried out.

The statement execution and input scanning is carried out cyclically at high speed.

In view of the high speed involved, as compared to reaction times of elector-mechanical switching elements and further with the aid of storage of output commands, the logic executin is continuous as in hard-wired logic and faster than relay logic.

For programming the statements of the logic to be performed, the user is given a simple, easy to understand and implement programming language.

The program for the interlock protectin logic is stored in the program memory provided in the memory chips of the memory module.

Each statement is entered as one step in the programming language.

The logic processing unit fetches and executes the instructins sequentially from the memory unit.

Thus the entire program is cyclically executed.

A perfect galvanic isolation can be maintained between field and internal logic by use of optical isolators. Use of optical isolators improves the noise immunity of the system and protects the internal processing circuitry from harsh field environment.

Features of plc

Shortest time installation

Flexibility : plcs provide flexibility of changes in logic requirements even at field by change of only the program.

Simple instruction set : the user is given a programming language which is easy to understand and simple to implement.

No necessity of splitting a logic equation : there should not be any need of splitting even the complicated interlock logic equation.

On-line programming capability.