What is a PLC (Programmable Logic Controller)?

A Programmable Logic Controller, or PLC, is more or less a tiny computer with a built-in operating system (OS).

This OS is highly specialized to handle incoming events in real time, i.e. at the time of their occurrence. The PLC has input lines where sensors are connected to notify upon events (e.g. temperature above/below a certain level, liquid level reached, etc.), and it has output lines to signal any reaction to the incoming events (e.g. start an engine, open/close a valve, etc.) The system is user programmable. It uses a language called "Relay Ladder" or RLL (Relay Ladder Logic). The name of this language implies the fact that the control logic of the earlier days, which was built from relays, is being simulated.

Difference between surge arrester and lightning arrester?

This is mainly a matter of semantics. Historically, the motivation for using arresters was to deal with lightning, so they were called 'lightning arresters'. But as system voltage increased, it became apparent that there could be switching-induced surges that were more damaging than lightning, so today, the preferred term is probably 'surge arrester'.

There is a technical distinction between a 'surge suppressor' and a 'surge arrester' that has to do with the energy dissipation capability built into the device. The device that most people plug computers into is a 'surge suppressor' and has a fairly low energy dissipation capability. It also has a voltage rating that is only slightly greater than the normal system voltage. A typical 'surge arrester' has a far greater energy dissipation capability, and the voltage rating is driven more by the surge voltage withstand capability of the insulation being protected than by the rated system voltage.

My Fine Science

 


 
Science is nothing but trained and organized common sense differing from the latter only as a veteran may differ from a raw recruit: and its methods differ from those of common sense only as far as the guardsman's cut and thrust differ from the manner in which a savage wields his club.

MY Fine Quotes

 



Watch your thoughts; they become words.
Watch your words; they become actions.
Watch your actions; they become habits.
Watch your habits; they become character.
Watch your character; it becomes your destiny