Tuesday, April 28, 2009

Why do we need Single Point and Double Point Status?

The Single Point signals and Double Point signals for Status monitoring of e.g. circuit breaker switch positions in substations are quite special in process control systems. The basic definitions have been made in fifties of the last century - when remote monitoring was realized with electro-mechanical relays, contacts, wires and relay inputs.

The most simplest approach was to use a single contact to signal a switch position: 110 V on the input contact = Switch closed; no voltage = Switch open. This single contact is called "a Single Point Signal":

SinglePointStatus

This solution was not very safe. Think of broken wire that was not detected. Or of a contact that was not closed for a long time. After that time the contact may have been corroded so that after closing the contact mechanically there is no electrical contact! The receiving device would not be informed that the switch has been closed!

Engineers found solutions to solve the problem: They invented the "Double Point Signal":

DoublePointStatus

Single Point and Double Point Data Types are still in use in serial communication systems like DNP3, IEC 60870-5-104 or IEC 61850. The IEC 61850-7-3 defines these types -- so-called Common Data Classes (CDC).

The reason for Double Point Status in serial communication systems (like the IEC 61850-7-3 CDC DPS) is quite different: the two bits for the representation of "open" and "close" make the status message secure. A single bit error does not cause a wrong interpretation of the value! The value will not be valid anymore.

More details on the basic modeling approach of process information and information exchange is provided in the comprehensive and neutral seminars and training classes of NettedAutomation GmbH (details). Now is the time to get more education.

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