Immediately with the ?change-over? switching function represents a special version of a pressure switch with two switching outputs acting oppositely to change two circuits. A ?change-over? closes one circuit and simultaneously opens another circuit. Thus a ?change-over? simultaneously fulfils the event of a ?normally open? and also a ?normally closed? contact. Therefore, functioning, a ?change-over? can only ever have one connected circuit closed and one energised.
Application types of pressure switches with ?change-over? switching function
?Change-over? contacts are used to a much lesser extent in industrial applications compared to the traditional ?normally open? and ?normally closed? variants. However, many applications with simple safety features could be implemented with ?change-over? contacts.
Let?s look at simple machine monitoring, e.g. a coolant unit on a machine tool with visual fault indication (see figure). A pressure switch with a ?change-over? switching function monitors the coolant supply to the tool. If the supply pressure of the coolant is sufficient, then the switching circuit for a green light is manufactured (green light ?on?). If the pump fails, however, and the supply pressure of the coolant drops off, then the switch changes its switching status. It opens the switching circuit for the green light (green light ?off?) and simultaneously closes the switching circuit for the yellow warning light (yellow light ?on?).Thus the device operator gets a primary visual feedback of any interruption at the coolant supply unit. Naturally, with the correct connection, the ?change-over? could switch other connected loads in place of a warning light, that could, for example, shut the device down or sound a warning hooter.
Use of mechanical versus electronic pressure switches
Through an ever-increasing level of automation, even in simple machinery, within the last years there has been a greater use of mechanical switches with ?normally open? and ?normally closed? as redundant safety monitoring for the continuous monitoring with sensors. The most recent developments in modern electronic switches provide operator, through the simultaneous provision of switching and analogue output signals, a lot better range of opportunities in the design of safety control. Thus, a control with the help of electronic switches can ? through the analogue signal ? continuously monitor system pressure, temperature or level, while also, at the same time ? through the evaluation of the switch contacts ? fulfilling an independent safety function through the monitoring of critical states, whereby hazards to personnel and machinery can be prevented.
Apart from ?change-over?, manufacturers of switches provide a variety of switching functions for all applications, such as for example ?normally closed? or ?normally open?. Using these switching functions, operators can fulfil almost 100 % of most simple control and safety tasks within machine building.
Note
Further information on mechanical pressure switches and electronic pressure switches can be found on the WIKA website. In the event you need any advice on the selection of a suitable switch, your contact will gladly assist you to.
Also read our article
What is meant by the ?normally open? switching function with pressure switches?