JohdotaA rotary switch stays in the position it is turned to. It is used for maintained commands, such as switching a drive on or choosing an operating mode.
Switch types
- Maintained rotary switch (0-I). Stays in the selected position. Contact I is closed only in position I.
- Spring-return switch. A spring returns it to the zero position as soon as it is let go. It works like a push-button, but you turn it instead of pressing it.
- Hand-Off-Auto selector switch. In position H (Hand) the equipment is run by hand, in position A (Auto) the automatic control runs it, and in position 0 it does not run.
Switching table
In a switch with several contacts, each contact has the positions in which it is closed. The manufacturer gives them in a switching table, where a cross or a black dot means that the contact is closed in that position. A Hand-Off-Auto switch typically has one contact that is closed only in position H and another that is closed only in position A.
Hand-Off-Auto in a circuit
The H contact is wired so that it controls the contactor directly. The A contact feeds a branch that runs through the automatic control, for example a level sensor. Protection contacts, such as the motor protection auxiliary contact, are wired into the common part of the circuit so that they stop the motor in both modes.
In your region
Showing notes for
United States, Canada
The Hand-Off-Auto selector switch is marked HOA, and other selector switches SS.
On ladder diagrams the switch positions are shown with the contacts, and a mark, often an X, shows the positions in which each contact is closed. Some diagrams put these marks in a separate table, called a target table or truth table. Below, the contact is closed in H (hand).
Source: Siemens: Basics of Control Components, part 1 (STEP 2000 course) · Siemens: Basics of Control Components, part 4 (STEP 2000 course) · Eaton: Comparison of NEMA and IEC schematic diagrams, MZ081001EN (university copy)
UK & Ireland, Europe, Australia & New Zealand, Other IEC countries
No regional notes on this page yet. If something is different in your country, please send feedback.
Frequently asked questions
What is a Hand-Off-Auto switch?
A three-position selector switch for choosing the operating mode. In H (Hand) the equipment is run by hand, in A (Auto) the automatic control runs it, and in 0 it does not run.
How do you wire a Hand-Off-Auto switch?
The H contact controls the contactor directly. The A contact feeds a branch through the automatic control, for example a level sensor. Protection contacts go in the common part before the switch, so they stop the motor in both modes.
What contacts does a Hand-Off-Auto switch have?
Typically one contact that is closed only in position H and another that is closed only in position A. In position 0 neither of them is closed.
How do you read the switching table of a rotary switch?
The table has a row for each contact and a column for each position. A cross or a black dot means that the contact is closed in that position.
What is a spring-return selector switch?
A switch that a spring turns back to zero when you let go. It works like a push-button that you turn instead of press. A maintained switch stays where it is set.