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The limit switch can be connected as a normally open (NO) or a normally closed (NC) switch. It can also be used on a machine in such a way that the machine travel will hold the switch in the NO or NC position before the machine starts its travel. Figure 1 shows the four ways one will find the electrical symbol for a limit switch. The top left switch symbol is for an NO limit switch. In this symbol notice that the switch contact arm is shown below the terminal on the right side of the switch, and that the switch is shown as an NO switch. The NO held closed limit switch is shown in the bottom left diagram. Notice this is similar to the NO switch in that the switch contact arm is shown below the switch’s output terminal on the right side. The major difference with this switch is that it's shown with NC contacts because the machine motion will keep this switch in its NC position.
It's important to remember when purchasing limit switches that the switch is only available as an NO or as an NC switch. The held open and held closed conditions occur when the switch is mounted into place on machinery and the location of the switch causes the machine to hold the switch in the activated position.
The switch in the top right section of this figure is an NC limit switch. This symbol is different in that the switch contact is shown on top of the switch’s output terminal. When the machine motion activates the switch, it moves the contact arm upward to its open position. When this switch is wired in the field, one should select the NC contacts.
The symbol for the NC held open limit switch is shown in the bottom left corner of this figure. This symbol shows the switch contact arm in the open position and it's shown above the output terminal. When one wires this switch in the field, one would use the NC set of contacts, and the position of the machine when it's at rest would keep the switch held in the NO position.
|Limit Switches and Other Pilot Switches|