Connecting and Interpreting Limit Switches

All about limit switches explicitly explained by Walter Diredger.

Introduction
There is a great variety of possible combinations for installing and connecting limit switches on valves. The number of switches depends on the particular control objective and may be influenced by redundancy considerations. The way they are connected depends on the safety and reliability requirements. In order to clarify this discussion, diagrams like Figure 1 will be used. All signals, switch positions, etc. are shown with the valve at the center of travel. No limit switches are actuated, all are shown in their shelf position as determined by their internal springs. Imagine the valve to be like a guillotine where the stem travels upward to open the valve and downwards to close it. The limit switch that is actuated when the valve is fully open is labeled ZSO. The one at the extreme opposite end is labeled ZSC. The terminals on the electrical switches are labeled Common (C), Normally Open (NO), and Normally Closed (NC). This unfortunate choice of terminology has nothing to do with either the state of the valve or even the “normal” position of the switch. It refers to the state of the switch when nothing is pushing on it.


Single Switch, Direct Approach
A single limit switch at the OPEN end of the valve (ZSO), as shown in Figure 2, will tell us when the valve is fully open. It cannot tell us if the valve is fully closed. The problem is that the term “open” is a bit ambiguous. Question: Is a half-open valve open, closed, neither open nor closed, or both open and closed? This discussion will use the following definitions:
OPEN = Partly or fully OPEN CLOSED = Partly or fully CLOSED
Not OPEN = Fully CLOSED Not CLOSED = Fully OPEN According to these definitions the half-open valve is both open and closed. A single ZSO switch can only tell us if the valve is “fully open” and“not closed”. It cannot tell us if the valve is partly open. Example 1: We need a limit switch and a status light to tell the operator that the fuel gas to a furnace is OPEN. If so, it is not safe to begin the light-off sequence. A ZSC switch at the closed end of travel is used so that we can be sure the valve is “fully closed” and “not open” even a little bit. The correct contact is NC. If the valve is even the slightest bit open, the OPEN light comes on. Example 2: We need a limit switch and a status light to tell the operator that the fuel gas to a furnace is CLOSED. If so, it is safe to begin the light-off sequence. This is exactly the same limit switch as before: ZSC. We want to know if the valve is “fully closed”. The only difference is that Example 1 uses the NC contact of the switch to turn off an OPEN light when the valve is fully closed while Example 2 uses the NO contact to turn on a CLOSED light when the valve is fully closed.

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