Question:

We are putting two level switches on a storage tank (in addition to a level transmitter) for alarm purposes. The switches will be located at a high and low level and will be input into the DCS. Logic in the DCS will display the words HIGH, NORMAL, and LOW on the screen, with the HIGH and LOW shown as alarms (red, flashing until acknowledged).

We are considering two types of level switch. The cheaper type is loop powered (simple switch with no electronics). These switches have a three screw terminal strip, with the terminals labeled Normally Closed, Normally Open, Common. They connect to a flanged connection on the tank with a float sticking into the tank. When the float rises the switch changes state. They have up and down side labels for correct mounting.

The other type of switch requires separate power and uses ultrasonic means to determine if there is liquid on the other side of the tank wall. It can be mounted anywhere on the tank (the reason for considering a more expensive level switch). That switch has the same NC,NO, Common terminal strip and also a switch marked “failsafe” with the positions labeled High and Low.

One switch will alarm if the level is too high, the other will alarm if the level is too low. Can you explain how to connect to these devices? I assume normally open or normally closed is the position corresponding to neither low nor high.

Answer:

First, let’s discuss the meaning of Normally Closed and Normally Open. Normal refers to the shelf or “out of the box” state of the contact. In the case of the float based switch “normal” should refer to the position of the contact when the switch is not exposed to liquid. That is, if the liquid level is below the switch the common terminal will be connected to the Normally Closed terminal. As the liquid rises above the switch, the Normally Open terminal is connected to common.

In your case (high and low level) you should arrange your logic so that the “high” input is open when the level is above the higher switch, and the “low” input is open when the level is below the lower switch. This “fail safe” approach will assure that the most likely failure, loss of the circuit due to a power supply failure, blown fuse, or broken wire will cause an alarm. If the level is in the normal range--between the switches--both loops are conducting. Therefore, you should wire the low input to the normally open contact and high level input to the normally closed contact. Your logic should provide four possible states.

High Switch - connected to Normally Open
Low Switch - connected to Normally Closed

High Low State
Input Input
Off On HIGH
On On NORMAL
On On LOW
Off Off FAILURE (level is both high and low, so there must be some failure)

For the unpowered switches normal is the shelf or “out of the box” condition. Whether you select the Normally Open or Normally Closed contact depends upon whether the shelf state is the safer or less safe state.

If the switch is powered, then the normal state is the power off state. That is, with no power to the instrument the relay will be de-energized. The Normally Open contact will be open and the Normally Closed contact will be closed.

Such instruments usually have a fail safe switch or jumper. This determines whether the relay will be de-energized if the level is above or below the probe. To achieve the results from the table above the fail safe switch is placed in the position that would represent an alarm. So in the above (high and low switch example) the fail safe position for the upper switch would be High and for the lower switch would be Low.

The configuration would be different if the switches were both used for high level, the lower for High and the upper for High-High (emergency high). In that configuration both the high and high-high switches are wired to the normally open switch. If the switch is powered, then both fail safe switches would be in the High position.