What is a level relay? Features and applications!

How to control the liquid level of a reservoir  using a level relay? In this article the World of Electrical shows how a system works that controls the level of two reservoirs simultaneously, besides teaching step by step, in a very simple way, the correct way to assemble this system.

There are several types of automatic level systems for controlling a water tank. The upper tank level control supplied by the lower tank is the most common. This system is widely used to fill water tanks in places where artesian wells or cisterns exist, being used very often in rural properties. However, it can also be used in places where there is a need to take water from dams to some reservoir. There are still countless other applications within the industrial sector, such as, for example, the control of fluid systems in machinery and equipment.

What is Level Relay?

The level relay is used for monitoring and automatic regulation of the liquid level, measuring the electrical current of the liquid in the reservoir by means of a set of electrodes, which act as sensors for the presence or absence of liquid. The operation is based on an electronic circuit that compares the current that circulates between the electrodes connected to the level relay, thus changing the output contacts of the level relay to the working or resting position when the liquid covers or uncovers the electrode.

Level relay for reservoir control.

Level Relay – Assembly

To demonstrate the assembly of this level system, we will use a level relay model that allows controlling simultaneously the water levels of the lower and upper reservoir. This system operates with several types of electrodes, capable of detecting liquids of different conductivities. The level relay has a frontal adjustment for different resistivities of the liquids used.

In this automation system, six sensors will be used, which are also called electrodes, used to control the level of the reservoirs. Of these six sensors, three are sensors for the lower reservoir and three for the upper reservoir, each reservoir having two-level electrodes and a reference electrode. In the assembly of the example below we have two water reservoirs, the lower reservoir and the upper reservoir. The objective is for the upper reservoir to be supplied by the lower reservoir, as if it were an artesian well.

The level controller we are using has three LEDs that signal the situation that the electrodes are in, in addition to indicating the actuation of the relay contact. The first LED on indicates that both the upper reservoir level electrodes are uncovered by the water. The second LED on indicates that both the lower reservoir level electrodes are submerged, and the third LED on indicates that the relay contact has been activated. When the lower reservoir level electrodes are submerged and the upper reservoir level electrodes are uncovered, the relay contact is activated.

The connection of this system is very simple, first supply the terminals A1 and A2 of the level relay, then connect the electrodes. There are five terminals for connecting the electrodes which are ER, ES1, ES2, EI1, and EI2. We have six electrodes in our level system and only five terminals in the electrode level relay, this is because the reference electrodes must be connected to the same terminal (ER).

The high-level electrode of the lower reservoir is connected to the terminal EI1 of the relay. The low-level electrode of the lower reservoir is connected to the terminal EI2 of the level relay. The high-level electrode of the upper reservoir is connected to terminal ES1. The low-level electrode of the upper reservoir is connected to terminal ES2. The reference electrodes are connected to the ER terminal.

Supply of A1 and A2, and connection of electrodes.

Now it remains to connect the contacts of the level relay where the water pump will be connected. First, we feed contact A1, then we make a jumper in phase R that goes to the common contact (11) of the relay. To power the pump, we connect the normally open contact on a terminal (14) directly to the water pump and then make a phase S jumper that goes on terminal A2, connecting directly to the water pump.

Connecting the water pump to the level relay.

In the initial situation, the LED indicator for the lower reservoir level relay is lit. This indicates that the lower reservoir level electrodes are submerged and the upper reservoir high-level electrode is uncovered, in this position the relay contact is not activated.

However, when consumption occurs, the water level in the upper reservoir decreases, causing the upper reservoir level electrodes to be uncovered. In this condition, the LEDs of the upper reservoir and the relay contact light up, and the water pump starts immediately, starting the filling of the upper reservoir.

When the low-level electrode of the upper reservoir is submerged, the LED of the upper reservoir goes out. Thus, the supply of the upper reservoir is complete, the water pump is turned off and the LED of the relay contact goes out.

Another situation that causes the water pump to shut down is when the lower reservoir level electrodes are uncovered. In this case, the shutdown occurs even if the supply to the upper reservoir is not complete. In this situation, the system acts as a protective element, preventing the water pump from being damaged due to lack of water, a situation that is known as a dry failure.