Level switch

Level switch-

On detecting high level, the contact changes from Normally close to open, and for detecting low level, the contact changes from normally open to closed state.

Any special reason for this type of configuration in level switches? Is this for safety?

Level is one of the easiest process variable to monitor. In commercial applications, level switches are more common. In industrial application on the otherhand, level sensors/transmitters are more common. Now to answer your question. Normally closed contacts are used in high level to prevent possible overflow. The normally closed contact allows power to flow to the pump. Once the high level is reached, the contact becomes open and thus disrupting the power. This will stop the pump to continously pumped water into the tank. The same is true to the low level contacts.

You are correct. The reason why this configuration is used for Tank protection is to ensure the circuits are "Fail Safe". This is the principle used on all Instrumentation High and Low Pressure switches, for example are NC high and NO low. Loss of voltage, cut cables, loose wires all protect the system and shutdown or stop pumps, etc.

Dear Prakash,

High Level----------> closed contact (without liquid inside the tank)
Low Level-----------> open contact (without liquid inside the tank).

Both are because fail safe state, when a high level switch is wired to closed contact, it will be always closed and the PLC/BMS can see if the wire is opened or fuse is bad nobody can touch it without the system activating. The low level is wired to the open contact because always the level of the tank/drum will be higher that the low level of the switch, therefore the open contact will be closed and it is possible to detect any problem with the wire or fuse, etc. If you use a closed contact for low level this will be open, because the liquid level gets its normal level inside the drum and this closed contact will open. Anybody can loosen the wire or take off the fuse and the system doesn't actuate it.

In my country we call that fail security (falla segura).

Regards,
J.Pinzon