In static, a body is said to be in equilibrium when the force system acting upon it has a zero resultant.
 

Conditions of Static Equilibrium of Concurrent Forces

The sum of all forces in the x-direction or horizontal is zero.

  or  

The sum of all forces in the y-direction or vertical is zero.

  or  

Important Points for Equilibrium Forces

• Two forces are in equilibrium if they are equal and oppositely directed.
• Three coplanar forces in equilibrium are concurrent.
• Three or more concurrent forces in equilibrium form a close polygon when connected in head-to-tail manner.