Engineering Expert Witness Blog

      When we discussed basic electric relays last week we talked about their resting in a so-called “normal state,” so designated by industrial control parlance.  It’s the state in which no electric current is flowing through its wire coil, the coil being one of the major devices within a relay assembly.  Using Figure 3 of my previous article as a general reference, in this normal state a relaxed spring keeps the armature touching the N.C. switch contact.  While in this state, a continuous conductive path is created for electricity through to the N.C. point.  It originates from the wire on the left side, which leads to the armature pivot point, travels through the armature and N.C. contact points, and finally dispenses through the wire at the right leading from the N.C. contact.

     Now let’s look at an alternate scenario, when current is made to flow through the coil.  See Figure l, below.

Figure 1

     Figure 1 shows the path of electric current as it flows through the wire coil, causing the coil and the steel core to which it’s attached to become magnetized.  This magnetization is strong, attracting the steel armature and pulling it towards the steel core, thus overcoming the spring’s tension and its natural tendency to rest in a tension-free state.

     The magnetic attraction causes the armature to rotate about the pivot point until it comes to rest against the N.O. contact, thus creating an electrical path en route to the N.O. wire, on its way to whatever device it’s meant to energize.  As long as current flows through the wire coil, its electromagnetic nature will attract the armature to it and contact will be maintained with the N.O. juncture.

     When current is made to flow through the wire coil, an air gap is created between the armature and the N.C. contact, and this prevents the flow of electric current through the N.C. contact area.  Current is forced to follow the path to the N.O. contact only, effectively cutting off any other choice or fork in the road as to electrical path that may be followed.  We can see that the main task of an electric relay is to switch current between two possible paths within a circuit, thereby directing its flow to one or the other.

     Next time we’ll examine a simple industrial control system and see how an electric relay can be engaged with the help of a pushbutton.

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