Transformer is galvanic isolation device which transfer AC electrical power by mutual inductance. Primary and secondary winding are insulated from each other. so there is no electrical connection between primary and secondary winding. Transformers are main part of our electrical power transmission system. Transformer are use to step-up and step-down AC voltage for long distance power transmission.
Consumer electric connection
Every domestic customer get electric power connection from 11KV-220V or 110V step down transformer. This transformer is shared for limited numbers of customer connections. One terminal of transformer secondary output is connected to earth pit by electric supply corporation. This terminal line is called neutral. And second or third secondary terminal lines are Phase. Third terminal exist where transformer is center tapped. Center tapped terminal use as neutral.
Difference between Ground and Neutral
Every consumer have own separate Earth pit/connection. In house wiring have Neutral, Phase and Earth lines. Earth and Neutral does not electrically connected but there should be nearly equal voltage between Neutral-Phase and Earth-Phase terminal. Good earthing give less then 10V AC voltage difference. This phenomenon prove that our earth is good conductor.
What is isolation transformer / isolated transformer consumer connection
When a customer need non-earthed electric connection. Then electric supply corporations provides electric connection from an isolated transformer which is not shared and specifically designed for only one consumer-connection with highly insulated from earth.
Reasons behind isolated electric connection
Customer may desire isolated electric connection for the sake of safety from electric shock. Or may be for special requirement of electric appliances or electricity control requirement.
How we get Electric shock
Human body consist on many minerals and water. So human body is mildly electrical conductive. Small electric signals flows through human body. But when a certain amount of electric current pass through human body human body feel electric shock. Due to sudden tissue burn and damage by electric current. Following picture describes how human body pass electric current.
we can note that one body point is touched to phase and second point is touched to earth or floor. So current will flow from easiest path between two body points in touched with electric circuit.
How isolating transformer connection protect from electric shock
Following picture show that there is no return path for electric current to pass through human body. Because both outlet terminals are isolated from earth. and there are 0V between earth and electric outlet terminals.
Disadvantages of Isolation transformer connection
Initial cost of transformer and connection is very high then domestic connection.
How much current and voltage is lethal
Above 70V AC or DC may cause a death or serious injury or an unrecoverable damage to human body. Human body resistance change by many factors. For example humidity surface area of body which is connected to electric circuit. more than 10mA current is considered lethal. However current path is important. In some case low current pass through Heart, lungs or brain area, this low current may cause death. In some case high electric current pass through a short path and just burn the body. Body can be burned by electric current due to high power dissipation in body. Electric current burn tissues, minerals and tissues residual carbon make body more conductive. which increase speed of burning human body.