What is the earthed & unearthed system?.....

Hi Vishnu,
Where first priority is the protection against ground fault,there we use earthed system,it can interrupt your supply in case of fault.While in many industrial application we cannot tolerate the power failure there we use un earthed system,its advantage is that it will give you continuous supply,and disadvantage of no protection against ground fault.

Hope this work and let other professional to give their view.

Earthed system is system in which neutral conductor is connected with earth conductor. By other side, unearthed system is system in which neutral conductor isn't connected with earth conductor.

An unearthed system is defined as a system of conductors with no intentional
connection to earth.

Contrarily, an earthed system is a system in which there is an intentional connection of conductors (normally the neutral of the power source), either solidly or through an impedance, to earth.

Earthed and Unearthed system terms implies to a Star connected system.
In Earthed system Neutral point of Star connection is earthed, whereas in Unearthed system it is not carried out.

Hi Nuli, That's a wee bit incorrect. There could be grounded & ungrounded systems with delta connection too. Delta, inherently, is ungrounded, as you do not have a neutral. But, even in delta connections, one can have the system grounded either by:

i) Grounding thro an earthing transformer

ii) Corner of Delta Earthing

iii) Mid-point Earthing

Pl. refer to the IEEE GREEN BOOK.

Normally in Olden Days Generators & Transformers are less size. So Fault Current also less. The Star Point or Neutral Point is solidly earthed, so this system is called Earthed System.
In this system, If Earth Fault Occurs on any of the phases the voltages of other two healthy phases, with respect to the earth remains the same.

Now a days Generator & Transformer sizes are very large, so fault current will also be high. So To reduce the Fault Current the star point is earthed through resistance, reactance or ZigZag Transformer. If fault occurs in any one of the phase then the insulation of the particular phase will goes down. so rest of the healthy phases voltage will rise 1.732 times with respect to earth. So this system is called Un-Earthed System.

In addition V.R V. the technical term is Grounded and Ungrounded system. To enhance safety to the explanation given by V.R V, lines are protected by CT's, relays, reclosers and fuses and other such like protective devices. These devices are important due to the fact that fault current these lines could be significantly high.

Vishnu, you use the term "earthed", so I'm going to assume you have a supply transformer, 415 V, phase to phase, 240 V phase to neutral. The supply transformer winding on the secondary side is Y , or some people call it Star.
Each point on the Y represents the 3 phases, the Center point is zero vector sum. The Center point can be physically connected to Mother Earth, via copper earth rods, as under normal conditions, no current should be flowing. Electrical equipment, such as a 3 phase motor will be connected to each of theY points. However, single phase devices, TV, toaster, kettle will be connected between a point, and the Center, as that TV only wants 240 V, not the 415 version.
Life is good provided, the 3 phases are balanced.
However life is not always normal.

So we design for what happens when a short circuit fault happens.
Imagine, an earthed Y system. Imagine the TV connected to phase L1. Imagine a short circuit on Phase L2. The equipment on the 2 good phases is OK, because the Y Center point is connected to earth.
If the Y was NOT connected to earth, the other two phases, will raise their Voltage by an additional root 3 , (1.732 ), so the connected TV, will sizzle and usually burst into flames. Not good. Especially a transformer supplying a street full of houses.

That is the simplest version of an earthed Y supply transformer, and why the centre point is connected to Earth; and hopefully demonstrates the two different system configuration.
Does that make sense?

There are many methods for earthing a delta system:

i) By earthing one corner of the Delta
ii) By earthing the mid-point of one of the three delta windings

The above two are mostly practised in USA.

iii) By earthing the bus of a delta connected system by using an earthing transformer like:

a) A zig-zag earthing transformer
b) A star-delta earthing transformer
c) A star-broken delta earthing transformer

Hi Everyone,

In one of our project we stepping up the DG-generated emergency supply volatge to 11kV from 415v through a step-up xmer 415v/11kV to reduce the Vd as the load center has a considerable long length from the source. And stepping down the voltage back to 415v by using step down 11kV/415V Dyn11  on the Load side.

The step up xmer is 415V/11kV and star-delta connected. The vector group used is YNd1 and We are solidly grounding the star point of 415v primary winding. I believe by doing this the Primary side of the Xmer falls under earthed system, however I'm confused whether the system is earthed of un-earthed on the secondary delta side as it is not connected to ground by any means.

Request your remarks on the above problem.

Prashant Nimbade

If primary 415v earthed means the system is earthed system.