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(elimination of one switchgear pole and one conductor).
Disadvantages
1. If not multi-point earthed, and the neutral earth broken, the
exposed metallic part may have float voltage.
2. High earth fault level.
3. Intervene the operation of earth fault protective device.
4. Current operated type device is not appropriated, voltage
detected type could be employed.
5. Third and multiples of third harmonics circulate in the
protective conductor (TNC system).
6. The fire risk is higher and, moreover, it cannot be used in places
presenting a fire risk (TNC system).

(2b) TN-S:
• In TN-S system (the third letter S = Separate Neutral and Earth
Conductor) neutral of the source of energy is connected with earth
at one point only, generally near to the Source. The neutral and
Earth conductors are separately distributed to load.
• In This System Earthing connections must be evenly placed
along the length of the Neutral-(Earth) conductor to avoid potential
rises in the exposed conductive parts at Load Side if a fault occurs.
• This system must not be used upstream of a TNC system.

System characteristics
1. Low earth fault loop impedance.
2. High earth fault current.
Advantages
1. Use of over current protective devices to ensure protection
against indirect contact.
2. Earth fault protection device operates faster.
3. Allow multi point earth, better earthing continuity; minimize the
use of earth fault relay because of low earth fault loop impedance.
Disadvantages
1. Switching on occurrence of the first insulation fault.
2. The TNC system involves the use of fixed and rigid trunkings
3. Requires earthing connections to be evenly placed in the
installation so that the protective conductor remains at the same
potential as the earth.
4. A tripping check on occurrence of the insulation fault should be

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