Earthed and Unearthed System

Introduction:

Earthing is the process of connecting the current carrying and non-current carrying metal parts of an electrical system to dissipate the currents to the ground. Even though earthed system has so many advantages, the unearthed system is preferred for safety-critical applications, such as intensive care units, where a failure of the power supply would have disastrous consequences.

IS 3043 explains about the factors influencing the choice of earthed or unearthed system and the details are as follows.

Earthed or Unearthed System:

IS 3043 explains about the following factors for comparing the earthed and unearthed systems. They are,

  1. Service Continuity
  2. Multiple Faults to Ground
  3. Arcing Fault Burndowns
  4. Location of Faults
  5. Safety
  6. Abnormal Voltage Hazards
  7. Cost

The main objective of earthing is to ensure the safety of operating person & equipment, reference for the system voltage, dissipating the impulse lightning current and discharge the static charges.

Service Continuity:

Protection against electric shock during any fault can be achieved by automatic disconnection of supply during the fault conditions. Earthed systems are in most cases designed so that circuit protective devices will remove the faulty circuit from the system regardless of the type of fault.
A number of industrial plant systems have been operated unearthed at one or more voltage levels due to the thought of gaining an additional degree of service continuity varying in its importance depending on the type of plant.
But the greater service continuity has been experienced with earthed-neutral than with unearthed neutral systems.

Multiple Faults to Ground:

When a ground fault occurs on an earthed system, the circuit protective devices will remove the faulty circuit from the system and multiple ground faults are very rarely experienced.
In an unearthed system, if ground fault on one phase remains for a longer duration, chances for fault to occur in another phase are very high. A ground fault on one phase of an unearthed system does not cause a service interruption. The occurrence of a second ground fault on a different phase before the first fault is cleared, does result in an outage. Hence an organized maintenance is very essential in an unearthed system to locate and handle the faults at the earliest.

Arcing Fault Burndowns:

Arcing ground faults are extremely destructive when not detected and rectified properly. In typical cases, an arcing fault becomes established between two or more phase conductors in an unearthed system or between phase and ground in a solidly earthed-neutral system.
The arcing fault current levels are very low that phase overcurrent protective devices do not operate to remove the fault quickly. It is generally recognized that protection against the arcing fault current can be achieved by fast and sensitive detection of faults and interrupting the fault within 10-20 cycles.
In solidly earthed neutral systems, an arcing fault would produce a current in the ground path and by sensing the current we can detect and trip the circuit against phase-to-ground arcing fault breakdowns.

Location of Faults:

In an earthed system, an accidental ground fault is both indicated at least partially located by an automatic interruption of the accidentally grounded circuit or piece of equipment.
In an unearthed system, a ground fault does not open the circuit. Some other means of detecting the presence of a ground fault requires to be installed. Insulation Fault Location Systems can be used to detect the faults in an unearthed system.

Safety:

Whether a system is grounded or not, protection of personnel and property from hazards require thorough grounding of equipment and structures.
Safety of operating person and equipment is one of the main objectives of earthing,
Proper grounding results in less likelihood of accidents to personnel.
Other hazards of shock and fire may result from inadequate grounding of equipment in unearthed and earthed systems.

Abnormal Voltage Hazards:

The possible over-voltages on the unearthed system may cause more frequent failures of equipment than in the earthed system.
A fault on one phase of an unearthed or impedance grounded system places a sustained increased voltage of about 1.73 times the normal voltage on the insulation of ungrounded phases in a 3-phase system.
The sustained over-voltages on the unearthed system may not immediately cause failure of insulation but may tend to reduce the life of the insulation.

Some of the more common sources of over-voltages on a power system are the following:

a) Lightning,
b) Switching surges,
c) Static,
d) Contact with a high voltage system,
e) Line-to-ground fault,
f) Resonant conditions, and
g) Re-striking ground faults.

Lightning is a natural source of over voltage. Surge arresters are recommended for lightning protection. Neutral grounding is effective in reducing transient build up by reducing the neutral displacement from ground potential and the destructiveness of any high frequency voltage oscillations following each arc initiation or restrike.

In an unearthed system resonant over-voltages and arcing, re-striking or vibrating ground faults can produce surge voltages as high as 6 times the normal voltage.

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