Solidly Neutral Grounded Systems

Solidly Neutral Grounded Systems:

§  Solidly grounded systems are usually used in low voltage applications at 600 volts or less.

§  In solidly grounded system, the neutral point is connected to earth.

§  Solidly Neutral Grounding slightly reduces the problem of transient over voltages found on the ungrounded system and provided path for the ground fault current is in the range of 25 to 100% of the system three phase fault current. However, if the reactance of the generator or transformer is too great, the problem of transient over voltages will not be solved.

§  While solidly grounded systems are an improvement over ungrounded systems, and speed up the location of faults, they lack the current limiting ability of resistance grounding and the extra protection this provides.

§  To maintain systems health and safe, Transformer neutral is grounded and grounding conductor must be extend from the source to the furthest point of the system within the same raceway or conduit. Its purpose is to maintain very low impedance to ground faults so that a relatively high fault current will flow thus insuring that circuit breakers or fuses will clear the fault quickly and therefore minimize damage. It also greatly reduces the shock hazard to personnel

§  If the system is not solidly grounded, the neutral point of the system would “float” with respect to ground as a function of load subjecting the line-to-neutral loads to voltage unbalances and instability.

§  The single-phase earth fault current in a solidly earthed system may exceed the three phase fault current. The magnitude of the current depends on the fault location and the fault resistance. One way to reduce the earth fault current is to leave some of the transformer neutrals unearthed.

§  Advantage:

1.     The main advantage of solidly earthed systems is low over voltages, which makes the earthing design common at high voltage levels (HV).

§  Disadvantage:

1.     This system involves all the drawbacks and hazards of high earth fault current: maximum damage and disturbances.

2.     There is no service continuity on the faulty feeder.

3.     The danger for personnel is high during the fault since the touch voltages created are high.

§  Applications:

1.     Distributed neutral conductor.

2.     3-phase + neutral distribution.

3.     Use of the neutral conductor as a protective conductor with systematic earthing at each transmission pole.

4.     Used when the short-circuit power of the source is low.