Very low-impedance ground connection
As is evident from the name, a solidly grounded system is one where the neutral of the system is directly connected to ground without introducing any intentional resistance in the ground circuit.
With appropriate choice of the type and number of grounding electrodes, it is possible to obtain a very low-impedance ground connection, sometimes as low as 1Ω.
A solidly grounded system clamps the neutral tightly to ground and ensures that when there is a ground fault in one phase, the voltage of the healthy phases with reference to ground does not increase to values appreciably higher than the value under the normal operating conditions.
The advantages of this system
1. A fault is readily detected and therefore isolated quickly by circuit protective devices. Quite often, the protection against short circuit faults (such as circuit breakers or fuses) is adequate to sense and isolate ground faults as well.
2. It is easy to identify and selectively trip the faulted circuit so that power to the other circuits or consumers can continue unaffected (contrast this with the ungrounded system where a system may have to be extensively disturbed to enable detection of the faulty circuit).
3. No possibility of transient overvoltages.
The main disadvantage is that when applied in distribution circuits of higher voltage (5 kV and above), the very low ground impedance results in extremely high fault currents almost equal to or in some cases higher than the system’s three-phase short circuit currents.
This can increase the rupturing duty ratings of the equipment to be selected in these systems.
For these reasons, use of solid grounding of neutral is restricted to systems of lower voltage (380 V/480 V) used normally in consumer premises. In all the other cases, some form of grounding impedance is always used for reducing damage to critical equipment components.
2014 NEC – Grounding Electrode System (VIDEO)
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Reference: Practical Grounding, Bonding, Shielding and Surge Protection G. Vijayaraghavan, Mark Brown and Malcolm Barnes (Buy hardcopy from Amazon)