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In transmission line faults, roughly 65% - 70% are asymmetric line-to-ground faults. [2] double line-to-ground fault - two lines come into contact with the ground (and each other), also commonly due to storm damage. In transmission line faults, roughly 15% - 20% are asymmetric double line-to-ground. [2]
A line-to-ground fault will cause a voltage sag until the protective switchgear (fuse or circuit breaker) operates. [2] [11] Some accidents in power lines such as lightning or a falling object can cause a line-to-ground fault. [11] Sudden load changes or excessive loads [11] Depending on the transformer connections, transformers energizing [6]
(The principle of double fault to danger requires either basic and additional insulation to fail or basic insulation and the connection to the protective earth to fail simultaneously before danger arises.) A typical example for a PELV circuit is a metal cased computer with a Class I power supply.
A ground conductor only carries significant current if there is a circuit fault that would otherwise energize exposed conductive parts and present a shock hazard. In that case, circuit protection devices may detect a fault to a grounded metal enclosure and automatically de-energize the circuit, or may provide a warning of a ground fault.
Lowering line sag at high temperatures can prevent wildfires from starting when power lines touch dry vegetation. [34] Although advanced lines can cost 2-4x more than steel, total reconductoring costs are less than half of a new line, given savings in time, land acquisition, permitting, and construction. [33]
A ground fault protection relay must trip the breaker to protect the circuit before overheating of the resistor occurs. High-resistance grounding (HRG) systems use an NGR to limit the fault current to 25 A or less. They have a continuous rating, and are designed to operate with a single-ground fault.
If after the first, swift reclose, the recloser closes onto a fault, it is likely that the fault is a secondary class of fault, vegetation contact or equipment failure. An overcurrent fault would indicate a line to line class fault, which can be confirmed by negative phase sequence overcurrent protection, whereas an earth fault can indicate a ...
Presence of uninterrupted ground fault may pose a significant safety risk: if the current exceeds 4 A – 5 A an electric arc develops, which may be sustained even after the fault is cleared. [28] For that reason, they are chiefly limited to underground and submarine networks, and industrial applications, where the reliability need is high and ...