It can't be defined without more information as to the application.
In high impedance system grounding, the grounding impedance is sometimes selected to roughly equal the parasitic capacitive impedance to ground. The impedance is usually high enough that normal overcurrent protection will not trip.
Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.
A high impedance fault is a fault characterized with a system impedance ratio between the zero sequence reactance and the positive sequence reactance greater than 10.
Xo/X1 > 10
A ground fault of a high impedance fault is associated with a low short circuit current injected into the ground and relative high overvoltages in the unfaulted phase that can reach up to 2.73 PU.(1pu= [SQRT(2)/SQRT(3)]*Vmax)
Is the OP talking about a high-impedance fault or a high-impedance grounded system? cuky has defined a high-impedance grounded system.
A high-impedance fault is just a fault where the fault impedance (as opposed to the system impedance) is high. For example the equation for a ground fault is:
I0=I1=I2 = Vf / (Z0 + Z1 + Z2 + 3Zf)
The Zf term represents the impedance at the fault location. When this value is high (10ohms?, 40ohms?) it is sometimes described as a high-impedance fault.
I do not know of any official limit that makes it 'high-impedance'.
High impedance faults may not be common but cause problems for protection descrimination as it can be difficult to distinguish between load current and fault current in some cases.
In both high and low resistance grounding, the goal is to limit the magnitude of ground fault current to prevent equipment damage.
Low resistance grounding is generally for systems above 4.16kV where you want to limit the bolted fault current to some value no less than 50A. Generally where ground fault relay sensitivity is a concern.
High resistance grounding is generally for systems 4.16kV and below (distribution equipment) where you want detection only (generally not for breaker tripping) and you have a continuous industrial process. Generally, ground current is limited to 10A or less. This system is generally employed at the 480V level specifically. Helpful info is in the link below:
Any fault that does not allow enough current to flow to operate a protective overcurrent device might be classified as high impedance. Special types of relays such as AFCI or GFCI are needed to detect the faults.
Examples:
A 115 kV conductor falling on bedrock.
A 12.5 kV conductor falling on concrete or asphalt pavement.
A 240V street light conductor energizing a metallic handhold or vault in the sidewalk.
