The Physics of... MV System-Neutral Grounding

I've taken the liberty of submitting this topic to address CSA's, Cephas', and Robbyk's questions... paraphrasing, "Why is the MV system-neutral grounded, especially when a 4th or neutral-conductor is never required!" If it were possible to insulate electrical equipment with glass, this discussion would hardly be necessary.

Early in electrical history ungrounded poly-phase systems were used somewhat optimistically, that is, it was possible to maintain operation with a one-phase grounded. That is, until such operation leads to over-voltages, both transient and continuous, resulting in a substantial increase in subsequent damage. (The Control List Archive contains numerous cases.) Although, somewhat to this discussion it is interesting to note that LV ungrounded systems are mandated by the USA NEC (National Electric Code), but only in specific

The premise behind system-neutral grounding, at any voltage level, is to mitigate the effects of ground-fault current flow, i.e., burned-insulation, or melted magnetic-iron. A LV system-neutral can certainly be grounded through an impedance to limit fault-current magnitude (of the order of ten's of thousands of kVA) but solid-grounding eliminates the need to install additional ground-fault detectors or breakers. The phase-over-current protective device is sufficient. Repair or replacement of LV equipment is easily handled, both in material and cost!

Any MV system-neutral can be solidly-grounded, but the resultant ground-fault current is much greater than LV systems (of the order of 100's of thousands of kVA.) Also, if damage is kept low, repair is possible. If, however, current magnitude is very large, the magnetic structure damage may preclude repair, but instead will require total replacement. Thus, impedance-grounding of a MV system-neutral is more of an economic choice. Almost with certainty, the Benefit-to-Cost Ratio justifies its implementation.

Cephas... an earthing (or Zig-Zag) transformer is considered an impedance-earthed system

Regards, Phil Corso (cepsicon [at]
Many an electrician of mine have asked me this question, i.e. if neutral and earth theoretically do the same function, then why two separate conductors? I say to them practical considerations and electrical standards of safety determine them to be separate.

Responding to Voltman's 15-Jun-09 (15:17) comments... the Earth (Ground-ING conductor) and the Neutral (Ground-ED conductor), purposely hyphenated to conform to NEC jargon, do not have the same function in power systems.

The former, the Earth, performs a safety function, and by Standard can not carry current continuously. The operative-word for power systems is "continuously!"

The latter, the Neutral, is designed to carry unbalanced current present in virtually every LV, 3-phase, 4-wire system.

See Control List Thread # 102628374 for detail.

Regards, Phil Corso (cepsicon [at]

Thank you, Phil Corso.

I understand a little more than I did before.

I hope the originator of the thread that precipitated this one will find this.
The flames are getting hotter!! In the past, I have observed this strange fondness exhibited by Mr. CSA towards Mr. Corso. I still remember the reference to the latter as, 'Exclamation Pointer'.