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#133185 06/21/02 03:45 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
The IEConsult website has some interesting technical papers. I thought these might be of particular interest:

#172: Earthing Systems in LV
A good overview of the various grounding methods used in Europe.

#173: Earthing systems worldwide & evolution
Some very interesting notes on the development of grounding through the years, with particular reference to British & French methods.

#178: IT System (Unearthed neutral)
An unusual distribution arrangement. Apparently used in Norway.

These are English translations of French academic papers, PDF format.

[This message has been edited by pauluk (edited 06-21-2002).]

#133186 06/22/02 04:59 PM
Joined: Oct 2000
Posts: 5,392
Paul, is #178 this( In the NEC)? it would seem so.....I find this system interesting in light of all the earthing we are usually held to....
I find it interesting that a level of R is to subscribe to a certain 'capactive coupling' (in #178 it's 1700 ohms) that would occur anyways....

250.36 High-Impedance Grounded Neutral Systems.
High-impedance grounded neutral systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a low value shall be permitted for 3-phase ac systems of 480 volts to 1000 volts where all the following conditions are met:

Section 250.36 covers high-impedance grounded neutral systems of 480 to 1000 volts. Systems rated over 1000 volts are covered in 250.186. For information on the differences between solidly grounded systems and high-impedance grounded neutral systems, see “Grounding for Emergency and Standby Power Systems,” by Robert B. West, IEEE Transactions on Industry Applications, Vol. IA-15, No. 2, March/April 1979.
As the schematic diagram in Exhibit 250.20 shows, a high-impedance grounded neutral system is designed to minimize the amount of fault current that can flow during a ground fault. The grounding impedance is usually selected to limit fault current to a value that is slightly greater than or equal to the capacitive charging current. This system is used where continuity of power is required. Therefore, a ground fault results in an alarm condition rather than in the tripping of a circuit breaker, which allows for a safe and orderly shutdown.

The conditions of maintenance and supervision ensure that only qualified persons service the installation.
Continuity of power is required.
Ground detectors are installed on the system.
Line-to-neutral loads are not served.
High-impedance grounded neutral systems shall comply with the provisions of 250.36
(A) through (G).
Grounding Impedance Location.

The grounding impedance shall be installed between the grounding electrode conductor and the system neutral. Where a neutral is not available, the grounding impedance shall be installed between the grounding electrode conductor and the neutral derived from a grounding transformer.
(B) Neutral Conductor
. The neutral conductor from the neutral point of the transformer or generator to its connection point to the grounding impedance shall be fully insulated.
The neutral conductor shall have an ampacity of not less than the maximum current rating of the grounding impedance. In no case shall the neutral conductor be smaller than 8 AWG copper or 6 AWG aluminum or copper-clad aluminum.
The current through the neutral conductor is limited by the grounding impedance. Therefore, the neutral conductor is not required to be sized to carry high-fault current. The neutral conductor cannot be smaller than 8 AWG copper or 6 AWG aluminum.
(C) System Neutral Connection.
The system neutral conductor shall not be connected to ground except through the grounding impedance.
FPN:The impedance is normally selected to limit the ground-fault current to a value slightly greater than or equal to the capacitive charging current of the system. This value of impedance will also limit transient overvoltages to safe values. For guidance, refer to criteria for limiting transient overvoltages in ANSI/IEEE 142-1991, Recommended Practice for Grounding of Industrial and Commercial Power Systems.
Additional information can be found in “Charging Current Data for Guesswork-Free Design of High-Resistance Grounded Systems,” by D. S. Baker, IEEE Transactions on Industry Applications, Vol. IA-15, No. 2, March/April 1979; and “High-Resistance Grounding,” by Baldwin Bridger, Jr., IEEE Transactions on Industry Applications, Vol. IA-19, No. 1, January/February 1983. [/b]

(D) Neutral Conductor Routing.
The conductor connecting the neutral point of the transformer or generator to the grounding impedance shall be permitted to be installed in a separate raceway. It shall not be required to run this conductor with the phase conductors to the first system disconnecting means or overcurrent device.
(E) Equipment Bonding Jumper.
The equipment bonding jumper (the connection between the equipment grounding conductors and the grounding impedance) shall be an unspliced conductor run from the first system disconnecting means or overcurrent device to the grounded side of the grounding impedance.
(F) Grounding Electrode Conductor Location.
The grounding electrode conductor shall be attached at any point from the grounded side of the grounding impedance to the equipment grounding connection at the service equipment or first system disconnecting means.

[This message has been edited by sparky (edited 06-22-2002).]

#133187 06/23/02 08:35 AM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
That certainly looks like the same basic arrangement.

The NEC rule 250.36 seems to be limiting the system to 480V and above, whereas the French document is talking about it for all LV systems. (Note that according to the document the IT system is required in French operating theatres.)


(4) Line-to-neutral loads are not served.
This seems to be adding an extra restriction which isn't mentioned in the French paper either.

Maybe the NEC reasoning is that if no line-neutral loads are served then there's no need to distribute the neutral on branch circuits and there's therefore less chance of the neutral being grounded out by a fault. Sound plausible?

By the way, the relatively new European designations such as IT, TT, TN-C etc. might be a little confusing to Americans, so perhaps I should explain the system:

1st letter indicates the grounding arrangements of the neutral at source: T means solidy grounded, I means insulated from ground or grounded via an impedance.

2nd letter is the grounding arrangement at the building: T=separately grounded to earth, N=grounded to the neutral.

Subsequent letters indicate whether the neutral and protective ground paths are separate or combined, or a combination of both.

Thus in the U.K. all public LV supplies have a grounded neutral, so the first letter will always be T. We have:

TT = Building grounded just to a local rod.

TN-S = Grounding to neutral point of xfmr, but ground path is kept separate from the neutral throughout its length (i.e. a 5-wire distribution system).

TN-C-S = Building ground is to the incoming neutral, but separate from that point onward.

Under this system, a normal American residential or commercial wye supply would also be classed as TN-C-S.

[This message has been edited by pauluk (edited 06-23-2002).]

#133188 06/23/02 03:21 PM
Joined: Oct 2000
Posts: 5,392
yes Paul....very plausible.

It would also seem that the diverse nature of European configurations would accomadate many a scenario the NEC ( now lauding itself as 'international'in the 02' edition) cannot.....

#133189 06/23/02 04:41 PM
Joined: Aug 2001
Posts: 7,520
pauluk Offline OP
Although the IT (ungrounded neutral) system has been banned for public supplies here since about the 1920s, our IEE Regs. certainly cover it, mainly as it might relate to a private generator plant.

Our Regs. changed their format radically when the 15th edition came out in 1981. That was supposedly in order that they be rearranged to suit an IEC international format, paving the way toward eventual adoption of a common set of rules right across Europe. It hasn't happened yet, but things are certainly still moving in that direction, and at a ever-faster pace.

I have a copy of the 14th edition (1966) and its re-issue with metric conversion (1970), and to me the older format was far more logical and easy to understand.

#133190 06/23/02 08:07 PM
Joined: Apr 2002
Posts: 2,527
For the sake of US readers… The "#178 IT system" has some US counterparts and applies to 240-600V systems, but most often 480V.

Nota Bene: They are useless without {human-monitored!} ground detectors.

Commercially available hi-resistance systems {with fancy ground-detection in these two} are discussed at:

It is used extensively in Petrochem: John P. Nelson and Pankaj K. Sen, High-Resistance Grounding of Low-Voltage Systems: A Standard for the Petroleum and Chemical Industry, IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 35, NO. 4, JULY/AUGUST 1999

#133191 06/24/02 05:53 AM
Joined: Oct 2000
Posts: 5,392
interesting stuff here. the 1-3.5 amps surely is'nt an arbitrary figure...this must somehow be adjusted to the capactive coupling relevant to each individual system?

#133192 06/24/02 05:31 PM
Joined: May 2002
Posts: 1,716
Bjarney, we actually have a 120 volt counter part to the unearthed version of the IT system. Our medical isolation system. Our monitors are called LIM's verses PIM'S. the fault and alarm values are a little different also.


#133193 06/24/02 10:15 PM
Joined: Apr 2002
Posts: 2,527
Oh boy, sparky, you've struck a nerve—but not an excruciatingly painful one. You ask an intuitive question. It’s SoapBox time!

Some electrical folks [across the board in experience] seem to have misconceptions about these systems, even though they have been in heavy use for a good part of the last century. Warnings of their shortfalls have been published for at least half that time.

First—recognize the variations of "size" in ungrounded 480V systems. On the low end—15kVA bank serving an ancient 10hp hollow-shaft river-lift pump, a 5hp process-garbage screw conveyor, or a 2hp gear-drive fish ladder {via ~2 miles of tape-shielded EPR from the 13.8kV station bus in a 500kV switchyard.}. On the other extreme—an urban automobile- assembly plant with dozens of 3750kVA articulated secondary-unit substations; where each feeds several thousand feet of ‘sandwich’ bus duct.

Now—consider the correspondingly wide range of 480V-sysytem xero-sequence [“ground”] currents, based on the three naturally occurring capacitors that form from each phase to ground. Rule of thumb—the inescapable “charging” current that continuously flows from this characteristic is ROUGHLY 300 milliamperes to maybe 3 amperes per 1000kVA of serving-transformer capacity.

The resistor should be able to effectively damp overvoltages on the 480-volt gear. Note that increased grounding-resistor current in excess of charging current is not normally of concern, but it allows unneeded additional resistor power dissipation with no other advantage.

Roger—there are some real gnarley looking toroidal bobbin-wound transformers made for that purpose.

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