ungrounded system

A quote from 250.4 (B)UNGROUNDED SYSTEM (2) , "[equipment bonding] shall be connected together and to the supply system grounded equipment."

Does the "supply system grounded equipment" indicate that the supply system is system-grounded? Or does that just mean that the suuply system's equipment bonding is connected to earth? (because an ungrounded system's equipment bonding gets connected to earth, but not to the power source)

 

Even if you're working with an electrically ungrounded system, the equipment enclosers must be grounded. NEC 250.4(A)(2) says, "Non-current carrying conductive materials enclosing electrical equipment, or forming part of such equipment, shall be connected to earth so as to limit the voltage to ground on these materials." 250.4(B)(2) is essentially looking back at 250.4(A)(2), saying that any enclosures, conduit, etc, that forms the parts of your ungrounded system, must be connected to the supply system grounded equipment. The important words are "grounded equipment", which do not imply that the supply system has to be electrically grounded. What they want is a permanent, continuous, low impedance path for ground fault, from your equipment enclosures and parts, all the way back to the supply system's enclosures.

Even so, you may also be required to have additional grounding at your new / remote equipment, depending on the application. For instance, a feeder supplying an outbuilding disconnect from a grounded or ungrouneded supply system. This would require the disconnect enclosure to be bonded to the supply system enclosure ground, AND also requires a new, seperate grounding electrode at the disconnect itself.

hope this helps

 

thanks Erin for your answer.
I thought that is along the line of what the code "could" be saying.

It still annoys me why the people who write the code could not "uniform" the words they choose. For example, by "supply system grounded equpment" they open its definition wide open. I mean, without specifying that the supply system must be an ungrounded system, the neutral conductor could be a grounded equipment, as well as equipment grounding system and the electrode system. No wonder grounding is one of the most confused subject in electricity.

again thanks for help.

sung

 

Glad I was able to help. Here's a little more help: Reading the NEC code one section at a time, here and there, can lead to confusion. You have to remember that the code builds on itself. Think of it as a pyramid, with chapter 90 being the bottom, biggest block. Anything in any section of the code, is subject to restrictions in chapters prior to to the section you're reading.

With regard to your example of the neutral, the code defines the neutral as a groundED conductor, not grounded equipment. The ground wires are defined as groudING conductors, and hot wires are UNgrounded conductors. Your grounding SYSTEM could be a grounding electrode, a concrete encased ground or any other of about 7 approved grounding system methods (250.52).

Before you go any further in the NEC, read all of chapter 100 - Definitions. It'll only take about an hour, (if you're a slow reader like me), and you'll find that it's a great help in understanding what the heck they're talking about in later chapters.

 

Erin
thanks for the advice. I didn't know that the code specifically excluded neutral conductors from "grounded equipment."

I am afraid I still have the same question--the kind of situation where you thought you got the answer but later realized you really didn't.

Okay, what if the supply system is grounded (the neutral and electrode and equipment grounding are bonded), and the "supplied system" is to be ungrounded. According to the code, the ungrounded system's equipment grounding is to be connected to its own electrode AND TO THE supply system's grounded equipment.

If I am right so far, when I have a ground fault in the ungrounded (supplied) system, wouldn't that trip the supply system's overcurrent protection device, (since the equipment bonding of the secondary is ultimately bonded to the supply system's neutral conductor) therefore shutting down the supplying and supplied system?

thanks

 

The ungrounded system does not always have to have it's own grounding electrode. If the supplied system was not to be electrically grounded, then you would only have a hot and neutral, with no ground wires at all. The most common place you would find something like this would be a seperately derived system, where say a transformer is between your supply system and your ungrounded system. In this configuration you would not need the ungrounded system to have it's own grounding electrode. You'll find this quite a bit in manufacturing plant control power circuits.

In this case, the fault protection devices on the supplied side of the seperately derived system (transformer) would trip, with out effecting the fault devices from the supply side. Current is always trying to find it's way back to earth in a grounded system, and back to it's source (the transformer) in the ungrounded system. Therefore, if you've fused things correctly, you shouldn't have a "cascade" trip all the back to the original source, even though the equipment grounds are all connected to the same earth.

Are you working on a specific project?

Hope this helped.

 

why the neutral grounding is used in transformers and especially in current transformers