Ungrounded System

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don_resqcapt19 is right.

When I'm challenged why this is needed since it is ungrounded; my response is that the code requires it, and I also present the situation where phase A faults to ground on one part of the system (nothing happens since it is ungrounded; just alarms) and at a latter time phase B faults to ground on a different part of the system. At this point you have a line-line-ground fault. The EGC at each fault location will have LLG fault current until the upstream protection trips. The EGC sizing needs to be the same as if it was a grounded system.
 
What do you mean that there is no bonding jumper? Should it be bonded in the service equipment? Or the the frame of the transformer?
 
poweringtech said:
What do you mean that there is no bonding jumper? Should it be bonded in the service equipment? Or the the frame of the transformer?
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There is no bonding jumper period, that it is why it is called a "Ungrounded Sytem. The whole idea is to prevent unnesecary outages from L-G faults. Ungrounded systems are used for mission critcal systems or life safety things like process control, drug manufacturing, elevators, pipelines, etc.
 
"Exactly the same as in a grounded system, except that there is no main bonding jumper." Don

What do you mean about "except there is no bonding jumper"?

How is equipment grounding done in an UNGROUNDED SYSTEM? Where will the bonding conductor terminate? Will it be in the service equipment or in the case of the transformer?
 
What do you mean by a main bonding jumper? Please define what is main bonding jumper for you?

If you say the equipment grounding is the same for all types of system grounding, please explain how it is bonded together? Where will it terminate? Will it be in the service equipment?
 
poweringtech said:
What do you mean by a main bonding jumper? Please define what is main bonding jumper for you?
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I'll do one better, and post the NEC definition in Article 100:
Bonding Jumper, Main. The connection between the grounded circuit conductor and the equipment grounding conductor at the service.
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If you say the equipment grounding is the same for all types of system grounding, please explain how it is bonded together? Where will it terminate? Will it be in the service equipment?
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Poweringtech, the key is that without a neutral, the first energized fault to ground will not kick a breaker. All metal in the structure becomes energized.

A second fault will kick a breaker, because it is a short circuit from phase to phase. Look at 250.4(B)(4):
(4) Path for Fault Current. Electrical equipment, wiring, and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low-impedance circuit from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices should a second fault occur on the wiring system. The earth shall not be considered as an effective fault-current path.
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The main bonding jumper is the wire, bussbar, etc., that bonds the grounded conductor (neutral) and the gounding conductor (equipment ground). In an ungrounded system this is intentionally left out so if any type of fault happens in a process critical situation. (a process that can be detrimental if not shut down in an orderly fashion) The first fault will trigger an alarm system to alert machine operators, and maintenance personelle to properly shut down the equipment in order to fix the fault condition. This first fault essentially creates a grounded system, the second fault will operate the overcurrent protection, resulting in shutting down the equipment.

Everything in an ungrounded system is wired exactly the same, EGC, GEC, service entrance, all non-current carrying metal parts must be effectively bonded together as to reduce all shock hazards. The only difference is, the neutral is not intentionally bonded to ground (aka main bonding jumper)

Hope this helps

Gerry
 
The main bonding jumper is generally a green machine screw that inserts thru the "neutral" bar into the enclosure to "bond" the enclosure to the grounding system or a copper "S" shaped strap that achieves the same by connecting to both enclosure and " neutral" bus bar.
 
georgestolz said:
. . . without a neutral, the first energized fault to ground will not kick a breaker. All metal in the structure becomes energized.
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George, wouldn't it be more accurate to say that the first fault to ground will 'lock' the otherwise-floating two non-grounded phases to a voltage-to-earth equal to the phase-to-phase voltage?
 
George,
That was a little harsh, wasn't it?
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Maybe so, but the poster asked a NEC question, but does not seem to have any knowledge of the document.
the key is that without a neutral, the first energized fault to ground will not kick a breaker. All metal in the structure becomes energized.
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No more so than when a system conductor is intentionally grounded.

Don
 
Larry, that sounds good.

Don said:
No more so than when a system conductor is intentionally grounded.
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You're right. I've never dealt with an ungrounded system, so I tend to forget that without a earth connection at the utility side, there is no return path via earth to the source. A single phase touching the EGC would not "energize" the EGC in the way I commonly think of it.

Thanks for the correction. :)

Don said:
Maybe so, but the poster asked a NEC question, but does not seem to have any knowledge of the document.
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On the brighter side, he didn't seem to be asking how to build his own, so... ;)
 
George, back when I was a little bitty helper, we installed a 480-to-208/120 transformer in a warehouse to feed a panel for new fluorescent lighting. We energized the panel before installing the neutral bond "because we need the light," as I was told. (I know, I know, but I was only a helper; it's not a habit I picked up.)

When the connection was made, there was a spark, and one of the lights went out. I immediately knew what happened, and it impressed the guy I was helping. I said that a ballast wire was pinched in that fixture, and the neutral was at the ballast's output voltage until the bond was made, which shorted that lamp's voltage.

Capacitance to earth is what maintains a floating system's balanced voltage, as long as a relatively high-input-impedance voltmeter is being used. Grounding a phase instantly makes the other phases snap to the line-to-line voltage to earth, and with a much lower impedance than capacitance alone could provide.
 
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