480V 3W delta ungrounded

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tryinghard

Senior Member
Location
California
I understand for some specific circuits, that are usually critical, the ungrounded system is used or wanted. And apparently with a ground grid a ground fault to case on a motor will have the same potential as ground therefore unless the potential is different there is no electrocution possibility? In other words it?s like a bird on a wire.

Are there other reasons like power factor or efficiency or? Why would the 3W be used rather then a 4W wye?
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The only reason for ungrounded systems is to prevent an OCPD from operating on the first "case fault".

Efficiency and power factor have nothing to do with it.
 

tryinghard

Senior Member
Location
California
Curious, what if a new air line was installed and not bonded, would it be safe to touch the shorting motor and the new air line?

I’m looking at 250-4(B)(3), if there were items not bonded wouldn’t there be a difference in potential causing possible electrocution?
 
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benaround

Senior Member
Location
Arizona
tryinghard,

With the 'shorted motor' , all the metal in the building that is bonded including

all the electrical equipment will have that phase voltage on it. But. it is an

ungrounded system so no circuit will complete to ground including the air line.
 

tryinghard

Senior Member
Location
California
benaround said:
tryinghard,

With the 'shorted motor' , all the metal in the building that is bonded including

all the electrical equipment will have that phase voltage on it. But. it is an

ungrounded system so no circuit will complete to ground including the air line.

Wow, that?s hard for me to grasp! I thought if the potentials are the same there is no chance for electrocution, but if the air line is not bonded it won?t have the same potential, the air line being bonded or not doesn?t matter?!!?

What if the ground grid, which bonds, has a 1 ohm resistance to earth and a 480V 3W ungrounded 5 HP motor shorts wouldn?t this enable a 480A draw (I=E/R) causing the 20A breaker to trip? Isn?t this shorting voltage going to want to travel through the 1 ohm resistance ground?
 

Smart $

Esteemed Member
Location
Ohio
tryinghard said:
What if the ground grid, which bonds,...
Bonds to what?

If no point of the supply has been bonded to ground intentionally, the first intentional bond or unintentional short to ground only acts as a voltage reference for the system. The balance of the system will then have a voltage to ground. prior to such grounding, there is no true voltage-to-ground differential. Connecting a high impedance meter may yield a reading of voltage, but it will be due to reactive coupling of the system t ground and not a "hard" connection.


...has a 1 ohm resistance to earth and a 480V 3W ungrounded 5 HP motor shorts wouldn?t this enable a 480A draw (I=E/R) causing the 20A breaker to trip? Isn?t this shorting voltage going to want to travel through the 1 ohm resistance ground?
Not if the short is the first "hard" connection of the system to ground. This would just create an open circuit path, as there would also need to be a path for shorting current to go through ground and back to the source?which is typically the last transformer's secondary windings.
 

K2500

Senior Member
Location
Texas
tryinghard said:
I thought if the potentials are the same there is no chance for electrocution, but if the air line is not bonded it won?t have the same potential, the air line being bonded or not doesn?t matter?!!?

I read that to mean that your air line is floating. That is, there is not a hard connection between your air line and the system. In that case, it would have no potential until it makes contact with the system. If it were bonded(building steel ect) it would have a potential equal to the phase voltage, but only in reference to the other two phases.
 

tryinghard

Senior Member
Location
California
K2500 said:
I read that to mean that your air line is floating. That is, there is not a hard connection between your air line and the system. In that case, it would have no potential until it makes contact with the system. If it were bonded(building steel ect) it would have a potential equal to the phase voltage, but only in reference to the other two phases.

But what if the air line has high impedance continuity to the bonding system?
 

tryinghard

Senior Member
Location
California
Smart $ said:
If no point of the supply has been bonded to ground intentionally, the first intentional bond or unintentional short to ground only acts as a voltage reference for the system. The balance of the system will then have a voltage to ground. prior to such grounding, there is no true voltage-to-ground differential .
(and your reply to my example of a low impedance ground receiving a short)
Smart $ said:
Not if the short is the first "hard" connection of the system to ground. This would just create an open circuit path, as there would also need to be a path for shorting current to go through ground and back to the source—which is typically the last transformer's secondary windings.

I have no trouble with “a bird on a wire” scenario because I can visualize the isolation. But I am having trouble visualizing the isolation with an ungrounded system while standing in a world of grounded items. At the moment I understand that because the source is ungrounded, a branch downstream can have one ground fault and be safe even though other systems may/may not be grounded nearby.

So because of the reference to ground on an ungrounded system, can a live voltage line lay on the ground with no hazard or usage?

I really appreciate everyone’s help with this! I’m trying to learn here and because of this ungrounded system I feel like I’m starting all over!
 
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K2500

Senior Member
Location
Texas
tryinghard said:
But what if the air line has high impedance continuity to the bonding system?

If you are refering to an impedance grounded system:

Current limiting resistors are used in HRG(high restance ground) equipment. So, that being said, you have current and you have continuity. Just not as much of either.

If not then:

You have current and continuity.

tryinghard said:
So because of the reference to ground on an ungrounded system, can a live voltage line lay on the ground with no hazard or usage?

There is always a hazard when electricity is involved, thats why I still have a job.
 

tryinghard

Senior Member
Location
California
Post #7
K2500 said:
I read that to mean that your air line is floating. That is, there is not a hard connection between your air line and the system. In that case, it would have no potential until it makes contact with the system. If it were bonded(building steel ect) it would have a potential equal to the phase voltage, but only in reference to the other two phases.
And Post #10
K2500 said:
If you are refering to an impedance grounded system:
Current limiting resistors are used in HRG(high restance ground) equipment. So, that being said, you have current and you have continuity. Just not as much of either.

No I am referring to a grounding system that includes electrodes connected to equipment grounding conductors that bond to conductive items. This building includes a grounding system even though it has 480V 3W ungrounded system; it also has other grounded systems as well.

The air line I?m using as example represents various mechanical lines and conductive items that are not floating intentionally but are also not bonded intentionally as per 250-104 & 250-4(A)(4)-(5). If these are not intentionally bonded yet thoroughly exist throughout the concrete and steel building there will be continuity but it will be unknown how much and accidental. Do these allow dangerous touch voltage in the event a short occurs from an ungrounded system?

K2500 said:
If not then:
You have current and continuity.

So does my scenario have dangerous touch voltage if ground fault occurs on an ungrounded system while surrounded by conductive items that are not know to be bonded?


K2500 said:
There is always a hazard when electricity is involved, thats why I still have a job.

I am currently understanding from all who have responded on this posting that dangerous touch voltage does not exist with 1 ground fault on a 480V 3W ungrounded system, even though other conductive items exist.

With that said, can a live bare wire lay on the ground or metal catwalk and not allow dangerous touch voltage and also not have current flowing?
 

benaround

Senior Member
Location
Arizona
Tryinghard,

Not recomnded but it is no different than the first fault situation. I'm going

to try in my own way to make you better understand this, remember I'm

just an Electricain also.

Use two cars for an example, each have a 12v system with the negitive

'grounded' or all the metal parts are 'bonded'. If you test from car1 pos. to

car2 neg. you get nothing ( ungrounded ) now jumper car1 neg. and car2

neg. ( first fault ) and you get potential from either car positive ( 2nd fault)


Like I said 'in my own way' , but maybe this will get you thinking how the

real ungrounded system is isolated from completing a circuit to ground.

The bird on the wire is more at risk the larger the wingspan it has.
 

tryinghard

Senior Member
Location
California
benaround said:
Tryinghard,
...Use two cars for an example, each have a 12v system with the negitive
'grounded' or all the metal parts are 'bonded'. If you test from car1 pos. to
car2 neg. you get nothing ( ungrounded ) now jumper car1 neg. and car2
neg. ( first fault ) and you get potential from either car positive ( 2nd fault)...maybe this will get you thinking how the real ungrounded system is isolated from completing a circuit to ground.

The bird on the wire is more at risk the larger the wingspan it has.

Thanks your example is what I needed it?s helpful! Now I understand with one short dangerous touch voltage is not a problem and current will not flow through ground causing overload. Voltage will be present and a 2nd short will cause a phase-to-phase short circuit. I just keep seeing the current wanting to flow to ground but I understand with an ungrounded system it won?t :-? :roll:

Notice what Mike Holt has to say about ungrounded system:
250-4(B) Ungrounded Systems.

Author?s Comment: According to IEEE 242, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (Buff Book), if a ground fault is intermittent, or allowed to continue on an ungrounded system, the system wiring could be subjected to severe system overvoltage, which can be as high as six or eight times the phase voltage. This excessive system voltage can puncture conductor insulation and result in additional ground faults. System overvoltage can be caused by repetitive charging of the system capacitance or by resonance between the system capacitance and the inductances of equipment in the system [7.2.5].

In addition, ANSI/IEEE 142, Recommended Practice for Grounding of Industrial and Commercial Power Systems (Green Book) states, ?One of the dangers of an ungrounded system is that system overvoltages can occur during arcing, resonant or near-resonant ground faults [1.4.2].? And, ?Field experience and theoretical studies have shown that arcing, restriking, or vibrating ground faults on ungrounded systems can, under certain conditions, produce surge voltages as high as six times normal.

The conditions necessary for producing overvoltage require that the dielectric strength of the arc path build up at a higher rate after each extinction of the arc than it did after the preceding extinction. This phenomenon is unlikely to take place in open air between stationary contacts because such an arc path is not likely to develop sufficient dielectric recovery strength. It may occur in confined areas where the pressure may increase after each conduction period.

Neutral grounding is effective in reducing transient voltage buildup from such intermittent ground faults by reducing neutral displacement from ground potential and reducing destructive effectiveness of any high-frequency voltage oscillations following each arc initiation or restrike [1.2.14].? Figure 250?27

(1) Grounding Electrical Equipment to the Earth. Metal parts of electrical equipment must be grounded to the earth by electrically connecting the building or structure disconnecting means [225.31 or 230.70] with a grounding electrode conductor [250.64(A)] to a grounding electrode [250.52, 250.24(D), and 250.32(A)].


Author?s Comments:

Metal parts of the electrical installation are grounded to the earth to reduce voltage on the metal parts from lightning so as to prevent fires from surface arcs within the building or structure. Grounding equipment to the earth doesn?t provide a low-impedance fault-current path to the source to clear ground faults. In fact, the Code prohibits the use of the earth as the effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].


Grounding metal parts to the earth doesn?t protect electrical or electronic equipment from lightning voltage transients on the circuit conductors. To protect electrical equipment from high-voltage transients, proper transient voltage surge-protection devices must be installed in accordance with Article 280 at service equipment, and in accordance with Article 285 at panelboards and other locations.
(2) Bonding Wiring Methods to the Metal Enclosure of the System. To remove dangerous voltage from a second ground fault, metal parts of electrical raceways, cables, enclosures, or equipment must be bonded together and to the metal enclosure of the system.

(3) Bonding Conductive Materials to the Metal Enclosure of the System. Electrically conductive materials that are likely to become energized must be bonded together and to the metal enclosure containing the system.

(4) Fault-Current Path. Electrical equipment, wiring, and other electrically conductive material likely to become energized must be installed in a manner that creates a permanent, low-impedance fault-current path from any point on the wiring system to the electrical supply source to facilitate the operation of overcurrent devices should a second ground fault occur on the wiring system.

Author?s Comment: A single ground fault cannot be cleared on an ungrounded system because there?s no low-impedance fault-current path to the power source. However, in the event of a second ground fault (line-to-line short circuit), the bonding path provides a low-impedance fault-current path so that the circuit-protection device will open to clear the fault.
 
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