240 volt ungrounded Delta

[charlest]

There are 6 ungrounded Delta seperately derived systems in our new Tec Ed Center. Do you think that ungrounded systems may be installed in a school?

 

[George Stolz]

Yes.

Why do you think they shouldn't?

 

[hardworkingstiff]

Yes.

Why do you think they shouldn't?

If I understand this correctly, any ungrounded system will allow one phase to have a fault to ground without opening an overcurrent device. So I believe the only safe way to install an ungrounded system is with a ground fault detection system.

I believe the cost of the ground fault detection system would be much higher than grounding the system. Then there is the maintenance of the ground fault system. It's no good if it doesn't work.

 

[iwire]

If I understand this correctly, any ungrounded system will allow one phase to have a fault to ground without opening an over current device.

Yes.

So I believe the only safe way to install an ungrounded system is with a ground fault detection system.

It might even be a requirment...250.21

I believe the cost of the ground fault detection system would be much higher than grounding the system.

Actually the 'system' can be very cheap, perhaps just 6 standard lamps and sockets.

That said I don't believe a school is the place for an ungrounded system.

 

[infinity]

I'm curious as to why someone would design a ungrounded system for a school? Seems like the purpose of using an ungrounded system does not fit into this application.

 

[hardworkingstiff]

Yes.

Why do you think they shouldn't?

The more I think about this, I second Georges' question.

Actually, I have another one. Is an ungrounded system more dangerous than a grounded system? If yes, why?

 

[iwire]

As I understand it the problems come when no one maintains the system.

I was under the impression that you must have 'conditions of maintenance' to ensure only qualified persons work on the ungrounded system.

Once a ground fault happens with no one to fix it you end up with corner grounded delta....almost.

I say almost as this connection to ground will not likely be a reliable solid connection as a bonding jumper would be.

I think the trend now is moving to impedance grounded systems.

 

[dlhoule]

What happens when A leg goes to ground on 1 piece of equipment and B leg goes to ground on an adjacent piece of equipment. I've seen the metal on 2 adjacent pieces of equipment have 460 Volts between them.

 

[iwire]

I've seen the metal on 2 adjacent pieces of equipment have 460 Volts between them.

That should not happen as the metal equipment is still required to be bonded together.

The second fault should open a OCPD.

That said I still can't see the reason to go this route in a school.

 

[George Stolz]

To be honest, I only said "yes" to help stir a discussion.

I cannot envision a school meeting the requirements that the NEC puts onto that system. Then again, who knows what could qualify as a "school"?

One thing I'd like to be certain of (which will betray my lack of knowledge of the system): an intact ungrounded system with a ground fault will still not shock someone, correct? If the source has no connection to earth, there is no circuit path that way, so there is no shock hazard in that way, right?

 

[iwire]

To be honest, I only said "yes" to help stir a discussion.

Trouble maker

Then again, who knows what could qualify as a "school"?

I agree.

an intact ungrounded system with a ground fault will still not shock someone, correct?

Basically what was ungrounded delta becomes corner grounded delta.

Even when installing ungrounded delta all the 'metal stuff' we normally bond together and earth are still required to required to be bonded and grounded.

That being the case there should be no chance of shock between earth and the metal stuff.

They would get a line to line voltage shock if they somehow contacted one of the remaining ungrounded phases and earth / metal stuff.

If the source has no connection to earth, there is no circuit path that way, so there is no shock hazard in that way, right?

I thought that as well....don't try it.

There is enough capacitance coupling of the system to earth that it will still 'light you up' if you get between a hot and grounded objects.

But it is not enough coupling to open and OCPDs.

So it saves equipment downtime but will still put you six feet under.

 

[George Stolz]

Trouble maker

That's my job.

There is enough capacitance coupling of the system to earth that it will still 'light you up' if you get between a hot and grounded objects.

But it is not enough coupling to open and OCPDs.

So it saves equipment downtime but will still put you six feet under.

Can you explain more?

 

[wireman3736]

I don't believe I have ever seen an ungrounded system in my area?, two questions come to mind, is this type of installation still allowed and if it is what would be the benefits of this type of system.

 

[winnie]

Think of it as 'phantom voltage' measured on an open conductor sitting next to an energized conductor in a conduit run.

Between all of the conductors of the system and 'earth' you will find parasitic (unintentional) capacitance, simply by the nature of having and electrode (all of the conductors of a phase) separated by insulation from another electrode (the earth and all bonded metal). A connection between any one phase and ground actually forms a closed circuit to the other phases and back to the source via this parasitic capacitance.

So an ungrounded system may present a shock hazard, depending upon how large it is and how much the 'capacitive charging current' is.

-Jon

 

[iwire]

I don't believe I have ever seen an ungrounded system in my area?

I only remember running into it once when I was still an apprentice.

two questions come to mind, is this type of installation still allowed

Yes.

and if it is what would be the benefits of this type of system.

The benefit is that when a circuit conductor shorts to ground none of the over current devices open.

If you have a factory you don't want a ground fault in a circuit shutting down equipment necessary for production.

However, there should be fault indicators and a people on site who know what the indicators mean and how to get the problem resolved.

With the correct tools and skill the ground fault can be located and repaired with little to no downtime.

If the fault is not corrected the next fault on an opposing phase will open at least one overcurrent device and will cause downtime.

I believe that High-Impedance Grounded Neutral Systems are becoming more popular than ungrounded delta but I have no hands on experience with either.

Don't bet your life on the above, it comes mostly from what I have read on the forum.

 

[George Stolz]

I don't quite get it, but I can sorta picture what Jon described. A chunk of unbonded metal connected to remote earth (from the GES's perspective) would be a conductor back to the GES, at a slightly different potential, hence the shock hazard.

Does that sound right?

 

[jboardman]

A good example of ungrounded systems is found in hospitals (isolated power). It is sometimes used to provide an extra level of safety in operating rooms and ICUs.

 

[dlhoule]

That should not happen as the metal equipment is still required to be bonded together.

The second fault should open a OCPD.

That said I still can't see the reason to go this route in a school.

Isn't the green wire on a 3 phase plug supposed to be doing this. When you are going through about 30' of #8 to get to 60 A fuses in Main Panels with 300 to 600 amp mains being fed from a bus plug with each machine being fed from a different bus. Which over current device do you think would operate?
Now keep in mind these may or may not be a real low resistance to ground where the ground is occurring.

 

[ardy]

Would this be the same system that the Navy used back in the 60's & 70's on shipboard? I think I heard it called a "radial ungrounded system"

 

[al vander ploeg]

240 volt ungrounded Delta

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There are 6 ungrounded Delta seperately derived systems in our new Tec Ed Center. Do you think that ungrounded systems may be installed in a school?

Just a Question.
Are they feeding delta- wye transfomers located locally thru-out the tech-ed center?