ungrounded delta systems

[Charlie Bob]

I've never worked onsuch a system but as i'm reviewing grounding and bonding something came to my attention.
as the name discribes it, the system is ungrounded, therefore a ground fault will not tri any OCP but a second will cause it will be a short fault of the two phases. So if all the metal parts of the system are connected together by a EGC to ground:
- how safe this system is against ground faults?
- I guess one should check potential between metal parts and ground before
working on them?
- Am i missing anything?

Thanks.

 

[Pierre C Belarge]

Typically an ungrounded system is supposed to have a monitor/alarm system to notify the maintenance team of the first ground fault.

But...safety is paramount.

 

[Charlie Bob]

Where do they instll them,Pierre, i mean the monitors.

 

[hurk27]

Where do they install them,Pierre, i mean the monitors.

Generally at the service or a SDS.

Most commonly use in industrial, where ground faults can pose an increase in danger of injury or critical production failure.

the alarm give a maintenance crew time to fix a fault with an orderly shut down.

 

[erickench]

An ungrounded system means that the EGC is not connected to the neutral however, the non-current carrying conductive materials are grounded to the earth. The earth should not be used as a return path back to the utility transformer which is also grounded to earth.

 

[tryinghard]

Ungrounded systems - as described in 250.2(B) - are delta 3-W systems with no neutral involved, not wye or delta 4-W system (see 250.20 (B)). They are intentional and created for use for critical machinery, to allow fault on non-current carrying items while only annunciating the fault to a monitored location. The fact there is no grounded transformer allows a fault to conductive items while no potential with these items, no reference like a bird on a wire.

A delta 4W transformer supplying distribution that eventually supplies an electrical component that is missing its ground is a violation the components circuit is not an “ungrounded system”. In this case if a short occurs there will be potential.

All the equipment grounding is the same as that of a grounded system.

 

[jim dungar]

Maybe a better name would be: un-bonded system.

After all, if it is metallic and could become energized, it must have a grounding conductor. But, if it is a normal current carrying conductor it does not always have to be bonded to a grounded conductor.

 

[tryinghard]

A while back I was at an old existing industrial cabinet shop. This shop is served by a wye system, 3 PH 4W XFMR, about 100 yards UG to an old 480V 3W distribution panel. This distribution panel supplies some 120/240V 3PH 4W panels through XFMR?s. The main distribution panel is grounded (its equipment anyway, again 3W only). This is an example of a grounded system in violation because its deviod of its grounded neutral conductor from its source.

This serving XFMR is grounded correctly again it is a wye 4W 480/277 with the XO neutral bonded to earth ground. In the example above if a fault occurs at the distribution or its feeds/branches current will try to return to the serving XFMR via all paths according to each level of resistance and in this case earth. Potential will exist on all non-current carrying items that are bonded and ohms law can prove a fault will remain without opening breakers leaving extremely dangerous touch voltage on all conductive items. This is a metal building as well.

The service lateral tapped to another building that included 4W correctly, and the cabinet shops service has since been corrected to include the grounded conductor.

My point is to show there may be systems out there that appear to be truly ungrounded but may not really be correct. The system is qualified by its source/supply not its presents or lack of the grounded conductor.

 

[sandsnow]

Ground detectors were always a good idea, but not required until recently. You may see older systems without detectors.