3phase delta ?

[bure961]

we have a transformer labeled as a 3 phase Delta on our service. A phase to ground is zero volts b and c phase read 480 v. I believe this is called a corner ground connection but i am not sure . The grounded a phase is never changed after the service . Is it required to be changed to b or c phase at some point ? And is it hot phase that can course shock or worst ? Any information will be very help full.

 

[brian john]

Operates no differently than a Grounded Wye. You can operate a wye with any phase grounded if you lift the neutral to ground bnond. That would be a violation of the NEC but it would work and you would not have any more risk from shock.

Other than the level of voltage.

 

[Smart $]

we have a transformer labeled as a 3 phase Delta on our service. A phase to ground is zero volts b and c phase read 480 v. I believe this is called a corner ground connection but i am not sure . The grounded a phase is never changed after the service . Is it required to be changed to b or c phase at some point ? And is it hot phase that can course shock or worst ? Any information will be very help full.

I believe most corner grounded systems are wired similar to a 1? with neutral... only the ungrounded phases have ocpd's and the grounded conductor is connected via busbar. I am not aware of any requirement for line to letter correlation. The grounded conductor to ground potential will be similar to any grounded conductor system.

 

[meternerd]

Corner grounded 480

Corner grounded 480

You have a corner grounded 480 volt system. Three wire delta. The grounded phase has zero volts to ground, the other two phases have 480 volts to ground. The utility will install a Form 12S meter, which is set up so that the 5th jaw is connected to B phase, so that is normally the one that is grounded. But in actuality, any phase labeling is meaningless as long as the grounded phase is connected to the horizontal 5th jaw of the meter socket. Downstream labeling should be consistent with the service entrance phasing.

As discussed in many previous posts, corner grounding 480 is a practice that has differing opinions as to its safety. In the industrial field I work in, which is a drinking water and electric utility, 480 delta is never grounded and is replaced with 277/480 Wye whenever finances permit. Reasons for grounding are that a ground fault will trip the breaker if either ungrounded phase faults to ground. But...in incedental contact between ground and an ungrounded phase is basically a phase to phase event. The drawback to an ungrounded system is that Code requires ground fault indication and if the system is not monitored, that fault can remain undetected. The upside is that an incidental contact will not be nearly as serious. As an electrician working around this stuff, I kinda hate both setups. 277/480 Wye is a better way to go, but more expensive due to the extra wire for the neutral. just my opinion.

 

[augie47]

It might be worth investigating or inquiring if the one phase is intentionally grounded or has accidentally gone to ground.
As noted above, if it's intentionally grounded then the wiring would be installed in a manner indicating such.

 

[bure961]

The utility that serves the company I work for had a service from a pole then road service removed the pole and installed a pad mounted trans with no changed in the 3 phase corner grounded service. Should the utiliy have insisted or changed to a 4 wire service or insisted on a ground detection ? Once a generator was installed because of utility failure and a 3 wire delta was supplied from the generator and it was not a corner grounded service. Does anyone recommend this be changed to 4 wire 480volt service ?

 

[Hv&Lv]

The utility that serves the company I work for had a service from a pole then road service removed the pole and installed a pad mounted trans with no changed in the 3 phase corner grounded service. Should the utiliy have insisted or changed to a 4 wire service or insisted on a ground detection ? Once a generator was installed because of utility failure and a 3 wire delta was supplied from the generator and it was not a corner grounded service. Does anyone recommend this be changed to 4 wire 480volt service ?

I would think the problem here would be separating the grounds if you had a wye installed. Then would you pull new conductors throughout the buildings to supply the separate ground?

 

[brian john]

I would think the problem here would be separating the grounds if you had a wye installed. Then would you pull new conductors throughout the buildings to supply the separate ground?

Why would seperating the grounds be difficult or required?

 

[augie47]

The utility that serves the company I work for had a service from a pole then road service removed the pole and installed a pad mounted trans with no changed in the 3 phase corner grounded service. Should the utiliy have insisted or changed to a 4 wire service or insisted on a ground detection ? Once a generator was installed because of utility failure and a 3 wire delta was supplied from the generator and it was not a corner grounded service. Does anyone recommend this be changed to 4 wire 480volt service ?

If the building is wired for a grounded delta, then changing to a 4 wire could be an expense (3 pole vs 2 pole etc) and had POCO insisted on 4 wire they would be the "bad guy". Ground detectors are not needed as you have a ground,,,,it happens to be one phase.
If the generator output is delta, you should have no problem matching it to your corner grounded system by grounding one phase,

 

[Hv&Lv]

Why would seperating the grounds be difficult or required?

Separating the grounds was a bad choice of words. Making the grounded phase conductor to a ground only conductor will require another conductor. To change a buildings entire setup to a four wire 277/480 when it is a 480 CGD? Pull in another conductor, conduit to small, change the conduit. It wouldn't be simple or cheap.