Corner gounded Delta Question

[doof]

Hey everyone.
I was up on a roof today looking at a 4160/480v 3 wire corner grounded delta or grounded B delta system. The cables were in open air and missing the sheath in many places but thats not why I am here. This is the first time I have had any experience with the system. My question is more theoretical than anything else. So lets assume phase B is the grounded phase. I under stand how it works, 480 phase to phase and phase to ground except B which is zero phase to ground. so my questions are:
1. If I came into contact accidentally with the grounded phase B, nothing would happen correct? Since I would be grounded and there is zero potential between myself and the B phase?
2. Also would the same be true for an ungrounded system? If you "fault" an ungrounded system it more or less becomes a corner grounded delta. What is the potential to ground on an ungrounded system?

I only ask because if I am right, then wouldnt corner grounded delta and ungrounded systems be much safer systems? Are there hazards that arise which are not obvious at first?

That's all, thanks!

 

[infinity]

1) Yes.
2) Yes, that's one of the reasons you would use an ungrounded system, so that one phase ground fault would allow the system to continue operating.

 

[augie47]

Theory

Theory

in theory , you are correct...in the real world don't try it....(the touch trick)

 

[jim dungar]

I only ask because if I am right, then wouldnt corner grounded delta and ungrounded systems be much safer systems? Are there hazards that arise which are not obvious at first?

It depends on how you define safer? If you touch a current carrying conductor, you become part of the current path.

 

[doof]

When I say safer, I mean as far as getting wacked accidentally when working on the system. Is it a much safer system to maintain and generally be in proximity to?
Thx!

 

[don_resqcapt19]

You will get "wacked" if you touch a phase conductor of an ungrounded system. There is enough capacitance to create a potential to ground in most systems. Most of the ungrounded systems that I have worked on would pull in my solenoid voltage tester and that takes about 20 to 30 mA. More than enough current to be a serious shock hazard.
Don

 

[LarryFine]

1. Doof, the grounded conductor of a corner-grounded Delta is akin to the grounded neutral of a 3- or 4-wire grounded system. It will be at GEC voltage minus any voltage drop.

The biggest difference here is that a grounded neutral has at least a chance of actually being near zero volts, because of the tendency to at least approach having little load on it.

The corner-grounded Delta conductor, on the other hand, carries full line-to-line current, and is more likely to have an appreciable potential to earth, increasing shock hazard.

My recommendation: even though it's colored like a neutral, and bonded like one (once), treat it like a circuit conductor, which it is, like you should all grounded circuit conductors.


2. Read Don's post.

 

[doof]

Hey Larry, I guess your answer is why I am a little confused on the matter. When I put my wiggies on it, it reads zero volts to ground. It may have a ton of current on it, but it still has no potential in reference to me. In theory the only place that current will flow is to one of the other phases because that is the only place it has potential. Is that not true?

 

[brian john]

At the service the "B" phase has zero potential to ground. Due to voltage drop in the system (depending on load) the further you get from you get from the "B" phase to ground bond at the service the higher the potential between "B" phase and any grounded objects. Much like the numerous post on this site "Why do I have Voltage between my neutral and ground".

The ungrounded system as noted can have a capacitive ground and once again touching any exposed/open connection from any phase to ground can lead to a possible shock.

There are advantages and disadvantages to both systems. Corner grounded has a higher potential from ground to "A" or "C" phase, no neutral for lighting loads, Ungrounded systems have issues with over voltages, locating faults should one phase short to ground. There are some other issues others will chime in about regarding these systems.

The biggest advantage to the ugrounded system as I see it is the first faault does not shut down production. With the corner grounded system the only advantage I can think of is that is the system or transformer you have and must use, I dislike this type of installation, mainly from the fact it befuddles electricians.

 

[doof]

Ah that explains it very well. Thanks a bunch guys!