Grounded "B" phase, is it safe?

[Paul G]

Is there anything unsafe about the old grounded "B" phase 480 3 wire systems?

 

[charlie b]

It is neither more safe nor less safe than any electrical system. Do you have a specific concern in mind?

Welcome to the forum.

 

[jim dungar]

Is there anything unsafe about the old grounded "B" phase 480 3 wire systems?

People often use standard equipment, which can lead to equipment failure, rather than special stuff designed for it.

But as a general rule, there is nothing that makes a grounded-B phase system more "unsafe" than other electrical systems.

 

[Paul G]

It is neither more safe nor less safe than any electrical system. Do you have a specific concern in mind?

Welcome to the forum.

We are upgrading a well site and the engineers are choosing to use the existing 3 wire 480 disconnect and meter panel. We had the power company power down so we could take a good look at the panel the other day. It is basically in decent condition. There is light cracking of the insulator material and some of the sealant covering recessed bolt heads is melted out. We cleaned and lubed the moving parts. It works, but we would like to have it replaced and switched over to a 4 wire with grounded nuetral. The transformer on site is already a 4 wire. The power Co. has the nuetral buss un-used, and grounded the B phase in the transformer secondary cabinet. So it is still 3 wire. I would assume wye secondary?

Safety is really the only way to trump the bean counters on this one. I dont really see anything unsafe about the panel as it is other than being almost 50 years old and sitting in the hot desert sun all that time.

 

[iwire]

It is very common even today to feed pump houses with corner grounded delta, making it a wye only adds costs not safety.

If the panel was bad I would simply replace it with another straight three phase panel.

 

[brian john]

We are upgrading a well site and the engineers are choosing to use the existing 3 wire 480 disconnect and meter panel. We had the power company power down so we could take a good look at the panel the other day. It is basically in decent condition. There is light cracking of the insulator material and some of the sealant covering recessed bolt heads is melted out. We cleaned and lubed the moving parts. It works, but we would like to have it replaced and switched over to a 4 wire with grounded nuetral. The transformer on site is already a 4 wire. The power Co. has the nuetral buss un-used, and grounded the B phase in the transformer secondary cabinet. So it is still 3 wire. I would assume wye secondary?

Safety is really the only way to trump the bean counters on this one. I don't really see anything unsafe about the panel as it is other than being almost 50 years old and sitting in the hot desert sun all that time.

Did you megger the bus phase to phase phase to ground?
Did you do any testing of the OCP's

Minor cracks can be MAJOR issues, though the desert may be hlping with the excessively dry air.

50 years old IMO may be excellent, but is working against you as far as any replacement components go.

 

[LarryFine]

We are upgrading a well site and the engineers are choosing to use the existing 3 wire 480 disconnect and meter panel.

It works, but we would like to have it replaced and switched over to a 4 wire with grounded nuetral. The transformer on site is already a 4 wire.

The power Co. has the nuetral buss un-used, and grounded the B phase in the transformer secondary cabinet. So it is still 3 wire. I would assume wye secondary?

Are you saying that the transformer has a Y secondary, but instead of grounding the neutral, they grounded one phase? That's weird.

A 480v Delta can have a grounded phase, but there is no neutral. Are you sure there is a Y neutral point, and they didn't ground it?

Before energizing a previously-grounded line, I would make sure there are no other grounded connections. Assume nothing!

 

[ceb58]

What about the EGC? Is there one? And would the "B" phase not have to be the no-blow type fuse?

 

[chris kennedy]

The power Co. has the nuetral buss un-used, and grounded the B phase in the transformer secondary cabinet.

Wouldn't you have a violation of 250.24(C)?

 

[iwire]

Wouldn't you have a violation of 250.24(C)?

No, not as long as the XO is left floating it is not the grounded conductor. B phase is the grounded conductor and has been brought to the service disconnect.

I believe that Charlie the utility guy has mentioned that this is what can happen when the POCO use the transformer they have on hand.

 

[iwire]

What about the EGC? Is there one? And would the "B" phase not have to be the no-blow type fuse?

You never run an EGC on that side of the service disconnect.

You are right, the B phase should not be fused and it also should be marked white.

 

[chris kennedy]

B phase is the grounded conductor and has been brought to the service disconnect.

Yup. Stand corrected.

 

[charlie]

Good configuration

Good configuration

. . . instead of grounding the neutral, they grounded one phase? That's weird. . .

No Larry, it is not weird. It is permissible by the NEC and the NESC, it is unusual but it is not weird.

250.20(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts.
A 480Y would exceed 150 volts so this does not apply.

(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor
A 480Y is not using the center point as a circuit conductor so this does not apply.

(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor
A 480Y is not a delta so this does not apply.

Therefore, it is permitted to ground a phase and float the center point. In places where the customer wants a three wire delta service and we only have wye transformers available, this is a common practice.

 

[quogueelectric]

No Larry, it is not weird. It is permissible by the NEC and the NESC, it is unusual but it is not weird.

250.20(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:

(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts.
A 480Y would exceed 150 volts so this does not apply.

(2) Where the system is 3-phase, 4-wire, wye connected in which the neutral conductor is used as a circuit conductor
A 480Y is not using the center point as a circuit conductor so this does not apply.

(3) Where the system is 3-phase, 4-wire, delta connected in which the midpoint of one phase winding is used as a circuit conductor
A 480Y is not a delta so this does not apply.

Therefore, it is permitted to ground a phase and float the center point. In places where the customer wants a three wire delta service and we only have wye transformers available, this is a common practice.

The only real danger is an untrained electrician not understanding what he is looking at can cause a world of problems.

 

[charlie]

The only real danger is an untrained electrician [is this an oxymoron? CKE] not understanding what he is looking at can cause a world of problems.

To the untrained electrician, this will be a corner grounded 480 volt delta system. On the other hand, why would any untrained person be working on any 480 volt system? Upon further consideration, why would an untrained person be taking the cover off of anything?

 

[quogueelectric]

To the untrained electrician, this will be a corner grounded 480 volt delta system. On the other hand, why would any untrained person be working on any 480 volt system? Upon further consideration, why would an untrained person be taking the cover off of anything?

Hey this happens all the time. I have seen highly trained electricians freak out at a corner grounded delta in a power plant by me. Glenwood landings for lipa has one. It kind of freaks me out measuring 0 volts to ground on a open buss 4-5 thousand amps switchgear in the powerplant. I know it will kill me in a second yet It is reading 0 volts to ground. Kind of drops your guard. Then you slap yourself out of it and trust your training.

 

[LarryFine]

No Larry, it is not weird. It is permissible by the NEC and the NESC, it is unusual but it is not weird.

250.20(B) Alternating-Current Systems of 50 Volts to 1000 Volts.

Yep, I read through that after my comment, but just didn't retract my weird.

 

[LarryFine]

It kind of freaks me out measuring 0 volts to ground on a open buss 4-5 thousand amps switchgear in the powerplant. I know it will kill me in a second yet It is reading 0 volts to ground.

Why would it kill you?

 

[Paul G]

Thanks for all the reply's. I am learning a lot about this system.

Doing a little research,

http://ecatalog.squared.com/pubs/El...sin Encapsulated)/Export Model/2700DB0202.pdf

I am reading that the over load protection can not open the grounded leg unless it opens all legs together. Hence, a 3 pole circuit breaker. Is that correct? Currently, the main disconnect switch is a fused disconnect with 3 800 amp fuses. The 600 amp motor disconnect and the 30 amp feeder disconnect switches all have 3 fuses, all fed from the main in that same panel.

Should the main disconnect have a solid, un-fused B phase? With the feeders then having 3 fuses each?

 

[LarryFine]

I am reading that the over load protection can not open the grounded leg unless it opens all legs together. Hence, a 3 pole circuit breaker. Is that correct?

It is. An opened grounded conductor leaves the entire feeder or branch circuit energized.

Currently, the main disconnect switch is a fused disconnect with 3 800 amp fuses. The 600 amp motor disconnect and the 30 amp feeder disconnect switches all have 3 fuses, all fed from the main in that same panel.

In each case, with a grounded phase, that phase should have "dummy" fuses in the fuseholders.

Should the main disconnect have a solid, un-fused B phase? With the feeders then having 3 fuses each?

No. Main, feeder, or branch circuit, the same applies: no fused grounded conductor; must have simultaneous disconnect.