GROUNDED B PHASE

[VTP99]

Boy did i get a education on this one the other day.

[bunny]

Boy did i get a education on this one the other day.

Well, please share what you learned.

[Dallas Duster]

Well, please share what you learned.

Well he is still alive so I bet it was a valuable lesson.

[FrostyBeer]

Well, please share what you learned.

yes, please do.

[itsiceman]

[VTP99]

Well for one thing don't just reverse any two phases to reverse direction. Also a fused disconnect is a no no with the N.E.C.
The word needs to get out about this type of service.

[VTP99]

A phase top
B phase bottom left
C phase bottom right

Yes that B phase is grounded and will read zero volts to ground but she is still HOT.

[VTP99]

Well he is still alive so I bet it was a valuable lesson.

Yes i'm still here but that 3 phase motor is toast.

[SBEC]

The voltage of a delta connected system from B phase to ground is actually 240V. The grounded or nuetral conductor is derived by grounding the midpoint of 2 windings. these systems are also referred to as Hi leg, Red leg, Wild leg etc. The reason is between 2 phases to ground the voltage is 120V , but the the third phase to ground it's 240V. There are a lot of these systems that are not marked. A clue is when you see a panel with every third space blank. In a 3 phase application they operate like any other 3 phase connected system. The NEC requires an overload protective device in every phase. That could be an overload heater in a motor starter, a fuse or a common trip circuit breaker. In NJ, PSEG has some delta systems in the older cities where 1 phase is insulated from ground, but the voltage between phases is 240V. PSEG also makes the hi leg phase C phase in the meter enclosure and we have to transpose that conductor from C phase in the meter enclosure to B phase in the panel as per NEC. These conductors and bus bars should be marked with orange tape, paint etc.

[bobboan]

SBEC,
The type of service that you are referring to is a 4wire, mid point grounded Delta; VTP99 is describing a 3wire, corner grounded Delta. On this latter type, one phase is indeed grounded and is 0 volts to ground. However, the grounded phase is still current carrying just like a neutral is current carrying and is grounded. The drawing he attached is correct. This type of service is only found in commercial/industrial where there are only 3 phase loads.

A 4wire, mid point grounded Delta service is "hot" on all phases; 240v between all phases, and 208v to ground on the "stinger" or high phase.

[J/D]

Well for one thing don't just reverse any two phases to reverse direction.

How do you change direction then? I have only seen this configuration once and had no problem changing direction. Dont understand how it would toast motor if proper voltage. Motor is still being energized on all three legs.

[zw17]

I have never seen it. Is this a regional thing?

I need to look into this.

[J/D]

I have never seen it. Is this a regional thing?

I need to look into this.

Just very old system not used in new construction the building I seen it in was pre WWII.

[Icefixer]

If its a stinger leg transformer. You can still reverse any two legs to reverse motor on a three phase motor. Unless I'm missing something in the conversation.

[jammzzville]

Ran into this two years ago on a Carrier rooftop (forgot exactly which model, remember serial being from late 80s) with electric heat. Building service was 208Y/120. RTU nameplate was 460V. RTU had a dedicated step up transformer for 480V service.

First service call (no cooling, summer) the fuse block for the unit (not the electric heat) was completely toasted, absolutely fried. I still have it to this day to show people just how violent the arcing must have been. There was a non-fused disconnect at the RTU and a fused disconnect after the transformer in the electrical/mechanical room. Open up the disconnect after the transformer and the A and C phases were fused but the B phase was bypassing the disconnect by being attached to the ground lug of the disconnect. I think, "!!!!WHAT?!?" So I open up the transformer and the B phase lug has a 10GA green conductor ran straight to a grounding rod. After doing some research I learned that this was a corner grounded delta. Had to replace one of the two fuses in the disconnect but replaced both to be safe. AB, BC, and AC were all around 490V or so. A and C to ground were also 490V and B to ground was 0V. After understanding the corner ground this made sense. So, I replaced the fuse block and the fuses and the unit ran fine for the rest of the summer. We figured that the insulation of the fuse block must have broken down or something of that nature. There was no other explanation at the time.

Second service call (too little heat, fall) I find the fuse block, contactor, and conductors of one of the two heaters equally as toasted as the units fuse block a few months prior. Again, after investigation, no explanation as to the cause. Replaced fuse block and contactor on both heaters, both heaters running fine. Do further research that night and find out that electrical distribution and control components utilized on corner ground systems can not be "slash rated" such as "120/240" "277/480" If it is a 480V system the components must have a solid "480V" rating. I believe this is because with a 480Y/277 system the potential from any terminal to ground will only be 277V but when you have a corner grounded system the potential from the A and C terminals to ground will be 480V. Luckily the contactors I had in the van were rated up to 600V and were even slightly over sized for the heater KW. I went back the next day and checked the compressor and evap fan contactors. The evap fan contactor had been recently replaced with a 480V contactor and both compressor contactors were rated up to 600V. Too bad I'll never know what the the original fuse blocks and contactors that burned up were rated for because they were completely toasted.

Great learning lesson.

[itsiceman]

A phase top
B phase bottom left
C phase bottom right

Yes that B phase is grounded and will read zero volts to ground but she is still HOT.

You'll never forget the first time
I remember going WTF too

[FrostyBeer]

so I don't have a "first time", how does one safely work with grounded B phase?

[bobboan]

First,
Remember that it's the same voltage to ground as it is between phases. If it's 480v between phases, then it's 480v to ground.

Second,
Treat the grounded phase like you would a "live" neutral that's carrying current: don't be the path that makes or breaks continuity of it.

Third,
As pointed out, all components must be rated for the actual voltage to ground.

Fourth,
No fusing in the grounded phase.

There's probably more in the NEC, but it's too late to dig it up tonight.

Buenas noches, amigos.

[wolfdog]

This comes up from time to time. Here was the last discussion with some code references. http://hvac-talk.com/vbb/showthread....grounded+delta

[stanbyyourword]

I called the power company out . and to be completely honest I still don`t completely understand it. Nuetral in a single phase makes perfect sense to me , ie negative on the sine wave yet still carrying current, this just blows my mind, there has been other really long and informative discussions here in the past, I have to put my hand up and say I am still not 100% on it stan