High Leg Delta Trasnformer

[CuriousElectron]

Hi Gents,
Is high leg delta transformer considered a solidly grounded transformer? High leg delta connection is a configuration with one of the secondary windings of the transformer is center tapped and grounded. These type of connections are not common, but I believe you'd consider this transformer being solidly grounded.

Thanks,
EE

 

[davidbeach]

Who says they're not common?

If the center tap is solidly grounded (not permitted by the NEC to be otherwise), then the system is solidly grounded.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[jraef]

Well... it depends on why you are asking I suppose. Technically, the term "solidly grounded" is just defined as not having anything between the neutral and ground connections, as opposed to neutral being connected to ground through a resistance or impedance device.

But generally, when someone calls for a "solidly grounded system", they are expecting that the Line to Ground reference voltage is 150V or less. In a 240/120V 3 phase 4 wire "High Leg" delta transformer arrangement, the High leg is going to be 208V referenced to ground, so it would not fit that definition.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

This should be a delta/delta connection.
With a wye primary an open delta works much better.
I have encountered the remnants of wild leg systems in high end homes.
Many years ago, when distribution voltages were in the range of 2400 Volts and 4260 Volts, it was difficult to start large air conditioners on single phase.
With better motor designs and stiffer grids large residential A/C is more easily started on single phase.
The voltage has risen from 220 Volts to 240 Volts as well.
All of the remnants of four wire delta systems in homes that I encountered had been converted back to single phase, 120/240 Volts.
That has helped the starting issues.
In the Canadian code, systems below 150 Volts to ground must be solidly grounded.
Systems above 150 Volts to ground may be grounded.
However there is also a requirement in the Canadian code that if the system incorporates a neutral, the system shall be grounded.
So, a four wire or center tapped delta would be required to be grounded despite the 208 Volts to ground due to the neutral.
(In Canada)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Compositepro]

But are connections allowed using the 208V to neutral? I think not.

 

[davidbeach]

Come on Jeff, there can solidly grounded systems at any voltage. We have examples all the way up to 500kV.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[waross]

But are connections allowed using the 208V to neutral? I think not.

I am not aware of any rule that prohibits this, so are these connections allowed:
Yes.
Is it done? Never seen it done. There are not a lot of single phase 208 Volt loads and 240 Volt loads will be fed 208 Volts from a two pole breaker that may use the wild leg. Habit. That's the way we always hooked up water heaters. (A two pole breaker)
The range and clothes drier need a neutral so the wild leg is not used.
Electric heat? Two pole breakers balanced across all three phases.
The current balance is better using two phases versus one phase and a neutral.
An exception is the open delta four wire system where the wild leg capacity is often restricted and is only used for three phase loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CuriousElectron]

Thanks, guys.

 

[Palletjack]

. I am not aware of any rule that prohibits this, so are these connections allowed:
Yes.
Is it done? Never seen it done. There are not a lot of single phase 208 Volt loads and 240 Volt loads will be fed 208 Volts from a two pole breaker that may use the wild leg. Habit. That's the way we always hooked up water heaters. (A two pole breaker)
The range and clothes drier need a neutral so the wild leg is not used.
Electric heat? Two pole breakers balanced across all three phases.
The current balance is better using two phases versus one phase and a neutral.
An exception is the open delta four wire system where the wild leg capacity is often restricted and is only used for three phase loads.

Not a rule that prohibits it, but you won’t find (I think) a single pole breaker rated for 208 volts.

 

[waross]

> Not a rule that prohibits it, but you won’t find (I think) a single pole breaker rated for 208 volts.

You got me there.
I have never seen a 208 Volt rated single pole breaker.
I would use a 120/240 Volt rated breaker.
They are quite common.
(GE, FPE, Square D)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

I was thinking most molded case breakers in the US are rated for 600V. Try one of those.

 

[FacEngrPE]

Caution Slash rated breakers are intended to be used on center grounded systems.

"A slash-rated circuit breaker is one with two voltage ratings separated by a slash, such as 208Y/120 volt. The smaller of the two ratings is for overcurrents at line -to-ground voltages, meant to be cleared by one pole of the device."
Reference

http://www.cooperindustries.com/con...ry/BUS_Ele_Tech_Lib_Slash_Rated_Devices_1.pdf

And the national electric code

IE if your system is not solidly center point grounded do not use slash rated components.

It might be OK to consider a slash rated breaker on circuits connected to the bottom two legs of a high leg system, where the slash rated component is directly connected to the center tapped winding, but any circuit landing on the high leg exceeds the line to ground limit of a slash rated component.