On a step down XFMR 480 Delta to 208 Y you need to bond the XO to ground or there is no ground fault path on the output side. I saw a step UP XFMR today that was 208 Y on the input and 480 Delta on the output, there is nothing connected to the XO. There is just an equipment ground run through the conduits and bonded to the boxes. How is the fault current path made in this configuration ?
XFMR question
- Thread starter acrwc10
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- Placerville, CA, USA
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The fault current path on the input of the step up, where the wye is, is not through the transformer at all. It has to go to the source, whether service or SDS that is supplying the transformer. Bonding the X0 would server no purpose and, if the three phase lines are not properly balanced, could result in high circulating currents and very objectionable ground currents in the EGC.
On the output side you can either run an ungrounded delta, subject to all of the additional requirements of the NEC, or you can corner ground it if the load equipment can tolerate that.
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This set up is not corner grounded, its all motor load, but it does read voltage phase to ground on the output side. I am just trying to wrap my head around the "how does it have a ground path" if it is not physically connected to ground on the output side of the XFMR.
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- Retired PV System Designer
On the output side, the only possible fault current is from line to line when the delta is not grounded.
The first fault from anywhere in the circuit will NOT cause fault current to flow. It will just establish a ground point in the circuit.
It is the second fault that will result in fault current, and that fault current will flow from one grounded point in the delta to the other, completing the circuit through the windings.
Grounding the XO on the input side will have no effect one way or the other since that has no path to the delta side.
When you read stable and balanced line to ground voltages on an ungrounded (as far as you know) delta, you are usually seeing the effects of a capacitive voltage divider among lines and ground. It will not provide significant current and will not show up as line voltage with a wiggy or low impedance meter.
On your 480v side there is no "ground path" unless you ground one phase.
If it remains an ungrounded system 250.21(B) requires a ground detection device to alert if one phase does go to ground.
One of the advantages of the ungrounded system is that a phase can go to ground and not open an overcurrent device thus allowing the equipment to continue to operate.
You still need a reliable equipment grounding (bonding) path on the secondary side.
(posted in conjunction with Gold Digger)
If it remains an ungrounded system 250.21(B) requires a ground detection device to alert if one phase does go to ground.
One of the advantages of the ungrounded system is that a phase can go to ground and not open an overcurrent device thus allowing the equipment to continue to operate.
You still need a reliable equipment grounding (bonding) path on the secondary side.
(posted in conjunction with Gold Digger)
kwired
Electron manager
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- EC
I think you got it, but will add a little - "separately derived" is an independent source. All the primary winding does is provide a magnetic field to excite the secondary winding, the secondary is a completely independent source otherwise and must have it's own connection to ground established if it is to operate as a grounded system.
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- PE (Retired) - Power Systems
The manufacturer is technically correct. The transformer, by itself, does not need to be grounded, however the output voltage system created by the transformer needs to be installed per the applicable codes. Mandated ground detection schemes are relatively new (only a few code cycles) so people that do not do much ungrounded work may often 'forget' about them.
Since the xfmr is a step up xfmr and the egc on the input is larger than the required GEC for the SDS can the supply side EGC be used as the GEC? Or is a ground rod required at the new xfmr and another bonding jumper to the water line require?
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You don't need a ground rod rod for an SDS, you only need to tag to a water pipe or building steel.
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- Massachusetts
If neither the water pipe or building steel quilifiy as an electrode you would have to add one.
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- Licensed Electrician
Correct. I was just now scrolling up and down between these two posts after I had sent my reply to the one I quoted.
Between the two one could conclude a ground rod would be needed in this case, however, how many times do you see a transformer in a building without a qualifying water pipe or building steel nearby?
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- Massachusetts
I don't think we are allowed to use the water line as the GEC unless we go back to within 5' odpf the water lines enterance to the building. Typically the connection to the water line would be the bonding connection.
We run into this issue when working in old large brick and wood mill buildings. No building steel and we can't use the water line so we have to run a 3/0 GEC back to the service.
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- Licensed Electrician
Correct. That's why I said water line or building steel.
Sadly, around here most all of those kind of buildings have been torn down and replaced with condos.
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- Licensed Electrician
There is some back and forth between recent code cycles whether or not an EGC can also be a GEC. I've never been in a situation where I had to do it so I never gave it a lot of concentration.
I would rather run a wire back to the service than drive a ground rod.
kwired
Electron manager
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- NE Nebraska
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- EC
Depending on the type of work you do - I have worked on a lot of grain storage systems all throughout my career - no water systems are available at the site quite often, Buildings - I guess the grain bins themselves may be a building and are well grounded with all the concrete and steel they are built out of - but we don't install any wiring "in" them except an occasional individual branch circuit or two for equipment that may be inside (and even those have lessened over the years), most equipment requiring electrical components is on the outside. Most of the wiring for such facilities is either in a "control shed" or more often is just NEMA 3R or NEMA 4/12 enclosures mounted on posts or on the bin exterior.
Posts/independent structures is my preference if there is large or multiple enclosures (aux gutters are not made to fit on a round bin, and neither is any other enclosure much wider then 15" for that matter). This leaves the "building or structure" involved with nothing but the GEC associated with the feeder or service supplying it to connect to any SDS you may install there.
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