Corner grounded Delta supply

[SceneryDriver]

Need a double check on corner grounded 240V Delta:

Our new building came with a 240V corner grounded delta service. There seems to be lots of confusion on the part of the EC regarding proper hookup of transformers to a corner grounded Delta service.

We several have transformers that are 240V Delta -----> 208/120V Wye to serve convenience receptacles and office loads. I believe these are connected correctly, but will have to check.

We have several step-up trafos to supply some of our machine tools with 480V three phase (240V Delta -----> 480V Wye), and this is where I believe the EC is incorrect in his installation and connections:

1) He has run a #6 green from the transformer chassis back to building steel, but has also bonded the grounded leg of the three phase supply to the transformer chassis. I'm of two minds on this; I know this potentially creates multiple paths to ground (ground loops) but this is a very old building, and the portion of the building housing the transformer may or may not have continuous building steel back to the service entrance (we can't really tell). On the other hand, tying the grounded leg to the transformer chassis ensures a fault clearing path back to the service. Thoughts?

2) He has not tied H0 (the center point of the 480V Wye) to ground in the transformer. He says you don't do that. I say you need to in order to give the 480V side a ground reference and a fault clearing path. An ungrounded delta supply would need ground detectors, and could be the reason for damage the connected electronic control unit of the compressor unit being powered by this trafo. Thoughts?


Also, at our paint booth:

3) Our paint booth has (2) separate supplies. A 100A, 240V corner grounded Delta three phase supply needed to power the large VFDs for ventilation fans, as well as a 30A, 120V single phase supply (fed from a separate 208/120V panel) to separately feed control needs and lighting (this thing is rather strangely designed). He has tied the grounded phase of the delta supply to the green lug in the control panel, and has not run a separate ground conductor from the three phase service. Since the supply runs through nonmetallic conduit for part of its run, I say an appropriately sized ground conductor is needed; you don't get "credit" for grounding the machine through the grounded phase, correct?

3a) The 30A 120V single phase supply has no ground conductor run with it either (also through a section of nonmetallic conduit). Each feed to the machine must have an appropriately sized ground conductor back to it's supply for fault clearing, yes? You can't rely on the 3-phase supply to ground the single phase supply, yes?

Corner grounded Delta is a strange beast to many people these days, since it isn't commonly installed anymore. Thanks for the help!


SceneryDriver

 

[jim dungar]

Corner grounded Delta is a strange beast to many people these days, since it isn't commonly installed anymore.

You have some issues that appear to be against the NEC. First and foremost: the grounded phase should never be connected to the 'grounding conductor' except at the source.

For the most part, the grounded phase conductor should be treated as if it were a Neutral (e.g. single point of bonding to ground and no fuses).

 

[SceneryDriver]

You have some issues that appear to be against the NEC. First and foremost: the grounded phase should never be connected to the 'grounding conductor' except at the source.

For the most part, the grounded phase conductor should be treated as if it were a Neutral (e.g. single point of bonding to ground and no fuses).

Agreed.

I needed a double check regarding transformer grounding; am I correct that the H0 terminal on the step-up trafo should be tied to ground? That will give the 480V Wye SDS a ground reference, yes? The EC is hopping mad regarding that, and says it'll cause a short. I know it won't, but he's insistent.


SceneryDriver

 

[mpoulton]

Agreed.

I needed a double check regarding transformer grounding; am I correct that the H0 terminal on the step-up trafo should be tied to ground? That will give the 480V Wye SDS a ground reference, yes? The EC is hopping mad regarding that, and says it'll cause a short. I know it won't, but he's insistent.

You are correct on all counts, and your EC seems not to be up to handling this kind of job. The grounded phase conductor is treated just like a neutral. The NEC doesn't use the term "neutral", but instead calls a current carrying conductor that is earth-bonded a "grounded conductor", which is what that grounded phase is. Just like any other "grounded conductor", it gets bonded to the grounding electrode system at only one point, the service disconnect location. The transformer primary should have no bond. The secondary, on the other hand, must be bonded/grounded unless it's an intentionally ungrounded system. And that separate 120V circuit requires an EGC that returns to its source, just like any other circuit.

 

[texie]

Agreed.

I needed a double check regarding transformer grounding; am I correct that the H0 terminal on the step-up trafo should be tied to ground? That will give the 480V Wye SDS a ground reference, yes? The EC is hopping mad regarding that, and says it'll cause a short. I know it won't, but he's insistent.


SceneryDriver

Yes, in general you are correct. Unless you have a specific reason (very rare) and meet all the requirements to do so, a 480Y should always be installed as a grounded system and comply with 250.30(A). Based on your description I think you have all kinds of grounding and bonding violations in addition to the 480Y system and the EC is clueless.

 

[Saturn_Europa]

Yes, in general you are correct. Unless you have a specific reason (very rare) and meet all the requirements to do so, a 480Y should always be installed as a grounded system and comply with 250.30(A). Based on your description I think you have all kinds of grounding and bonding violations in addition to the 480Y system and the EC is clueless.

90% of the work I do is controls and automation. The transformers I wire up are usually 2 pole single phase 480 v to 120 single phase control transformers . So this is a straight up question:

I thought H0 on a 3 phase 480volt transformer was for the grounded (neutral) conductor and was to be left open if you had a 3 wire system. That the grounding (green) conductor was tied to the transformer housing only.

Edit: Disregard, just reread the post. In this case the 480 is the secondary side of the transformer.

 

[iwire]

I thought H0 on a 3 phase 480volt transformer was for the grounded (neutral) conductor and was to be left open if you had a 3 wire system.

If you are using a delta - wye transformer in reverse that is when you leave the HO open.

 

[Saturn_Europa]

If you are using a delta - wye transformer in reverse that is when you leave the HO open.

What do you do with H0 on a 3 phase 3wire 480 v wye to 3 phase 208 wye step down transformer?

 

[texie]

What do you do with H0 on a 3 phase 3wire 480 v wye to 3 phase 208 wye step down transformer?

It would not be typical to have a Y/Y transformer for this application. It would be a 480 delta primary/208Y secondary. In other words, you would not have an H0 to be concerned with.

 

[jim dungar]

What do you do with H0 on a 3 phase 3wire 480 v wye to 3 phase 208 wye step down transformer?

Unless you are a utility, the simple answer is to leave the neutral, H0 or X0, open when that winding is connected to the source.

This rule of thumb will cover the vast majority of typical dry type transformer installations in the US. Of course there are exceptions, but you need a starting point.

 

[SceneryDriver]

You are correct on all counts, and your EC seems not to be up to handling this kind of job. The grounded phase conductor is treated just like a neutral. The NEC doesn't use the term "neutral", but instead calls a current carrying conductor that is earth-bonded a "grounded conductor", which is what that grounded phase is. Just like any other "grounded conductor", it gets bonded to the grounding electrode system at only one point, the service disconnect location. The transformer primary should have no bond. The secondary, on the other hand, must be bonded/grounded unless it's an intentionally ungrounded system. And that separate 120V circuit requires an EGC that returns to its source, just like any other circuit.

An update:
The EC is coming to the site for a meeting tomorrow to discuss addressing our concerns. Apparently, it was the AHJ that demanded the grounded phase be bonded to the chassis of the transformers, panelboards, etc... His justification is kind of what I suspected: much of the building's infrastructure is old (50+ years) and he doesn't feel he can trust using the raceway as an effective bonding means. Since adding a green wire to the existing conduits isn't really feasible, he asked for the grounded phase to be bonded. Not code compliant, but I see his point. The engineering is pretty sound in my mind as well.

As for grounding the star point in our 240V delta ---> 480V Wye transformers, he has said he'll do it, but still thinks it will cause some type of damage. It might be a long meeting tomorrow...



SceneryDriver

 

[mpoulton]

An update:
The EC is coming to the site for a meeting tomorrow to discuss addressing our concerns. Apparently, it was the AHJ that demanded the grounded phase be bonded to the chassis of the transformers, panelboards, etc... His justification is kind of what I suspected: much of the building's infrastructure is old (50+ years) and he doesn't feel he can trust using the raceway as an effective bonding means. Since adding a green wire to the existing conduits isn't really feasible, he asked for the grounded phase to be bonded. Not code compliant, but I see his point. The engineering is pretty sound in my mind as well.

As for grounding the star point in our 240V delta ---> 480V Wye transformers, he has said he'll do it, but still thinks it will cause some type of damage. It might be a long meeting tomorrow...

AHJ apparently doesn't understand the consequences of bonding the grounded conductor at multiple points. Doing that WILL cause load current to flow on the conduits, and if there's much voltage drop on the conductors the current flow on the conduit can even be enough to cause heating at loose joints. Hopefully he gets Ohm's law and isn't one of those "path of least resistance" people. Adding an equipment grounding conductor is the only legal and technically sound solution if the raceways are not reliable grounding means. And how can an industrial EC not understand bonding an SDS??? Long meeting indeed...

 

[ActionDave]

An update:
The EC is coming to the site for a meeting tomorrow to discuss addressing our concerns. Apparently, it was the AHJ that demanded the grounded phase be bonded to the chassis of the transformers, panelboards, etc... His justification is kind of what I suspected: much of the building's infrastructure is old (50+ years) and he doesn't feel he can trust using the raceway as an effective bonding means. Since adding a green wire to the existing conduits isn't really feasible, he asked for the grounded phase to be bonded. Not code compliant, but I see his point. The engineering is pretty sound in my mind as well.

As for grounding the star point in our 240V delta ---> 480V Wye transformers, he has said he'll do it, but still thinks it will cause some type of damage. It might be a long meeting tomorrow...



SceneryDriver

If the AHJ is saying that rebonding the grounded conductor at different places in the building is a way to make the conduit safer just because it doesn't have a green wire in it he needs some Continuing ED. Everything about Parts I and II of Art. 250 is about keeping the current carrying parts of electrical system isolated from the bare metal. Using the AHJs reasoning why not just use bare wire for all the grounded conductors? Super bond everything that way.

That grounded conductor is a current carrying conductor and he wants you to use the building steel and all the metal that is tied to the electrical system to carry some of that current. I agree with mpoulton, it's the wrong thing to do and makes the building less safe. Ever open up a loaded neutral and see the sparks, that's what's gonna happen when you are working on an existing conduit run.

No wonder the EC is confused if he is working under an AHJ like this. Besides, metal conduit without a green is nothing to be afraid of even in old buildings. There are usually parallel paths aplenty for clearing a fault.

 

[SceneryDriver]

AHJ apparently doesn't understand the consequences of bonding the grounded conductor at multiple points. Doing that WILL cause load current to flow on the conduits, and if there's much voltage drop on the conductors the current flow on the conduit can even be enough to cause heating at loose joints. Hopefully he gets Ohm's law and isn't one of those "path of least resistance" people. Adding an equipment grounding conductor is the only legal and technically sound solution if the raceways are not reliable grounding means. And how can an industrial EC not understand bonding an SDS??? Long meeting indeed...

Agreed. It's gonna be a long meeting. I might need an extra-large cup of coffee... or a large scotch. Maybe both.


SceneryDriver

 

[SceneryDriver]

If the AHJ is saying that rebonding the grounded conductor at different places in the building is a way to make the conduit safer just because it doesn't have a green wire in it he needs some Continuing ED. Everything about Parts I and II of Art. 250 is about keeping the current carrying parts of electrical system isolated from the bare metal. Using the AHJs reasoning why not just use bare wire for all the grounded conductors? Super bond everything that way.

That grounded conductor is a current carrying conductor and he wants you to use the building steel and all the metal that is tied to the electrical system to carry some of that current. I agree with mpoulton, it's the wrong thing to do and makes the building less safe. Ever open up a loaded neutral and see the sparks, that's what's gonna happen when you are working on an existing conduit run.

No wonder the EC is confused if he is working under an AHJ like this. Besides, metal conduit without a green is nothing to be afraid of even in old buildings. There are usually parallel paths aplenty for clearing a fault.

I am in complete agreement, and if my boss wants to push it, we may appeal to the inspector's boss. We need to talk with the EC first; this has become a giant game of whispers. The EC wants to do the right thing, but it sounds like he's feeling stuck between the AHJ and code here. As for bonding the SDS center point, that's another discussion...



SceneryDriver