Fake Neutral!!

[ActionDave]

Inside of the VFD there are MOVs on the line terminals that are connected in Wye to ground. The VFD is essentially then acting as the Wye point, at least enough to be read on a DMM. This is a dangerous situation (for the VFD) because as soon as there is a ground fault ANYWHERE in that system, the MOVs in the drive will attempt to conduct ALL of the fault current to ground, making them explode and cause collateral damage, ie a flashover or component shorting etc. Most VFD mfrs will tell you EITHER to disconnect the ground reference to the MOVs (and common mode noise capacitors if included) when you connect to a delta service, or they just say “This drive can only be connected to a solidly grounded wye (star) service”, or words to that effect. Delta services are only used in North America, so some EU and Asian based drive mfrs do a poor job of explaining this up front.

So when you have the building load disconnected, you are reading the transformer voltages, but once the building load is connected, your meter is reading through the VFD front end. If there are any other 3 phase power electronics in the building, they will likely be the same, ie UPS, battery chargers, solar inverters etc.

What make and model of VFD is it.

I have been on troubleshoots where an ohm meter will give you a false reading with the power off because electronic components were present, but when real power is on something made a breaker go boom. In the case of the OP it sounded like the power stayed on.

 

[ActionDave]

Recently I saw a 480 corner ground SDS that was recently installed, and they did a spiral of white tape on one of the standard BOY colors. I remember thinking that it was kind of wrong, in that it should just be a "normal" solid white conductor. But I don't know, maybe that method is good as it might make it more clear that something "weird" is going on, I e. that it's corner grounded and not split phase?

I like this idea.

 

[Tulsa Electrician]

240 v three wire corner grounded delta

 

[kwired]

I installed a 600V corner grounded SDS to re- feed an old elevator after a service upgrade once. I used a fully white conductor for the grounded conductor on the new portion, but it refed old wiring where of course they were all just black.

Recently I saw a 480 corner ground SDS that was recently installed, and they did a spiral of white tape on one of the standard BOY colors. I remember thinking that it was kind of wrong, in that it should just be a "normal" solid white conductor. But I don't know, maybe that method is good as it might make it more clear that something "weird" is going on, I e. that it's corner grounded and not split phase?

If the conductors were 4 AWG or larger that should be acceptable. Isn't really any rule on the insulation color if tape or other white/gray means is used for such identification.

If they are smaller than 4 AWG they should be white or gray insulation tape can't be used to identify as the grounded conductor here.

 

[wwhitney]

240 v three wire corner grounded delta

Why are they using a 3 phase panel and putting OCPD on the grounded conductor? To facilitate a future changeover to 240V 4-wire hi leg delta? I guess that would just require removing a lot of white tape.

Cheers, Wayne

 

[ptonsparky]

Why are they using a 3 phase panel and putting OCPD on the grounded conductor? To facilitate a future changeover to 240V 4-wire hi leg delta? I guess that would just require removing a lot of white tape.

Cheers, Wayne

The only place we are not allowed OC protection is in the grounded conductor at the SE, after that it's normal for 3PH. Allowed under certain conditions for 1PH, but not normally done.

I would have used a lot less tape, but I'm lazy.

 

[wwhitney]

The only place we are not allowed OC protection is in the grounded conductor at the SE, after that it's normal for 3PH.

Right, but there's also the option to just use a two busbar panel of sufficient voltage rating, along with a grounded conductor bus bar. I.e. a panel that you might normally call a single phase panel. Unless I'm missing some NEC or other requirement?

Cheers, Wayne

 

[ptonsparky]

Right, but there's also the option to just use a two busbar panel of sufficient voltage rating, along with a grounded conductor bus bar. I.e. a panel that you might normally call a single phase panel. Unless I'm missing some NEC or other requirement?

Cheers, Wayne

I suggested that to one of our SED Inspectors several+ years ago. He suggested NO!

It wasn't worth the effort to argue the point. We saw so few of the corner grounded Delta.

 

[electrofelon]

I suggested that to one of our SED Inspectors several+ years ago. He suggested NO!

It wasn't worth the effort to argue the point. We saw so few of the corner grounded Delta.

I'm not sure if a single phase load center type panelboard would be listed/approved for a 240V corner grounded system, never dug into it, but for a full panel board, with Siemens at least, I know they have a letter code in the part number for that type of system, so I don't see what the issue would be.

 

[kwired]

I'm not sure if a single phase load center type panelboard would be listed/approved for a 240V corner grounded system, never dug into it, but for a full panel board, with Siemens at least, I know they have a letter code in the part number for that type of system, so I don't see what the issue would be.

Pretty sure many QO panels mention this use on labeling.

You do need to use the straight 240 volt rated breakers in it and not 120/240 rated breakers. Cost and availability of those makes it about as simple to just use a three phase panel and three pole breakers which are straight 240 rated.

 

[tortuga]

240 v three wire corner grounded delta

Thanks for posting those photos of a corner grounded delta. I can't remember ever having worked on one and I asked a coworker and he does not know of any around here. Are they common around there?
What would be the reason for the corner grounded?
If you still need the 3 pole breakers seems like 4 wire hi-leg delta would cost about the same and give you a 120V option.
or even a say a 240Y/139 or the more common 220Y/ 127, not that the 139 would be useful, 127 would though.

 

[ptonsparky]

My reference in post 28 was a fused disconnect, not a panel.

The only new corner grounded 480 volt I've seen was installed because the POCO only had two phases available, IIRC. Possibly a primary neutral, too. Rural area and it has been more than 30 years ago, so in that case it was monetary.

I asked the POCO staking engineer at the time how, and he didn't know.

 

[Tulsa Electrician]

Why are they using a 3 phase panel and putting OCPD on the grounded conductor? To facilitate a future changeover to 240V 4-wire hi leg delta? I guess that would just require removing a lot of white tape.

Cheers, Wayne

For this application it was designed that way. The GEC is at transformer.
Pretty sure 240.22. (1) allows this to have a three pole main breaker.
I was late to this party as they did not understand the wireing methods involved. Since I had experience with these installs I helped them square it away. I had recommended B phase be grounded and use black,white, blue and use all striped wire since there three other systems in the building.

They had already pulled branch circuits,BK,R,BL which had to be reworked. Then I did my best to explain the grounded conductor to them as well as do and don't.
the first hurdle was the word netural. Next was hooking a white wire to breaker. Then what to do at the equipment. This system was for special 240 volt only equipment. No vfd all across the line. Depending on the type of disconnect required a different hook up. Non fused break both ungrounded and grounded. All were either two pole or three pole.
fused required a dummy in the grounded conductor.
the panel and breakers all rated for the delta. I also told them to ring all wires you do not want a short in the grounded conductor to EGC.
the GEC had to be resized correctly as well.
And of course lable,labe lable.
As far as tape went I said you need to pull the correct color of conductors for that system all the way thru and don't use tape.

Any way the choice was made to go C grounded and use black, red, white on the feeders and remark the branch. Once they were brought up to speed I was removed from project. I also had to have them re work all GEC s etc to the correct size. Told them #6 is not per NEC. For the entire grounding electrode system and multi SDS's. #6 for rods only.
The funny thing ( sad really)was on a 480 volt feeder there was no netural to the 277/480 volt panle.
Then this a three section. The pics speak for them selves. I said tare it all out start over.
Sorry a little rant.

 

[Tulsa Electrician]

Thanks for posting those photos of a corner grounded delta. I can't remember ever having worked on one and I asked a coworker and he does not know of any around here. Are they common around there?
What would be the reason for the corner grounded?
If you still need the 3 pole breakers seems like 4 wire hi-leg delta would cost about the same and give you a 120V option.
or even a say a 240Y/139 or the more common 220Y/ 127, not that the 139 would be useful, 127 would though.

Your welcome.
I did ask the engineer why not a high leg because of the netural etc. He said it was customer requested for his equipment. They wanted 240 volt only. The young journey man was in a little over his head. He did learn a lot when they were done.
I have been lucky to have done a few 480 and 240 as well as both three phase and single phase set ups.
By that I mean three pole breakers and two pole fuses using netural bar as third leg. Service and SDS.
My first 240 was a service for an MCC center for a co-op. Then several for machines etc.
Every time I did one the other guys there would stand way back because they said you can't do that. I always sid well I did. It seams like every time one comes up I get a call.
They are getting harder to come by on the service side so most go on the SDS side which only makes sense.
I can't stress enough use the correct color coding and use it all the way thru and lable,lable, lable. Even all j- box lids get a grounded delta symbol along with circuit #. I even paint the covers white. Put a system list in the electric room.
Wish I would have taken more pics. Next one I will and post.

Sorry OP for off topic post.