Grounding a Corner Grounded Delta System, NEC 2020, (3min:56sec)
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- เผยแพร่เมื่อ 29 ส.ค. 2024
- NEC expert Mike Holt explains how to properly ground a corner grounded delta system and the special characteristics of these rare systems.
This video is based on the 2020 NEC and was extracted from Mike Holt's Electrical Live Video Series. For additional information on electrical safety and the NEC visit www.MikeHolt.com/Code.
For over 40 years, Mike Holt Enterprises has been providing quality electrical code training to help electrical professionals learn the code, prepare for exams, and improve their electrical knowledge.
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I think it's important for people to understand that ABC phasing is relative to whichever phase we decide is "A". All the phases are 120 degrees apart and so they will only peak in ABC order or CBA order. Same is true for a corner grounded delta only one phase is effectively zero volts with a 480 or 240 volts in reference to the other two phases and those two phases are 480 or 240 volts in reference to each other. Since we have to maintain a ground fault current path the grounded phase must not be able to open unless the other two phase are opened as well so a fuse in that phase will be replaced with round copper bus or the phase will terminate in a lug or splice. If corner grounding an ungrounded system verify single phase and three phase loads cannot loose reference to the ground while energized. Grounding the middle winding in a transformer only makes sense because the center tap single phase and open delta's are terminated likewise (although an open delta can be wired as a high leg or with an open corner ground and still work, either way it's KVA rating and respectively maximum current is reduced to 58%.) In a high-leg we ground between A and C because B has to be the high leg. The need for corner grounding comes as the use of large electronic VFDs, inverters, and the use of many electronic driver and ballast in lighting greatly increased creating harmonics. Windings experiencing increased peak currents from harmonics can actually start pushing apart, which we call telescoping. (IBEW journeyman/JATC instructor)
All I can say is 'wow' you know exactly what this is all about. Congratulations! Smart Cookie...
@@MikeHoltNEC Thanks, coming from you that is an honor. Thanks also for the content and passion you have for educating electrical professionals, its very inspiring.
😮
Coming from a metering background I can say you don’t want the ground on 1 or 3. If the ground is on either of those phases it could create a metering error. You do not want a neutral or ground on a phase with a current sensor. That condition is called SCN - series coil neutral. To meet Blondels Theorem a utility will typically have the meter connected 1 & 3 at the current sensors and beginning potentials for the potential sensors, and have the ending polarity of the potential sensors connected to 2.
I’m a retired metering theory instructor for Southern California Edison.
Hello Sir. I just want to ask if you have encountered a 3phase 3wire system with a connected solar for net metering?
@@kimmallari2658 I have not. Possibly might be able to help though or give some ideas. I can guess your question though 😜. Is the solar array connected Y?
@@tomhb9479 thank you for the reply. Actually it is basically 3 wire with no grounding or neutral wire. So we applied tha blondel's thorem making it a 3 phase 3 wire circuit. But the kwh reading on the meter and the output on its inverter were not equal. Also for an ungrounded delta, does the blondel's theory still applies? I was really curious about it and needed someone to discuss it. Thank you very much sir. I really appreciate your response.
@@kimmallari2658 Yes, Blondel still applies. So a two element meter monitoring two of the currents with those same phases being the beginning polarity potentials for the two elements, and the third phase should be non polarity ending potential for your two elements. No connection to ground other than perhaps a case ground for safety, lightning, etc.
In the oilfield the B phase is often not metered but A and C are.
how you measure the difference between single phase and three phase equipment? Two separate meters? How I can get fair metering in case or consumption between A and C. Can the meter distinguish the type by reactive power? Especially wen the consumption it's mixed and having reactive power like motors.
B phase is the correct phase to intentionally ground. These types of systems are referred to as Grounded B Phase Systems.
Grounding of "B" phase is common, but it doesn't matter what phase is Corner Grounded.
@@MikeHoltNEC I believe you are very likely correct but the trade slang I've always heard to describe this sort of system is indeed a grounded b phase system. For this reason I've always tried to maintain a grounded b phase. Honestly, it's so rare around here that if I referred to it as a corner grounded delta system people would think I was crazy... However, many electricians have at least heard of a grounded b phase system.
Most have zero idea of how to deal with it but they've at least heard of it. Just like a wild leg/crazy leg/high leg system, they've heard of it but most have almost zero interaction, let alone understanding of how they actually work, with these systems unless they deal with old antiquated services.
Superlatives almost always come back to bite you in electricity. I respectfully disagree any phase can be grounded unless a transformer is being installed. I'm currently working on a corner grounded delta 240 service. The utility Cts are located outside at the utility overhead. The cts are mounted on a metal bracket. Phase B is bonded to the metal bracket. That bracket is mounted on a metal building. In this case the MDP must be grounded to phase B.
This video is about the system grounding. Any phase can be grounded. However, if the utility wants you to ground the B phase. Okay... that doesn't take away the theory.
Does zero sequence pick up on a ungrounded delta during a ground fault condition when using solid state protective relaying?
No. When there is a ground fault at an ungrounded system, the system becomes a grounded system with no movement of electrons during the ground fault.
Very interesting, ive just energized a transformer like this one
We this is what I have, measuring from line to line I have 244,240,239.
Now measuring from line to ground I have
120,120,213 , and because I have a drive in place I'm getting a phase loose code. But why the 213 volts to ground, I understand it's a hot 🔥 leg. How can I balance this out. To 120,120,120 ????
Usually I come across line 3 for grounding
It's a common practice to ground the B phase, but it doesn't matter which is grounded.
@@MikeHoltNEC I agree. B PHASE would be my choice
Heyyyy mike! Wouldn’t you want to reserve L2 for orange in case of a wild leg?
Is Delta with center tapped considered grounding?
Watch - th-cam.com/video/mpgAVE4UwFw/w-d-xo.html
Can you do that in a 34.5 kV distribution system? or it only applies to a LV system?
No clue
So now you don't have 3 legs though. Usually I see brown orange yellow gray but with this configuration, you're grounding the yellow so now you're losing that 480v leg right?
The secondary voltage is 240V, so you can never get 480V in this example.
@MikeHoltNEC I guess I meant to say if it was 480 and you were just trying to get the grounded contuctor you would no longer have 3 phases right? Does that kind of thing happen? Say you had an old ungrounded 480v delta and you wanted to ground it to gain the benefits. Is it possible to do that without losing a phase?
@@robbysanchez6514 Let's not talk about phase or grounded conductor. There are three wires from the corner grounded transformer to the equipment, the voltage between any two wires is 240V (in this example), the three wires are 120 degrees out of phase with each other, and the three wires can supply a three-phase load. Does this help?
Good day Mike, what is the solution if we have existing three-phase corner grounded 240V secondary but we need only a Three-phase 240v. I hope you notice this question thank you in answering
A 240V corner grounded system gives you 240V three phase. So you are good to go.
@@MikeHoltNEC we have a equipment which does not work/faulty in the corner grounded system, any recommendations for this problem. thank you 😊
I don't see how that is possible. Get me more information, Mike@MikeHolt.com
@@ajsebastian5529 I've seen this before, most recently with a 3-phase SPD. Perhaps is has to do with electronics or controls since a big dumb 3 phase load, like a motor, doesn't know the difference. Transformer would be my solution.
Ok Mike maybe you can help explain this oddity I ran across in an old citrus packing plant. The system is three phase supplied by a generator it also is a corner grounded delta ( B phase grounded at Genset and at main disconnect) . This system however uses two pole disconnects and breakers and of course provides three phase in this manner . Single phase is provided by two pole devices on some circuits and by single pole devices and ground on others.120 volts is derived from transformers as required. Your illustrations show three pole devices as do all I have found in recent texts. Is this an old 50's -60' era way of wiring this type of system or is it something done by someone who wasn't fully aware of what they were doing ?
Yes, single phase equipment can be used in a corner grounded system. But the breakers must be "straight rated". For example, if you had a 240 volt corner grounded system, you would use 2-pole breakers rated at 240 volts, NOT 120/240.
I was asked where do you ground on a delta wye transformer, H1, H2, X1 or X2... It didnt show the answer but I put X2, is that correct?
Please watch the video again, and look at the location of the GEC connection, it's at the X0 terminal.
Can i put tape, paint, etc. on a green wire and use it as the ungrounded or grounded conducor?
No because 250.119 of the nec.
I'm confused if your C,B phase,High leg is a neutral is it effectively only a single phase system?
If you have A, B, and C phase conductors, you have a three-phase system.
There is no high leg. Two phases are 240V to ground and the third phase is grounded so 0V to ground. Voltage between any 2 phases is 240V. There is no neutral conductor, there is a grounded phase conductor. All neutrals are grounded conductors but not all grounded conductors are neutrals.
@@pld8993 Excellent reply, thank you.
Also, all grounded conductors must be white (or gray), including the grounded phase conductor, so while all neutrals are white not all whites are neutrals.
@@pld8993 Right again... not many understand the system and the NEC. You are amazing...
In electronics, when you ground one of the line conductors it's referred to as an unbalanced ground system. If you ground in between the line conductors with the secondary coil center tap it's referred to as a balanced grounding system. Is it the same way in power systems?
Nope, watch the video on what it's called for US power systems.
@@MikeHoltNEC I know you refer to a grounded line on a delta as "corner ground", but you don't mention single phase grounded line like you typically see in Europe. So I was wondering if line grounding was referred to as unbalanced and neutral grounding as balanced? I seem to remember reading something about that in the NEC regarding PV systems, not sure.
Good explanation.
@@Oldhogleg Hi, I service industrial equipment, mostly European brands with CE certification. On some of the older equipment it's common to have single phase 24VAC for signaling & operator controls. At the secondary side of the trafo, the ground is always permenantly bonded to chassis/equipotencial buss bar. I've never see the ground on the stepeddown voltage supplied through a disconnect.
Disconnecting the ground just does not sit right with me, especially after watch so many Mike Holt videos on grouding. Even when the disconnect is engaged, its contacts are still a potential point of failure or increased impedance.
These 24V signaling and control circuits are rated at higher amperage, anything beween 10 to 40 amps is typical.
@@trespire I believe I see what you're talking about. I think you may be thinking of equipment ground and a system ground as the same thing. You're right about not wanting a CB/fuse on an equipment ground path/conductor. And you wouldn't on a grounded line side of a single phase circuit either. But on a three phase power supply you would because of the need for three phase power to power three phase equipment like motors requiring three pole protection; but I could be wrong. I'm no "expert".
sir if my neutral cable gets destroyet will the current choose the path of the ground
Current is not seeking a path to ground, but it will use the earth as part of the return path to the source.
@@MikeHoltNEC if my neutral gets destroyed electricity will use the ground to complete the circuit like you said and this clear to me .....but also i need to have very low ohms ground resistance to run machime in case if my neutral gets destroyed correct?? like you did in that video on youtube placing 2 copper rods in parallel connected deep in ground the load test using ground as neutral ... i am asking about the ground because when i play with tesla coils high frequency the ground from earth gives a boost to tesla coil somehow accelerates electron flow any comments about this ??? .. at the end i want to thank you for your answear and thank you for helping us.
Can you install a single phase panel to serve a 3 phase loads with this system? I was reading ECM but I have not seen it in the field.
What a great question... yep.
Can a corner grounded delta be single phase?
If it's a delta system, then it's a three-phase system, and it has a 'corner.' If it's a single-phase system, then it's not a delta system, and it has no corner. Okay/
@MikeHoltNEC I am in a pickle and am not sure how to prove an argument. In my mcc I found a panel that was mislabeled in my opinion. It says it's 240 single phase. But it's not. It has 3 conductors going into the panel from the transformer. A and B are 240v each to ground. C is 0v to ground but 240v from phase to phase. C phase is grounded in the transformer. Which tells me it's a corner grounded delta 3 phase 240v panel. Common from back in the day and in sawmills and industrial plants. I brought the issue to my maintenance manager, supervisor and engineer. They hired an outside electrical contractor to evaluate the transformer and panel. They say it's 240v single phase. How do I prove to them they're wrong?
@@briananderson1987 It is a three-phase system called "240 Vac 3Ø Corner-Grounded Delta System" or 480 Vac 3Ø Corner-Grounded Delta System, see www.productinfo.schneider-electric.com/nadigest/5cc35b7ed6593c0001ba8661/Supplemental%20Digest/SD17703_MAIN%20(bookmap)_0000052852.xml/$/_SD17703021_48857
@@briananderson1987 I'd look at the output on the transformer nameplate. The transformer will definitively tell you what it's putting out. Because it's coming from an internal transformer and not the power company, it's likely that the power company does not provide corner grounded delta, most power companies don't (you can confirm this by looking at their website for some sort of electrician's handbook). If so, then I would bet that the electrical contractor has never seen a corner grounded delta system and does not understand what he was looking at. If he only put a meter on it and didn't bother to look at the transformer nameplate, that might explain the mislabeling. FYI, some contractors install single phase equipment on these systems to save costs. However, when using 2 pole breakers in a panel for a corner grounded system, you cannot use typical, off-the-shelf 2 pole breakers, which are only rated for 120V per leg. Last year I gave a customer a quote to replace a 240 CGD panel using 3 phase equipment rated for such use. Didn't hear from him and my HVAC friend, who initially referred me to him, sent me a pic of the new panel someone else had done. QO panel with 2 pole breakers right off the Home Depot shelf. I'm guessing he was much cheaper.
I came across a three phase system today 240v between the three legs and two of the legs read 240v and the third one, taped blue read 0v to ground. There is No neutral there. That’s what brought me here. They have an elevator motor circuit there. Would it be ok to run a 240v 3P stove on that 3P configuration?? I think the stove doesn’t require a neutral.
What kind of stove is 'three-phase?' Answer is 'yes.'
Can I connect/close the open delta with a smaller transformer for low power three phase equipment and lots of single phase appliances? palace. For example a commercial place with lots of lights, few 240V slicers and refrigerators; or a big house.
I don't know, post your question on MikeHolt.com/Forum
Mike, I’ve always looked at the grounded conductor in a corner grounded Delta as a “grounded phase” and not a grounded conductor like the neutral. Since it’s a ground phase and not a neutral, there wouldn’t be any unbalanced load to carry correct? Because of
this, Ive always stuck with common color codes(480-BOY 240-BkRB(unless high leg then BkOR). I can’t imagine how many problems it’d cause electricians/or maintenance guys if they saw a white conductor landed on a breaker. I’m sure they’d immediately think it’s a neutral, especially if they put a meter on it and saw it had 0V to ground. Is this common to use a white or gray for the grounded phase in a corner grounded delta? I’ve never seen this before.
Well all I can say is that you have been violating the NEC. I really suggest you watch the video and do it properly in the future now that you know the NEC requirements.
I was asked to install a 3 phase delta system of 240V at 12.5 kva, and does it needs to be corner ground, if the transformer system does not have a stinger leg set up? If I was told to use a 3 phase 240 V twist locking receptacle at 30Amps.
I do have concerns when I asked to install 3 phase floating arounding system.
Post your question on MikeHolt.com/Forum
have to be careful with grounds in delta systems..
How so?
Good day, I currently have a Delta High Leg transformer which gives me two lugs at 120v, one at 208, and a third used as a neutral. Is it possible to remove/disconnect the center tap function so I am left with just three lugs at 120v?
You're only getting the 120v at all because of the center tapped neutral, if you remove the center tap you would no longer have a neutral and you would have no way to get 120v, you would be left with 3 different phases at 208v of potential to each other.
I am not able to explain why someone (including the utility companies) use this system. it is just does not make sense unless you want to confuse someone and say , "hay look phase and line voltage are the same! " , of course some people love to play those games. The fact is, in the very same NEC code 230.75 it says ground must not be disconnected. So if there is a fused 3 phase disconnect switch in this system, you must put a copper bar in B phase because it is ground too ! If the fuse blows up in phase B then its nothing but danger zone ! . DO NOT USE IT !
See chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/media.distributordatasolutions.com/schneider2/2020q4/documents/f50bd47f84c77526d02eee3473c1215b7dd3585b.pdf. Just use a single-phase disconnect...
It’s not actually a ground.
MH!!!
What if I used B phase and across the street my neighbor used A Phace that will cause a problem
?
This guys is not an engineer
What is your point?