# Grounded vs ungrounded delta requirements



## joebanana

Ungrounded systems are usually semi-critical, as in they can tolerate a ground fault without bringing the whole system down. There should be a ground fault panel somewhere to indicate a fault so a planned shutdown can be scheduled before a second fault does bring it down. Sometimes there's a resistance between the ground, and neutral making it a high impedance grounded system, to reduce transient over-voltage problems, and lower the perspective ground fault levels found on ungrounded systems.


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## paulengr

mburtis said:


> So this is a spin off from my other post, decided to start new since I actually know what is going on now and have some specific questions just to help my understanding on these delta system grounding rules.
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> So we have a water plant with a 480 delta service, was a 240V service until a few years ago so all the old stuff is running off a 480V primary, 240V with 120V lighting tap secondary transformer. The 480 breaker that feeds this transformer has a piece of tape on it that says caution ungrounded system. The secondary from the transformer feeds an MCC, Im not 100% sure but all the loads in the mcc may be 3 phase so the center tap (neutral) may not be used.
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> I've been reading SOARES book on grounding and bonding, trying to research this stuff. If I'm understanding this right as long as the neutral is not being used as a circuit conductor, this is a 3 wire delta system and since its a transformer would be considered a new source. This means this is permitted to be ran ungrounded?
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> If the neutral is being used as a circuit conductor making it a 4 wire delta then it would be required to be a grounded system correct? Why it was left ungrounded even if permitted will need to be answered by the engineers.
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> Then a breaker in the MCC feeds another transformer. That isn't on any of the prints and is a 240V primary to 240V with 120V lighting tap secondary. This feeds a breaker panel running 120V outlets and other loads.
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> So neutral on the secondary of this transformer is definitely being used, and would be considered a new source, meaning it is required to be grounded? Currently this panel test as an open connection between the neutral and ground bus.
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> Reason i found all this out was that there was a ground fault on one of the circuits from this panel, only symptom was some gfci outlets that wouldn't reset. Had 120V from the ground to both hot and neutral with the breaker to the outlet shut off. I found the ground fault and got it disconnected so everything is behaving properly now, but discovering this entire system was ungrounded led me down the rabbit hole.
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> Just trying to understand the rules as applies to the grounded vs ungrounded systems.


Lots of questions. First look on the transformer name plate. It will show a Y or a delta for both primary and secondary side on a 3 phase transformer. If it's delta there is no neutral, period if it's wye then there is an X0. You can have an ungrounded wye used instead of a delta but it's more expensive and just not a good idea overall, except resistance grounded systems which are the best of both worlds.

Going from there Code requires that ungrounded and resistance grounded systems have ground fault detection which can be as simple as phase lights. You can't not have ground fault detection, not the same as ground fault tripping. Also kind of obvious but you can't the line to neutral loads on a delta because there is no neutral.

Transformers provide magnetic isolation. Not getting into whether or not a particular design blocks ground faults or not but from a Code point of view you have two choices. You can continue the ground of the primary side or start a new grounding/bonding system as a separately derived system. This is what your 240/120 system should be for two reasons. First so you can troubleshoot easier with separate systems and second because the neutral is at a different voltage (about 277 V) by nature compared to the 480 system. Since the 480 system isn't grounded it might work but it's not a good idea.

Also you're a water plant. Ungrounded systems have 4 times the failure rate of grounded ones, especially where corrosion and arcing are common. When you get a loose connection such as from corrosion or any kind of arcing like a pump failure, it creates voltages in your system that get up to 6 to 8 times the system voltage. These voltages dine on motors and eat electronics for dessert. Unless you are a Navy facility, get rid of that crap. You have to fix the Code violation anyways. But be careful. Just connecting X0 (if you have one) isn't all on the 480 side. The grounding and bonding might be sized for ungrounded so undersized for a "standard" solidly grounded wye. Also over a certain size you have to add ground fault tripping to the main breaker. So you might have a lot of expensive changes to make.

I'll give you a hint though. Ever seen what's left of something in your house if it shorts out? Ever seen the damage after a short on 480? A high resistance ground gives you the best of both worlds. You've already seen one of the issues of ungrounded systems...finding ground faults is a nightmare. The second one is it eats motors as it ages. The third rare one is burn downs.

Grounded systems avoid all these at the expense of extremely heavy ground fault currents. It's easy and it's the North American standard but not the best. Breakers do double duty...normal overload/overcurrent tripping catches ground faults most of the time.

Splitting the middle is a high resistance system. A small power resistor connects X0 to the ground rod. Typically i size it for 10 A so 27.7 ohms on a 480 V system. You can alarm and not just trip like ungrounded since all your motor wiring is at least #14 by Code and your existing minimum grounding and bonding is plenty big. Ground fault currents are low level, not much damage. You can easily find them by clamping an amp clamp around all 3 phases (ground faults mean I1+I2+I3 = ground current instead of 0). No shutdowns and opening and closing breakers trying to find it. But no heavy fault current damage from ground faults.

Sent from my SM-T350 using Tapatalk


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## mburtis

both transformers in the plant are marked as delta primary, delta secondary with a lighting tap (center tap on one phase) at least if i remember correctly. Now this is all stuff inside the plant after the service disconnect and such. I was under the impression that this center tap was considered the neutral, ie a 4 wire delta or high leg delta. 

After thinking about it i do remember ground fault lights over by the 480 breaker (the one that is marked as ungrounded) that feeds the one transformer. However I didn't notice any others, so it seems that after the secondary of this transformer there would either have to be another set of ground fault indicators or it would have to be grounded right? Doesn't seem it would be physically possible to indicate a ground fault on the secondary side of a transformer via indicators on the primary side.

The whole ungrounded thing makes me really nervous. Up until now, nobody even knew about it so that means we were most certainly not meeting the code requirements for maintaining such a system. Plus the possibility of major problems if more than one fault where to occur. Plus nothing at the water plant is so critical to justify it in my mind. Almost everything is redundant and we have a full SCADA with alarm callouts if plant performance were to go south. 

From what i have read the secondary side of the second transformer is for sure a code violation, due to it supplying the 120V loads from phase to neutral on a 4 wire delta. The secondary side of the first transformer may very well be in the same boat. My main concern is getting everything proper and safe. The 120V ground fault that i found had probably been active for 4 or 5 years. Ive only been there less than 2 (and i primarily work at a different plant, which is why i know very little about the place) but at some point an electrician removed some of the gfcis and installed regular receptacles so they would stop tripping, apparently he never tested it. 

Cant figure out why they would have bothered with an ungrounded hassle unless it has something to do with tying into the old 240V stuff some of which dates back to 64 or some engineer was trying to show off. Would be a lot simpler had they upgraded the entire plant to 480V like everyone wanted, but the engineers thought it was to much work.


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## mburtis

I have been reading some on the resistance or impedance grounded system. Will have to continue researching it as for the most part it sounds like the best of both worlds.


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## telsa

Simplicity beats perfection.

Look what you're going through.


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