Ungrounded System - Grounding Xo Term in Delta Wye

[strap89]

Q1: Say you had an industrial plant that was fed from a detla wye ungrounded system. If you grounded the Xo terminal on the transformer secondary, but didn't send the neutral conductor to the service equipment, would there be any benefit such as mitigating overvoltages? Would the building system still be classified as ungrounded?

Q2: Furthering the discussion say you carried the neutral to the service equipment and set up a grounding electrode system. If you only fed your downstream panels with three phase conductors and no neutral would this qualify as ungrounded? If someone touched one of the phase conductors would it act like a grounded system?

 

[LarryFine]

A1. Non-compliant, voltage to earth established, but no conductive pathway for local EGC and equipment ground faults, technically an ungrounded service.

A2. Compliant, a grounded service, neutral/EGC bond required, no feeder neutral required unless loads need it, yes to the shock hazard as a grounded system.

 

[zbang]

There a bunch of "it depends" here since the terms being used are a bit vague or contradictory (and I see that Larry just posted some succinct answers).

If the X0 is somehow bonded to a grounding electrode system (GES), you have a "grounded" system; that bond could be in the transformer or at the first disconnect (which could be the service disconnect). Whether or not a "neutral" wire is extended from that X0 to disconnect/panel/etc does not change that it's grounded.

However....
Are you talking about services (from power company owned transformer) or a separately-derived system from a customer-owned transformer? For instance, the PoCo doesn't bring an equipment ground (EGC) to the customer, they bring the neutral (which they may have tied to a GES; that's usually bonded to the GES at the service.

 

[GoldDigger]

A1. Non-compliant, voltage to earth established, but no conductive pathway for local EGC and equipment ground faults, technically an ungrounded service.

A2. Compliant, a grounded service, neutral/EGC bond required, no feeder neutral required unless loads need it, yes to the shock hazard as a grounded system.

A1. If it is a separately derived system no connection to a supply neutral is required. Depending on what the OP meant by "grounding" the X0, it might be compliant. No EGC is needed back toward the supply but the X0 needs to be bonded to something that is connected to the building GES, such as building steel. On the load side, there must be an EGC connection to the loads though.
A2. Agreed.

 

[strap89]

Non real world example FYI. I'm talking about a service transformer. The "benefit" as I understand it is that with an ungrounded system, an electrician could make contact with one of the phases and not get shocked. I'm trying to understand if there is a way to mitigate overvoltage due to transients such as lightning strikes and still maintain an ungrounded system. It sounds like the answer is no if you set up a GES at the service equipment. That makes sense. I'm less confident in the answer of connecting Xo to ground at service transformer but not running the neutral to the service equipment. There may be no benift. See sketch.

 

[don_resqcapt19]

Non real world example FYI. I'm talking about a service transformer. The "benefit" as I understand it is that with an ungrounded system, an electrician could make contact with one of the phases and not get shocked. I'm trying to understand if there is a way to mitigate overvoltage due to transients such as lightning strikes and still maintain an ungrounded system. It sounds like the answer is no if you set up a GES at the service equipment. That makes sense. I'm less confident in the answer of connecting Xo to ground at service transformer but not running the neutral to the service equipment. There may be no benift. See sketch.

Maybe if the transformer and the system is very small. An ungrounded system of any size will have enough capacitance to create a fatal shock hazard if you touch a phase conductor and earth.
If the utility has grounded the system at their transformer, you are required to run the grounded (neutral) conductor to the service equipment. 250.24. Your drawing would be correct for a separately derived system, but not for a service.

 

[winnie]

Non real world example FYI. I'm talking about a service transformer. The "benefit" as I understand it is that with an ungrounded system, an electrician could make contact with one of the phases and not get shocked. I'm trying to understand if there is a way to mitigate overvoltage due to transients such as lightning strikes and still maintain an ungrounded system. It sounds like the answer is no if you set up a GES at the service equipment. That makes sense. I'm less confident in the answer of connecting Xo to ground at service transformer but not running the neutral to the service equipment. There may be no benift. See sketch.

There is no such thing as a true ungrounded system. Every system has leakage current and capacitive coupling to ground. Any industrial scale 'ungrounded' system will have a significant shock hazard due to this sort of capacitive coupling. (What Don said!) Operating rooms have very small ungrounded systems with sensitive leakage detection where capacitive coupling is small enough to protect from shock hazard, but that is not what you are talking about.

The primary benefit of an industrial scale ungrounded system is 'continuity of service'. The system can suffer a single ground fault and keep on running as normal. Instead of a breaker tripping because of a ground fault, a fault indicator tells you that you need to schedule a shutdown to fix the problem.

Ungrounded systems are subject to overvoltages both because of external transients and because of internal fault conditions. In a 'restriking fault condition', the system inductance can combine with system capacitance to ground to form a 'boost' converter and actually pump system line-ground voltage up to more than L-L voltage.

The way to mitigate these issues is with an 'impedance grounded' system. This requires a wye source where the neutral is connected to earth via an impedance (usually a resistor). The impedance is selected to be high enough that during a ground fault _limited_ current flows and the system continues to operate, but low enough to limit the transient overvoltages.

-Jon

 

[strap89]

Maybe if the transformer and the system is very small. An ungrounded system of any size will have enough capacitance to create a fatal shock hazard if you touch a phase conductor and earth.
If the utility has grounded the system at their transformer, you are required to run the grounded (neutral) conductor to the service equipment. 250.24. Your drawing would be correct for a separately derived system, but not for a service.

250.24, that's what I was looking for.

The only place I can find a crystal clear requirement for a true ungrounded system is ship to shore power for Navy Vessels. Not sure if this applies to Non-Navy Vessels as well. I guess there would have to be a customer owned transformer, because most utilities won't permit their transformers to be ungrounded.

 

[kwired]

As has been mentioned if this is the utility supply and they have grounded anything - it is a grounded system.

AFAIK not to often will a utility provide an ungrounded system, most of those are separately derived on site. If the system in question is separately derived and you wish to take advantage of some the reasons to use an ungrounded system but is a wye system, another choice with similar aspects would be to set it up as an impedance grounded system.

Keep in mind that even though you might have an ungrounded system you still must install a grounding electrode system and still must run equipment grounding conductors to keep non current carrying components all at same potential, you just don't have a grounded conductor in the system.