Why did the designer Zig and Zag

[oldsparky52]

Question: Why didn't the designer just send a neutral and a ground from each utility which is on shore? Then have the neutral be switched at the ATS if there was any concern of the two utilities being connected at all?

That this isn’t a service. It’s a SDS. There is no utility involved.

Utility companies are involved.

 

[W@ttson]

That this isn’t a service. It’s a SDS. There is no utility involved.

Correct, the 208V Delta to 480V Delta step-up transformers (one on each side of the river) are client owned and are separately derived systems from the 208/120V utility supply.

 

[xptpcrewx]

If each is grounded, and the grounded conductor brought from each bank to the pier, then the grounded conductor would end up tying the two banks together.

Then this wouldn’t be a SDS, there wouldn’t be multiple grounding, and there is still no stray current involved.


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[W@ttson]

Utility companies are involved.

This would have been if they instituted a regular wye secondary. I missed that it was a delta and no connection to the utility since it is a SDS. Seems like the designer choose very specifically to have the step up transformer be a 208V Delta to 480V delta. So that no SDS grounding of neutral was required since no neutral exists.

 

[oldsparky52]

This would have been if they instituted a regular wye secondary. I missed that it was a delta and no connection to the utility since it is a SDS. Seems like the designer choose very specifically to have the step up transformer be a 208V Delta to 480V delta. So that no SDS grounding of neutral was required since no neutral exists.

And by design isolates all of the on premise grounding from any PoCo grounding.

 

[xptpcrewx]

Why did the designer Zig and Zag

Utility companies are involved.

I doubt it. Sounds to me like the utility feeds premises wiring then voltage gets transformed from premises wiring to pier.

 

[W@ttson]

Yes. But there are _two_ SDSs on opposite banks of a river.

If each is grounded, and the grounded conductor brought from each bank to the pier, then the grounded conductor would end up tying the two banks together.

As described, there are no grounded conductors and no EGCs brought with these feeders.

-Jon

If there were grounding conductors and if the ATS accepted the grounded conductors from each service at the pier, one way around connecting the two service grounding conductors would be for a switched neutral. Such that neither power source on each river bank would see the others neutral connection. I tried to allude to this in my original post. I called it two services but should have called it separately derived systems

 

[Wire-Smith]

How do you know utility is 120/208 if there is no neutral? I'm guessing zig zag was an afterthought with ground fault alarm

 

[xptpcrewx]

How do you know utility is 120/208 if there is no neutral? I'm guessing zig zag was an afterthought with ground fault alarm

There is a neutral on the 208Y/120-V service derived system; but we are saying there is no neutral on the 480-V separately derived system.

 

[Wire-Smith]

In that case, if pier is getting 480 I would see if it is ungrounded at transformer if so they likely used zigzag for ground connection for the drives. Systems supplying drives are supposed to be solidly grounded Wye systems, although with compromises you can use them on other systems

 

[xptpcrewx]

In that case, if pier is getting 480 I would see if it is ungrounded at transformer if so they likely used zigzag for ground reference for the drives.

Yes transformer is delta-delta ungrounded. Zig-zag was used for grounding the system most likely to simply ATS installation.

 

[electrofelon]

How the substation is grounded is not relevant; what we care about here is how/if the SDS where the feeder is supplied from is grounded. I’m still not understanding what you mean by parallel paths and why you think the water would have stray currents.

Assuming things are done properly, non current carrying metal parts of an electrical system, even if its an SDS with the secondary ungrounded, Will be connected to the utility MGN (if the utility supply is an MGN). So the EGc's and/or metal parts at the pier will provide a low impedance path for the MGN. That is the theory anyway. OP said there wasnt an EGC so maybe they deliberately skipped it to break the path for the MGN, although seems like the cable armor might be bridging the two systems.

 

[Wire-Smith]

Assuming things are done properly, non current carrying metal parts of an electrical system, even if its an SDS with the secondary ungrounded, Will be connected to the utility MGN (if the utility supply is an MGN). So the EGc's and/or metal parts at the pier will provide a low impedance path for the MGN. That is the theory anyway. OP said there wasnt an EGC so maybe they deliberately skipped it to break the path for the MGN, although seems like the cable armor might be bridging the two systems.

wouldn't that lower the difference of potential in the river? more of the current diverted to the wire ground instead of through the river. so long as neutral loads aren't used at the pier for the electrical system that connects to shore(and a neutral ground bond at pier and shore xfmr).

 

[xptpcrewx]

wouldn't that lower the difference of potential in the river? more of the current diverted to the wire ground instead of through the river. so long as neutral loads aren't used at the pier for the electrical system that connects to shore(and a neutral ground bond at pier and shore xfmr).

This stray current in the river thing is a bit ridiculous. The system is single point grounded at the pier by the zig-zag transformer. System charging current is pretty negligible for 600V systems (1A per 1MVA of system capacity). For all intents and purposes no stray current.

 

[kwired]

This would have been if they instituted a regular wye secondary. I missed that it was a delta and no connection to the utility since it is a SDS. Seems like the designer choose very specifically to have the step up transformer be a 208V Delta to 480V delta. So that no SDS grounding of neutral was required since no neutral exists.

Just because you have no neutral doesn't relieve you from grounding something else, or providing an ungrounded system with ground fault detection. And if you have ungrounded system it still doesn't relieve you from providing EGC's and bonding all non current carrying items like you normally do, that is what makes the link between services on each side, and even though you don't have any neutral current from the facility served, doesn't mean there isn't a path through your grounding network for utility neutral current to flow.

And by design isolates all of the on premise grounding from any PoCo grounding.

Yes, but you still need an EGC and ultimately that will tie both utilities together via main bonding jumpers on the service equipment.

I doubt it. Sounds to me like the utility feeds premises wiring then voltage gets transformed from premises wiring to pier.

EGC's still required and introduce a solid path between the two services grounded conductors.

There is a neutral on the 208Y/120-V service derived system; but we are saying there is no neutral on the 480-V separately derived system.

As I mentioned above, does not relieve grounding the secondary or using GF detection, and you still need GES and EGC's tied to non current carrying components.

Assuming things are done properly, non current carrying metal parts of an electrical system, even if its an SDS with the secondary ungrounded, Will be connected to the utility MGN (if the utility supply is an MGN). So the EGc's and/or metal parts at the pier will provide a low impedance path for the MGN. That is the theory anyway. OP said there wasnt an EGC so maybe they deliberately skipped it to break the path for the MGN, although seems like the cable armor might be bridging the two systems.

:thumbsup:

 

[xptpcrewx]

Why did the designer Zig and Zag

EGC's still required and introduce a solid path between the two services grounded conductors.

As I mentioned above, does not relieve grounding the secondary or using GF detection, and you still need GES and EGC's tied to non current carrying components.

Dude seriously? When have I ever said anything about EGC’s not being required? Also, thats bonding not grounding. It sounds like you are fixated on this point no matter who/what you are responding to.

 

[kwired]

Dude seriously? When have I ever said anything about EGC’s not being required? Also, thats bonding not grounding. It sounds like you are fixated on this point no matter what you are responding to.

It is bonding, and it does just including bonding one service to the other creating a path for potentially undesired current to flow on whether you added any additional grounding electrodes or not.

You can't avoid stray currents when your utility uses MGN and you are connecting to more than one point on their system unless you intentionally create isolation. General code rules still require equipment grounding which will defeat such isolation. There may be code that allows said isolation for OP's application somewhere, IDK for certain, but if so would likely have some conditions that must be met and is not something allowed as a general rule.

 

[xptpcrewx]

Why did the designer Zig and Zag

It is bonding, and it does just including bonding one service to the other creating a path for potentially undesired current to flow on whether you added any additional grounding electrodes or not.

You can't avoid stray currents when your utility uses MGN and you are connecting to more than one point on their system unless you intentionally create isolation. General code rules still require equipment grounding which will defeat such isolation. There may be code that allows said isolation for OP's application somewhere, IDK for certain, but if so would likely have some conditions that must be met and is not something allowed as a general rule.

Again with MGN. OP has nothing to do with this. For all intents and purposes currents will not flow between separately derived systems. This is a bogus concept unless you want to talk about specifics; i.e. system faults, charging currents, etc.

 

[kwired]

Again with MGN. OP has nothing to do with this. For all intents and purposes currents will not flow between separately derived systems. This is a bogus concept unless you want to talk about specifics; i.e. system faults, charging currents, etc.

I totally agree, no current will flow between the separately derived systems. But if you have a link between the grounded conductors of the two services (an EGC originating at each of the services will make that link), you still have a path for current from outside the SDS's to flow on. Said current is not selective on who it may shock when conditions are right just because it is not from the SDS, current is still current and it takes a voltage to drive it meaning there is voltage drop along the path, may only be a few volts dropped but will lead to voltage between the EGC and "earth".

 

[xptpcrewx]

Why did the designer Zig and Zag

I totally agree, no current will flow between the separately derived systems. But if you have a link between the grounded conductors of the two services (an EGC originating at each of the services will make that link), you still have a path for current from outside the SDS's to flow on. Said current is not selective on who it may shock when conditions are right just because it is not from the SDS, current is still current and it takes a voltage to drive it meaning there is voltage drop along the path, may only be a few volts dropped but will lead to voltage between the EGC and "earth".

If touch/step/transfer potential is a problem, then you need to engineer/design the bonding/grounding such that these currents create acceptable voltage-drops. The clearing times associated with ground-faults for solidly grounded systems are quick (around 50ms). And just in case anyone was wondering: This type of current flowing on the EGC is not considered objectionable current per the NEC. Furthermore, because we are providing the path with EGC’s, is it even worth calling it a stray current? In my mind the term stray current is probably more appropriately suited for ungrounded systems or situations where there is no intentional/effective return path.