Why did the designer Zig and Zag

[Wire-Smith]

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.

my post was not about stray current derived from the system on the pier or the system on the shore connected to the pier, its about the metal parts of the pier that we are connecting to the utility MGN acting as an antennae for utility current. this current is a normal problem, people get electrocuted by it, out in milk country cows won't milk, often shows up in pools where the pool equipotential bonding has deteriorated or wasn't sufficient to begin with.

 

[xptpcrewx]

Why did the designer Zig and Zag

my post was not about stray current derived from the system on the pier or the system on the shore connected to the pier, its about the metal parts of the pier that we are connecting to the utility MGN acting as an antennae for utility current. this current is a normal problem, people get electrocuted by it, out in milk country cows won't milk, often shows up in pools where the pool equipotential bonding has deteriorated or wasn't sufficient to begin with.

No one is connecting anything to the MGN. I think the discussion stops there.

What it sounds like you are talking about is the voltage gradient superimposed on earth due to the MV system neutral voltage drop.

 

[Wire-Smith]

No one is connecting anything to the MGN. I think the discussion stops there.

What it sounds like you are talking about is the voltage gradient superimposed on earth due to the MV system neutral voltage drop.

the reason MGN was brought up is some posters thought the designer may have been trying to not connect anything on the pier to the MGN like you are saying the install is done.

side-note, post 5 has left some uncertainty about it not being connected to MGN. again we are not talking about the piers electrical system creating stray current

 

[electrofelon]

the reason MGN was brought up is some posters thought the designer may have been trying to not connect anything on the pier to the MGN like you are saying the install is done.

side-note, post 5 has left some uncertainty about it not being connected to MGN. again we are not talking about the piers electrical system creating stray current

Right, its like the situation you have where you have two separate buildings each that has their own service. Say you run a circuit for a three way switch between the two buildings. Now, if the EGC for the switch interconnects with the buildings' EGC's , then you will have neutral current from the services travelling across the switch circuit EGC (Assuming buildings are fed by the same transformer or its an MGN system).

I'm not supporting this idea all the way to the grave or anything, its just plausible that is what someone was thinking and the physics are valid.

 

[oldsparky52]

No one is connecting anything to the MGN. I think the discussion stops there.

If the gear on the river is bonded back to the gear on the shore, then the grounding on the river gear has a connection to the neutral of the PoCo distribution system because of their grounding their neutral. No matter how many transformers you install along the way. This is a parallel path for current going back to the source (the substation). A low impedance connection to the river is much much easier than a low impedance connection to the earth.

Why are you fighting against this?

 

[Wire-Smith]

Right, its like the situation you have where you have two separate buildings each that has their own service. Say you run a circuit for a three way switch between the two buildings. Now, if the EGC for the switch interconnects with the buildings' EGC's , then you will have neutral current from the services travelling across the switch circuit EGC (Assuming buildings are fed by the same transformer or its an MGN system).

I'm not supporting this idea all the way to the grave or anything, its just plausible that is what someone was thinking and the physics are valid.

I agree, my post 33 was about how people were talking about causing current in the river. I think so long as the system supplying the pier is not double bonded, that bonding utility and pier together would actually lower gradient voltages in the river. I was wondering what everyone else thought about it. I think having a utility mgn in the vicinity alone gives the potential for gradients/stray current, and adding bonding to the river would reduce that in the river.

 

[Wire-Smith]

The reason I see for the design, is if system supplying pier is ungrounded on shore and only grounded at pier, then their is never a path when a ground fault occurs on the pier for their to be current from pier to shore except through wire. I speculate that the zigzag was an afterthought and the original plan was just completely ungrounded design.

 

[kwired]

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.

Stray current from POCO MGN is the concern.....

No one is connecting anything to the MGN. I think the discussion stops there.

What it sounds like you are talking about is the voltage gradient superimposed on earth due to the MV system neutral voltage drop.

Only if there is no EGC, ran with the feeder from each direction. Ordinarily you must run an EGC, it will ultimately be bonded to the MGN via the main/system bonding jumper as well as grounded service conductor.

I'd have to do some more digging through code to say for certain if there is any way possible to run these feeders without an EGC for this special circumstance.

Electrocutions at docks seem to involve a rise on the EGC more often than any electrocutions because of failed or missing GFCI's. Someone is swimming or otherwise in the water and they are at earth potential, then they come close to or even touch an object that is connected to the EGC of equipment on dock - and it is sitting there at same voltage to earth as what the voltage drop on the MGN/service neutral has on them combined because the EGC is also connected to them.

A GFCI will only respond to leakage current from the circuit it is protecting, it will not trip if there is stray current from outside the protected circuit on the EGC.

 

[W@ttson]

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.

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

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.

:thumbsup:

There is GF detection after the ATS (on the load terminals) on the branch that "feeds" the zig zag transformer. It monitors the 3 phases. I really do not think the Zig Zag was an after thought, I think it was intentionally done due to the requirement of the step up transformer being Delta to Delta. The designer also had a note that Electrical Ground Fault Protection is provided through the use of the Zig Zag group (i.e. the GF relay and the zig zag combo).

 

[kwired]

There is GF detection after the ATS (on the load terminals) on the branch that "feeds" the zig zag transformer. It monitors the 3 phases. I really do not think the Zig Zag was an after thought, I think it was intentionally done due to the requirement of the step up transformer being Delta to Delta. The designer also had a note that Electrical Ground Fault Protection is provided through the use of the Zig Zag group (i.e. the GF relay and the zig zag combo).

I understand what was done and why.

My remaining question is whether there is exceptions to general rules that would allow not using an EGC in the ungrounded feeders? That is where you still have a possibility of unwanted current flow from outside the system.

 

[dema]

Phase Angle

Phase Angle

I've had to specify a zig zag once - and that was due to not wanting a phase shift. In our case we had a 480V service and a 208V service that were in phase. We wanted to step down the 480V to feed the other side of a transfer switch. But most transformers are phase shift - delta wye transformers shift the phase. The zig zag was a zero phase shift transformer.

I can't tell if that applies in this situation - it isn't obvious that it does. But I don't know the characteristics of the two services nor of the transformers.

 

[newtonwb]

If's and assumptions...

Redundant feeders with ATS suggests that reliability is paramount.

If the motors are 3 phase 480v motors - then that would explain the need for stepping up to 3 phase 480.

Assuming that the zig-zag transformer is connected to ground via "high impedance ground" (ground fault detection mentioned above).

This configure could be for the 'reliability and safety' reasons that "high impedance ground" systems have to offer.

Just a thought...