Confused by array grounding

[timselectric]

I don't think that the solution is any different. But the situation becomes mute, in some cases.
As with a metal secondary building in full contact with the earth.
Then the decision on an auxiliary ground rod doesn't matter.
At that point, the best thing to do. Is run a GEC. Instead of an EGC.

 

[Shimmy]

I don't think that the solution is any different. But the situation becomes mute, in some cases.
As with a metal secondary building in full contact with the earth.
Then the decision on an auxiliary ground rod doesn't matter.
At that point, the best thing to do. Is run a GEC. Instead of an EGC.

It changes some when you get into commercial buildings and 480V service, although for here the subtleties don't really come into play.

 

[timselectric]

It changes some when you get into commercial buildings and 480V service, although for here the subtleties don't really come into play.

A 480v (or any voltage) 3-phase delta system has some differences. But, a 480v (or any voltage) 3-phase wye system is the same as anything else. When it comes to grounding.

 

[Shimmy]

A 480v (or any voltage) 3-phase delta system has some differences. But, a 480v (or any voltage) 3-phase wye system is the same as anything else. When it comes to grounding.

The grounding system for the building/campus changes by the fact that you have multiple separately derived sources (step-down transformer) requiring grounding electrode connections. How much it changes depends on the type of construction.

 

[timselectric]

The grounding system for the building/campus changes by the fact that you have multiple separately derived sources (step-down transformer) requiring grounding electrode connections. How much it changes depends on the type of construction.

Yeah, every time you go through an isolation transformer you start over.
But, it's still the same thing. (Lather, rinse, repeat)

 

[Hedges]

Isolation transformer (I've got a 25kVA 240/480 to 120/240 I plan to use) - that derives a neutral, which I will ground (by connection to an existing system which already has ground).

But ground of transformer chassis needs to tie to ground of source, and needs to tie to ground of derived system, so it is all one big happy ground, correct? And lighting strike to overhead neutral of source, which is bonded to ground, then conducts through ground of other system.

Imagine that the source is overhead utility and gets isolated by transformer, feeds some loads including something like Chargeverter which provides backup to a second underground utility system - if one grid connections goes down the other keeps power on through double-conversion. Not exactly my application, but what I want to do is avoid tying the neutrals of two utility drops together. After deriving neutral, those are tied together, but galvanically isolated from second source.

But it seems to to me grounds have to be connected through, can't "derive" a separate ground system at the transformer because both primary and secondary are on the core, either could fault to it.

 

[timselectric]

But ground of transformer chassis needs to tie to ground of source, and needs to tie to ground of derived system, so it is all one big happy ground, correct?

Yup

 

[timselectric]

But it seems to to me grounds have to be connected through, can't "derive" a separate ground system at the transformer because both primary and secondary are on the core, either could fault to it.

Everything needs to be safe to touch. So, you have to protect it from all sources.

 

[Hedges]

Right. Otherwise line to transformer chassis fault would put chassis at 120Vrms relative to a neighbor's ground rod. With typical 25 ohms resistance, 5A flows through the earth. If I touch any part of my ground system, I provide a parallel path to earth. So would my neighbor - hmm, how well do I like him?

I would like to think Neutral is bonded to ground at one place, but in the shared utility transformer secondary, one wire is "neutral" and each customer bonds it to his ground. OK so far, in that line to fault ground within a customer facility clears the fault by tripping breaker. but each of these customers has separate ground, and fault to earth somewhere (e.g. to metal parts of a double-insulated tool such as Bosch electric jackhammer) would be an electrified ground rod. (I'm pretty sure it is all metal from armature to jackhammer bit.) This would cause objectionable current in earth and in ground system. Think I'll make sure I use GFCI outlet.

 

[timselectric]

hmm, how well do I like him?

lol

Think I'll make sure I use GFCI outlet.

Always best for outdoor outlets.

 

[lapsmith]

Currently, both are required. Hopefully soon, they will drop the requirement for the rod. And just require the EGC. Because it creates more problems than it solves.

Well, it is easy enough to remove the GEC from the rod so that is what I will probably do. I can always put it back if I ever sell.

It is interesting though that depending on the type of building and structure the optimal solution can change significantly. Too much focus on one type or another skews good practice.

I'm wondering if my building falls into that category,. The barn has metal roof and metal siding, but the framing is wood. Even though the (painted) siding is a few inches from the soil, so no direct contact, there is bound to be some electrical continuity, such as via grounded metal outdoor outlet boxes screwed to the side of the building. I tried to measure resistance from soil to the siding, but not surprisingly, I got inconsistent readings; from kilo-ohms to meg-ohms. So based on all that, would you say if this is the type of building where a supplemental ground rod might be beneficial?

 

[Johneb]

If I may ask, in reading through this if I put up solar panels running to my mppt and my inverter is running a off grid system to run our heat pump. I can run a ground from the house panel to the off grid panel then on to the solar panel frames?

 

[FilterGuy]

If I may ask, in reading through this if I put up solar panels running to my mppt and my inverter is running a off grid system to run our heat pump. I can run a ground from the house panel to the off grid panel then on to the solar panel frames?

Yes

 

[Hedges]

All metal things and all power systems connected to your house should share a common ground connection.
You wouldn't want to touch a heat-pump air handler simultaneous with some other grounded object and belatedly discover there was a difference in potential.

 

[Johneb]

All metal things and all power systems connected to your house should share a common ground connection.
You wouldn't want to touch a heat-pump air handler simultaneous with some other grounded object and belatedly discover there was a difference in potential.

Ahhh, then I become the potentiometer. At least for a short time.

 

[lapsmith]

All metal things and all power systems connected to your house should share a common ground connection.
You wouldn't want to touch a heat-pump air handler simultaneous with some other grounded object and belatedly discover there was a difference in potential.

Since I was planning to disconnect the GEC from the ground rod at the barn, I could connect it to the metal siding instead in an attempt to ground the entire shell of the building. However, I don't know how effective that would be since the siding and roof panels are all painted (R panels) and only connected with sheet metal screws.

 

[timselectric]

I tried to measure resistance from soil to the siding, but not surprisingly, I got inconsistent readings; from kilo-ohms to meg-ohms.

That's preferable.

So based on all that, would you say if this is the type of building where a supplemental ground rod might be beneficial?

No, that would remove the above benefit.

 

[timselectric]

Since I was planning to disconnect the GEC from the ground rod at the barn, I could connect it to the metal siding instead in an attempt to ground the entire shell of the building. However, I don't know how effective that would be since the siding and roof panels are all painted (R panels) and only connected with sheet metal screws.

Use a star washer between the painted R panel and the lug. This should bight through the paint and make the connection. Without damaging more paint than necessary.

 

[lapsmith]

I like the simplicity of that, but just thought of something else. If I'm on the roof of the barn where the solar panels are and there is a fault in the inverter, for example, that causes the panel frame to be energized, wouldn't it be better if the metal roof were NOT grounded? Note that my panels will be mounted with treated wood tiltable frames so as long as it's not raining, the panels will be electrically isolated from the metal roof. As far as I can tell, using wood is okay since the panels will actually be located on an extension of the barn which is in fact a carport structure. Furthermore, my AHJ says they do not inspect the solar panel mountings.

And finally, I plan to individually ground each panel with the EGC I just ran to the house so for the panel to get energized, this ground wire would also have to fail.

 

[FilterGuy]

There's many failures that could energize the roof. Example: one of the wires freys or comes apart in a way that touches the metal. The roof should be grounded.