SDS Grounding/Bonding.. Ground Loop?

[Jerramundi]

Perhaps a dumb question...

Thinking about installing a 240V/120V Step-Down Isolation Transformer (i.e. SDS).

As it stands, the plan is to transfer from an underground Sch. 80 1/2" PVC to 1/2" IMC (not the part I want you to critique, haha) and land that on the input/primary side of of the SDS. Because of the PVC run, my initial thought is to pull a wire type EGC from the building metal conduit system/last interior j-box to the SDS outside and bond it to the enclosure... so that in the case of a primary side fault at the SDS, the primary OCPD can trip.

On the output/secondary side (i.e. same enclosure) will be an IMC threaded nipple to (1) 60A 2 Position Outdoor Load Center. Per "some NEC Code I don't feel like looking up right now," the (1) SBJ, (2) SSBJ, (3) GEC, and (4) EGCs for the SDS will terminate in this enclosure (i.e. the first disconnecting means of the secondary).

My question is... is there any potential for a ground loop here because I will have BOTH (1) a wire type EGC coming from the building to the SDS enclosure... and (2) a metal conduit bonding the SDS to the first disconnect, which will be grounded to a new ground rod.

I can't but wonder if I should avoid the EGC coming to the SDS from the building... and just let it be grounded at the first disconnect.

Thoughts?

 

[ActionDave]

....I can't but wonder if I should avoid the EGC coming to the SDS from the building... and just let it be grounded at the first disconnect.

Thoughts?

If you do that you will have no way to clear a fault on the primary feed of the transformer and that would be about one of the worst things you could do as an electrician.

 

[LarryFine]

There is no concern with parallel EGCs.

What you want to avoid is having the EGC/GEC system bonded to the neutral at more than one point.

 

[tom baker]

I sometmes see a post on a "ground loop", not sure where that comes from, its almost an electrical legend. The EGC does not normally carry current, so a loop is of no concern. Part of this issue may be a misuse of ground term, look up the definition of ground...

 

[texie]

Bottom line is that you must have an EGC all the way to the transformer case to handle any primary fault (as the OP stated in his second paragraph.

 

[Jerramundi]

If you do that you will have no way to clear a fault on the primary feed of the transformer and that would be about one of the worst things you could do as an electrician.

Yikes. "One of the WORST THINGS" ?? I guess I'm just overthinking the "Separate" part of "Separately Derived System."

 

[Jerramundi]

There is no concern with parallel EGCs.

What you want to avoid is having the EGC/GEC system bonded to the neutral at more than one point.

This is precisely my concern... but in an SDS this is supposed to happen, correct? EGC/GEC to Neutral on (1) the Primary Side and (2) the Secondary Side. My understanding is just that the two should be isolated from one another.

In the 1st Disconnect on the Secondary Side of the SDS, the Neutral will be bonded to the GEC and to ground.

This 1st Disconnect enclosure will be bonded to the SDS enclosure via the metal conduit nipple between them.

The SDS enclosure will be bonded to the Primary Side EGC running back to the building.... and thus to the Primary side Neutral/Ground connection.

In my mind, I'm seeing exactly what you're warning about here and that's why I made this post.

 

[don_resqcapt19]

This is precisely my concern... but in an SDS this is supposed to happen, correct? EGC/GEC to Neutral on (1) the Primary Side and (2) the Secondary Side. My understanding is just that the two should be isolated from one another.

In the 1st Disconnect on the Secondary Side of the SDS, the Neutral will be bonded to the GEC and to ground.

This 1st Disconnect enclosure will be bonded to the SDS enclosure via the metal conduit nipple between them.

The SDS enclosure will be bonded to the Primary Side EGC running back to the building.... and thus to the Primary side Neutral/Ground connection.

In my mind, I'm seeing exactly what you're warning about here and that's why I made this post.

In a code compliant installation, the primary and secondary EGC will never be isolated from each other. They both are required to connect to the transformer enclosure. This is not bonding a neutral twice, as the separately derived system has a "separate" neutral. Yes both neutrals are connected to EGCs, but neither neutral is bonded to the EGCs in more than one location.

I didn't see any comment about the system bonding jumper and the grounding electrode conductor and grounding electrod that is required for the secondary.

 

[texie]

Yikes. "One of the WORST THINGS" ?? I guess I'm just overthinking the "Separate" part of "Separately Derived System."

Well, it is pretty serious. Any metallic items in an electrical system you must always be sure there is a proper fault return path or they could become electrocution hazards.

 

[Jerramundi]

Well, it is pretty serious. Any metallic items in an electrical system you must always be sure there is a proper fault return path or they could become electrocution hazards.

I'm aware. I only suggested not doing the EGC to the SDS Case because I was concerned about a double ground to neutral connection. It's to tough to explain (see the following the post... w/ visual aid! )

 

[Jerramundi]

I didn't see any comment about the system bonding jumper and the grounding electrode conductor and grounding electrod that is required for the secondary.

The ORANGE highlighted path represents my concern.... (Here's your friggin' SBJ lol)

 

[winnie]

This is precisely my concern... but in an SDS this is supposed to happen, correct? EGC/GEC to Neutral on (1) the Primary Side and (2) the Secondary Side. My understanding is just that the two should be isolated from one another.

The neutral of a system is only supposed to be grounded once and only once.

But that is why we use the term 'separately derived system'. When you have a normal transformer with a separate primary and secondary, you create a new system with a new neutral, and that neutral also needs to be grounded. This is not a second grounding of the original system, but the initial grounding of the new 'derived' system.

-Jon

 

[Jerramundi]

The neutral of a system is only supposed to be grounded once and only once.

But that is why we use the term 'separately derived system'. When you have a normal transformer with a separate primary and secondary, you create a new system with a new neutral, and that neutral also needs to be grounded. This is not a second grounding of the original system, but the initial grounding of the new 'derived' system.

-Jon

I understand that. See the above image in Post #11 for my concern better expressed in visual form

 

[don_resqcapt19]

The ORANGE highlighted path represents my concern.... (Here's your friggin' SBJ lol)
View attachment 2553304

That is not two connections between the neutral and the earth. It is just multiple connections between the equipment grounding systems and the earth. Every system is like that.

 

[Jerramundi]

That is not two connections between the neutral and the earth. It is just multiple connections between the equipment grounding systems and the earth. Every system is like that.

What no props on my diagram put together in 15m w/ Microsoft Word? lol

 

[Jerramundi]

That is not two connections between the neutral and the earth. It is just multiple connections between the equipment grounding systems and the earth. Every system is like that.

So I'm overthinking that "ORANGE" shared pathway between the 2 points where Neutral connects to Ground?

 

[LarryFine]

This is precisely my concern... but in an SDS this is supposed to happen, correct? EGC/GEC to Neutral on (1) the Primary Side and (2) the Secondary Side. My understanding is just that the two should be isolated from one another.

There is no internal connection between neutrals in a transformer, which is why an isolated secondary is called separately derived.

In the 1st Disconnect on the Secondary Side of the SDS, the Neutral will be bonded to the GEC and to ground.

Unless done in the transformer, yes.

This 1st Disconnect enclosure will be bonded to the SDS enclosure via the metal conduit nipple between them.

So far, still yes.

The SDS enclosure will be bonded to the Primary Side EGC running back to the building.... and thus to the Primary side Neutral/Ground connection.

Yes, that will end up being true. However, no secondary current will attempt to flow back through the primary circuit. Grounding a single point on any system does not cause any neutral current to flow through EGCs. A second connection to ground, yes.

In my mind, I'm seeing exactly what you're warning about here and that's why I made this post.

Draw out what you see in your mind, and see if you can find the pathway you're picturing. Maybe you see something I don't.

As for multiple sources sharing EGCs, GECs, etc., remember that we only have one Earth. Some interconnections are unavoidable.

As an example, the power company's primaries and secondaries usually share single neutrals, even along the same pole pathway.

 

[Jerramundi]

Draw out what you see in your mind, and see if you can find the pathway you're picturing. Maybe you see something I don't.

Please see post #11 and let me know what you think...

Does the orange path I've highlighted not effectively connect the two neutrals together?

 

[winnie]

So I'm overthinking that "ORANGE" shared pathway between the 2 points where Neutral connects to Ground?

What you have there is one point on the neutral being connected to multiple points on the earth. This is a different issue from multiple points on the neutral being connected to the grounding system or earth.

Jon

 

[Jerramundi]

What you have there is one point on the neutral being connected to multiple points on the earth.

I suppose. I'm still tempted to argue that said multiple points to Earth create a connection between the two neutrals... but perhaps I'm thinking about it wrong. It sounds like my plan is good. I'm just having hard time NOT overthinking it.