Separately derived system transformer extra electrode hazard?

[dm9289]

the way I see it code wise you need a grounding electrode at a transformer for A separately derived system.
480v single phase input to transformer 2 wires and ground wire from 480v panel.
Output 240/120 to panel that uses incoming ground from 480v input
Why do you need to add electrode? What's the hazard?

 

[LarryFine]

A common conductor can be used, as long as it meets the requirements of each purpose.

 

[petersonra]

the way I see it code wise you need a grounding electrode at a transformer for A separately derived system.
480v single phase input to transformer 2 wires and ground wire from 480v panel.
Output 240/120 to panel that uses incoming ground from 480v input
Why do you need to add electrode? What's the hazard?

who knows? it is one of those curious rules around grounding and bonding that don't seem to serve any real purpose.

 

[augie47]

Are you asking why you need a grounding electrode on the LV side or why you can't use the equipment ground on the primary for your secondary grounding electrode ?

 

[jaggedben]

Or are you asking if you can ground the SDS to the same electrode system as the service? (Yes, you can.)

 

[dm9289]

who knows? it is one of those curious rules around grounding and bonding that don't seem to serve any real purpose.

Thank you

 

[dm9289]

Are you asking why you need a grounding electrode on the LV side or why you can't use the equipment ground on the primary for your secondary grounding electrode ?

yes why do you need one on low voltage side?
The 480v feed has a ground why would that not be good enough?

 

[dm9289]

Or are you asking if you can ground the SDS to the same electrode system as the service? (Yes, you can.)

Yes the grounding conductor that comes with the 480v service can that be the ground for the low voltage 120/240 side of transformer.
The way I was reading it you had to add another electrode at 240/120 side of transformer and i do not understand why?

 

[dm9289]

Are you asking why you need a grounding electrode on the LV side or why you can't use the equipment ground on the primary for your secondary grounding electrode ?

what is the hazard only using primary ground?

 

[dm9289]

A common conductor can be used, as long as it meets the requirements of each purpose.

A gnd runs from main disconnect to Main panel (no neutral}
Main panel has 2 sets of gnd plates 8" by 10ft brazed together buried in coke
Transformer primary fed from main 2 hots and gnd from main

Not sure if it meets requirements was having trouble navigating this in code. To me it seemed like they were asking for an additional electrode plus primary side gnd

 

[don_resqcapt19]

A gnd runs from main disconnect to Main panel (no neutral}
Main panel has 2 sets of gnd plates 8" by 10ft brazed together buried in coke
Transformer primary fed from main 2 hots and gnd from main

Not sure if it meets requirements was having trouble navigating this in code. To me it seemed like they were asking for an additional electrode plus primary side gnd

That is exactly what the code requires. The primary EGC, and a GEC from the secondary to the grounding electrode.

 

[Strathead]

Yes the grounding conductor that comes with the 480v service can that be the ground for the low voltage 120/240 side of transformer.
The way I was reading it you had to add another electrode at 240/120 side of transformer and i do not understand why?

A few things here. First no, the incoming "equipment grounding conductor" can't be used as the "grounding electrode conductor" of the separately derived system. In many, I would say, most cases it would be too small. The incoming does inadvertently connect to the outgoing, at least assumedly, as they both bond to noncurrent carrying metal throughout, including at the transformer, assuming the transformer isn't in a nonmetallic case. But they aren't joined together intentionally per any code requirement. Next, you have to have an additional grounding electrode conductor, but it can go to the same grounding electrode system as service does. Fore example, it can run back tot he ground bar you describe, or got to building steel, or to a qualifying portion of a water pipe, or even be permanently spliced to a grounding electrode conductor joining two or more of the services grounding electrodes. Just not to the equipment grounding conductor of the other system.

 

[LarryFine]

A gnd runs from main disconnect to Main panel (no neutral}
Main panel has 2 sets of gnd plates 8" by 10ft brazed together buried in coke
Transformer primary fed from main 2 hots and gnd from main

Not sure if it meets requirements was having trouble navigating this in code. To me it seemed like they were asking for an additional electrode plus primary side gnd

The primary EGC is unlikely to be large enough to act as the secondary GEC.

An additional electrode connection. All electrodes of a building should be joined.

 

[electrofelon]

Next, you have to have an additional grounding electrode conductor, but it can go to the same grounding electrode system as service does. Fore example, it can run back tot he ground bar you describe, or got to building steel, or to a qualifying portion of a water pipe, or even be permanently spliced to a grounding electrode conductor joining two or more of the services grounding electrodes. Just not to the equipment grounding conductor of the other system.

Actually the electrode for an SDS must be the "nearest" of either water pipe or structural steel if available.

 

[electrofelon]

The primary EGC is unlikely to be large enough to act as the secondary GEC.

Why does an SDS GEC, or any GEC for that matter, need to be so big? (rhetorical question )

 

[dm9289]

That is exactly what the code requires. The primary EGC, and a GEC from the secondary to the grounding electrode.

Thank you I just dont quite get the hazard in the case I speak of few a variety of odd circumstances the primary ground is large enough

 

[dm9289]

The primary EGC is unlikely to be large enough to act as the secondary GEC.

An additional electrode connection. All electrodes of a building should be joined.

Oddly enough EGC is in this case is large enough there are some unusual circumstances.
But my question has been what the hazard is so i guess the concern is ampacity of ground wire.

 

[dm9289]

A few things here. First no, the incoming "equipment grounding conductor" can't be used as the "grounding electrode conductor" of the separately derived system. In many, I would say, most cases it would be too small. The incoming does inadvertently connect to the outgoing, at least assumedly, as they both bond to noncurrent carrying metal throughout, including at the transformer, assuming the transformer isn't in a nonmetallic case. But they aren't joined together intentionally per any code requirement. Next, you have to have an additional grounding electrode conductor, but it can go to the same grounding electrode system as service does. Fore example, it can run back tot he ground bar you describe, or got to building steel, or to a qualifying portion of a water pipe, or even be permanently spliced to a grounding electrode conductor joining two or more of the services grounding electrodes. Just not to the equipment grounding conductor of the other system.

In this case odd circumstances primary gnd is large enough.
This transformer does have metal case, the primary gnd is connected to case and center tap directly.
In the 3 facilities I work in they are older buildings that have been repurposed a couple times newest one about 1990.
Some do have building columns attached in a few places to a large approximately 500kcm grounding conductor but as for what is tied together under concrete is unclear. Many of these panels and transformers are attached to metal beams, maybe the installers felt the physical connection or bolting on to beam was a sufficient GEC connection?

 

[dm9289]

Actually the electrode for an SDS must be the "nearest" of either water pipe or structural steel if available.

Water main in this case would be very far away hundreds of feet and to rely on piping as conductor would be very questionable.
Building steel beams are probably an option there are 500kcm cables that i have seen emerge from concrete in a few places. I guess I could get a long lead and test between building gnd and different columns far away from each other for a low resistance path.
The hazard is my main concern and it seems from input i got if primary gnd is large enough this should be OK. I guess burning thru an undersized conductor during a fault is the concern

 

[electrofelon]

But my question has been what the hazard is so i guess the concern is ampacity of ground wire.

The problem is the CMP are incompetent and don't know much about grounding particularly system grounding. The grounding electrode conductor does not need to be particularly big nor is it particularly important. The grounding electrode conductor does not carry faults, it just provides a ground reference for the grounded conductor during normal operation.