High Resistance Ground System and running grounds with derived phase conductors.

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Oly

Member
Location
Oregon
The following question is not for the squeamish.

High Resistance Ground System and the application of ground conductors in pvc conduit from transformer to power distribution center

We have a separately derived 480 wye connected secondary feeding the load side of a power distribution center (PDC) 4000A main disconnect. The system is using a Post Glover HRG installed remote from the PDC. The HRG will restrict current at 277V to <10A.
The HRG is connected with the "Grounding Electrode Conductor" at one end - and the X0 bushing "grounded system conductor" run isolated and connected at the opposite resistor end.
An Equipment Bonding Jumper connects the GEC resistor end to the PDC ground bus.
There is a 4/0 ground loop connection that bonds the transformer tank to the PDC ground bus.
We have 10 parallel feeders or "derived phase conductors" routed n buried PVC conduits.

I am having trouble justifying what ground related conductor is required to be in the conduit with the derived phase conductors and how to size them. I maintain that this conductor is considered a supply side jumper.

Can these derived phase conductors be run without a ground conductor in each conduit?
Does 250.122 apply as I have heard suggested? are these truly an equipment grounding conductors? (solution 500kcmil?)
Does 250.66 apply as per 250.36 (G)? The conductor in question are technically not an "Equipment Bonding Jumper". The (EBJ) is the link from the grounded resistor end to the PDC ground bus. (solution 1/0 or greater?)
Does 250.102 apply? this may seem to be the proper NECpath. (solution 1/0?)

Can anyone describe the condition where more than 5-10A will flow in this conductor?

Any help understanding this animal would be greatly appreciated!
Thanks!
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Treat the phase conductor conduits the same as you would any other 480 3-phase 3-wire circuit with equipment ground.
A single X0 to Resistor conductor can be run independently from the phase conductors. I believe this conductor never needs to be bigger than a #8.
A #8 conductor from the resistor to 'ground' completes the installation.
 

Oly

Member
Location
Oregon


OOPs...ment to say: We have a separately derived 480 wye connected secondary feeding the line side of a power distribution center (PDC) 4000A main disconnect.
 

Oly

Member
Location
Oregon
Treat the phase conductor conduits the same as you would any other 480 3-phase 3-wire circuit with equipment ground.

JIM, as a moderator I wonder if you could expand on this a bit. Suggesting that this conductor be treated as an equipment ground would imply that one should use table 250.122 for sizing.
Is this true?
This conductor is between the transformer secondary and the line side of the breaker. if you use 250.122What protective device do you size it to?

If not table 250.122 then which one?



A single X0 to Resistor conductor can be run independently from the phase conductors. I believe this conductor never needs to be bigger than a #8.
A #8 conductor from the resistor to 'ground' completes the installation.
Jim, this is valid extra information regarding HGR guidlines but it does not satisfy the core questions.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The conductor in which the high resistance element is located will carry a limited amount of current based on its resistance and the system voltage.
If for any reason a ground fault is not promptly dealt with, a second fault somewhere else in the system will cause fault current to flow which is not limited by the high resistance. So the EGCs which actually serve equipment need to be sized for the same fault current flow that you would have in a solidly grounded system (and use the same rules based on conductor size or OCPD size that apply for a grounded system.)
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
... as a moderator...

Moderators on this site, are not required to have any special knowledge or training. Please consider their postings the same as you would any other member's.



The moderator job primarily deals with the discourse between members.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
transformer secondary and the line side of the breaker. if you use 250.122What protective device do you size it to?

If not table 250.122 then which one?

When using a High resistance grounded system Line-Neutral loads are not allowed, therefore the NEC contains different rules for these 'neutral' conductors.

Have you read?
250.20(D)
250.36(G)
 

Ingenieur

Senior Member
Location
Earth
Each feeder should have a gnd cond based on CB size
each feeder must have a gnd from the grounded side of the NGR

under no circumstances will this see more than 277/NGR ohmic value
a double ph-gnd fault may see less

what protection is on the NGR?
i would sense V across it and trip at 50% with a small td
this will also detect an open ngr ie loss of grounding
and a ct to trip at 50%
if you can't trip at least alarm

I need to think about tying the xfmr frame to the ngr/frame gnd

the purpose of this scheme is to limit fault damage
Frame potential during a fault, basically gnd cond Z x ngr i value
allow operation during a fault
 
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