Y-Y transformer grounding

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mbrooke

mbrooke

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What code section(s) forbids connecting the primary wye H0 to the supply side EGC? Would having no L-N loads on the secondary make a difference?
 
mbrooke said:
What code section(s) forbids connecting the primary wye H0 to the supply side EGC? Would having no L-N loads on the secondary make a difference?
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H0 is a potentially current carrying conductor. As such it must be connected to a neutral rather than the EGC.
It does not matter if the loads are all balanced delta, there is a chance of current on H0 if the POCO voltages get unbalanced even slightly.
 
I don't think NEC addresses this, but do agree with GD in that it would be a current carrying conductor and would only be bonded back at the service or next upstream separately derived system.
 
GoldDigger said:
H0 is a potentially current carrying conductor. As such it must be connected to a neutral rather than the EGC.
It does not matter if the loads are all balanced delta, there is a chance of current on H0 if the POCO voltages get unbalanced even slightly.
Click to expand...

During a fault on the secondary it is- and in theory any phase differences would put current on the neutral- but I would imagine that some code section might restrict it.
 
kwired said:
I don't think NEC addresses this, but do agree with GD in that it would be a current carrying conductor and would only be bonded back at the service or next upstream separately derived system.
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Current carrying, but by which definition though? One could argue it only carries current during a fault on the secondary and is part of the effective ground fault current path.
 
mbrooke said:
Current carrying, but by which definition though? One could argue it only carries current during a fault on the secondary and is part of the effective ground fault current path.
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Connecting a conductor to H0 would make it a circuit conductor... so you cannot connect it [directly] to the primary EGC (I assume we're talking about a step-down transformer). The H0/neutral would be connected back to the source neutral point through branch and/ or feeder circuit wiring. Provided the primary is a grounded neutral system, connection to the equipment grounding system is through the main or system bonding jumper.
 
Smart $ said:
Connecting a conductor to H0 would make it a circuit conductor... so you cannot connect it [directly] to the primary EGC (I assume we're talking about a step-down transformer). The H0/neutral would be connected back to the source neutral point through branch and/ or feeder circuit wiring. Provided the primary is a grounded neutral system, connection to the equipment grounding system is through the main or system bonding jumper.
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Correct- step down.

But lets say I did connect the EGC- how would someone fail my prints? Not that I myself am planning on, or advocating on, doing this. :angel:
 
mbrooke said:
Correct- step down.

But lets say I did connect the EGC- how would someone fail my prints? Not that I myself am planning on, or advocating on, doing this. :angel:
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I'm wondering how you would even show connecting H0 to the EGC without it sticking out like a sore thumb. It's just not 'normal'. So how 'bout we not say? :D
 
mbrooke said:
Current carrying, but by which definition though? One could argue it only carries current during a fault on the secondary and is part of the effective ground fault current path.
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It should carry current any time there is voltage imbalance. Other then the difference in current level, wouldn't a line to neutral load on the secondary look much like a ground fault on the secondary to the primary?

Line to line single phase load also can easily create a voltage imbalance and result in some primary neutral current.
 
kwired said:
It should carry current any time there is voltage imbalance. Other then the difference in current level, wouldn't a line to neutral load on the secondary look much like a ground fault on the secondary to the primary?

Line to line single phase load also can easily create a voltage imbalance and result in some primary neutral current.
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I fully get this- but what explicit code section would fail someone?
 
Smart $ said:
Please cite reference(s).
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X. Grounding of Systems and Circuits of over 1000
Volts

250.184 (C):

(C) Multigrounded Neutral Systems. Where a multigrounded
neutral system is used, the following shall apply:

(1) The neutral conductor of a solidly grounded neutral
system shall be permitted to be grounded at more than
one point. Grounding shall be permitted at one or more
of the following locations:
a. Transformers supplying conductors to a building or
other structure
b. Underground circuits where the neutral conductor is
exposed
c. Overhead circuits installed outdoors

(2) The multigrounded neutral conductor shall be grounded
at each transformer and at other additional locations by
connection to a grounding electrode.

(3) At least one grounding electrode shall be installed and
connected to the multigrounded neutral conductor every
400 m (1300 ft).

(4) The maximum distance between any two adjacent electrodes
shall not be more than 400 m (1300 ft).

(5) In a multigrounded shielded cable system, the shielding
shall be grounded at each cable joint that is exposed to
personnel contact.
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The wording (ie, the use of "grounded" and "neutral") in this section leads me to believe that one can legally use a Y-Y transformer.
 
mbrooke said:
X. Grounding of Systems and Circuits of over 1000
Volts

250.184 (C):




The wording (ie, the use of "grounded" and "neutral") in this section leads me to believe that one can legally use a Y-Y transformer.
Click to expand...
But it is referring to the neutral of a solidly grounded system... not the center point of a wye load. You'd have to run a primary neutral conductor to H0 for 250.184(C) to be applicable.

Who is saying you can't [legally] use a Y-Y?
 
Smart $ said:
But it is referring to the neutral of a solidly grounded system... not the center point of a wye load. You'd have to run a primary neutral conductor to H0 for 250.184(C) to be applicable.

Who is saying you can't [legally] use a Y-Y?
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Correct- but that neutral is also your ground and is permitted to bond all exposed metal. The code uses the term neutral because it needs to be sized as a grounded conductor rather than a grounding conductor because no restriction exists on having L-N loads on the secondary. As such L-N loads may produce continuous current on the HO (of the same magnitude) as one of the phase conductor. Assuming Y-Y and no L-N loads, the code lets you connect the primary HO to an EGC from an electrical theory stand point once you exceed 1000 volts primary. Just that it must be sized as a neutral.
 
mbrooke said:
Correct- but that neutral is also your ground and is permitted to bond all exposed metal. The code uses the term neutral because it needs to be sized as a grounded conductor rather than a grounding conductor because no restriction exists on having L-N loads on the secondary. As such L-N loads may produce continuous current on the HO (of the same magnitude) as one of the phase conductor. Assuming Y-Y and no L-N loads, the code lets you connect the primary HO to an EGC from an electrical theory stand point once you exceed 1000 volts primary. Just that it must be sized as a neutral.
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This is the part I do not see. The transformer H0 is not a neutral when a primary neutral conductor is not connected to it. It is simply the center point of a wye load.
 
Smart $ said:
This is the part I do not see. The transformer H0 is not a neutral when a primary neutral conductor is not connected to it. It is simply the center point of a wye load.
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What in code says its a wye load though? Technically speaking the HO is the EGC when a fault occurs on the secondary because it complete the effective ground fault current path.
 
mbrooke said:
What in code says its a wye load though?
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Not stated in Code... and well, not necessarily a load, but certainly not a power source.


Technically speaking the HO is the EGC when a fault occurs on the secondary because it complete the effective ground fault current path.
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Without a primary neutral conductor, the fault path is earth. And if by chance the EGC runs all the way back to the power source, you are then using the grounding system to carrying neutral current under typical, non-fault operating conditions. The equipment grounding system is not a circuit conductor.
 
Smart $ said:
Not stated in Code... and well, not necessarily a load, but certainly not a power source.



Without a primary neutral conductor, the fault path is earth. And if by chance the EGC runs all the way back to the power source, you are then using the grounding system to carrying neutral current under typical, non-fault operating conditions. The equipment grounding system is not a circuit conductor.
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Not a circuit conductor under normal conditions.

But I get what you are saying, under normal conditions the HO will pass small amounts of current even without L-N loading.
 
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