3 phase 3 wire Separately Derived vs Non Separately Derived

mbrooke

Senior Member
Why would the neutral be kept ungrounded at the transformer and generator but only allowed to be grounded at the switchgear?

I can't figure this one out because in a 3 phase 3 wire system the neutral will not carry any load current and therefore objectionable current is no longer a concern.
 

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brian john

Senior Member
If I understand your question, you have two sources two utility feeds to a common bus or a generator (with 3-pole ATS) with a solid neutral to the common bus, 3-phase 4-wire with a 3-phase 3-wire distribution? If so if you bond the neutral in more than one location and have a ground fault you will have multiple paths back to the source, In the case of a switchboard with GFPE (Ground Fault Protection), you can negate the effectiveness of the GFPE. Depending on the settings of the GFPE Relay will depend on what the delay response is.
 

mbrooke

Senior Member
If I understand your question, you have two sources two utility feeds to a common bus or a generator (with 3-pole ATS) with a solid neutral to the common bus, 3-phase 4-wire with a 3-phase 3-wire distribution? If so if you bond the neutral in more than one location and have a ground fault you will have multiple paths back to the source, In the case of a switchboard with GFPE (Ground Fault Protection), you can negate the effectiveness of the GFPE. Depending on the settings of the GFPE Relay will depend on what the delay response is.

Both sources are 3 phase 3 wire as well as the load.
 

kwired

Electron manager
Both sources are 3 phase 3 wire as well as the load.
Then what it the fourth wire for ??

Ungrounded systems would have a 4th wire used for equipotential bonding purposes and installed as an EGC.

Three phase three wire that is grounded is a corner ground delta system.
 

synchro

Senior Member
IT does- and it would make sense for TN-S. But what if the noodle isn't used? Why must it still be insulated?
Even if the neutral is not used by a 3-wire load, there can still be current flowing across the neutral conductor that interconnects the two 4-wire sources if there's any imbalance between these sources. So in practice there will likely be some voltage drop and gradient across the neutral conductor. Therefore if the neutral conductor is not insulated and it get's connected to earth somewhere other than the desired connection to PE, unwanted current will flow through the earth and earthing system.
 

mbrooke

Senior Member
Then what it the fourth wire for ??

Ungrounded systems would have a 4th wire used for equipotential bonding purposes and installed as an EGC.

Three phase three wire that is grounded is a corner ground delta system.
4 th wire is the EGC, but if I wrote 3 phase 4 wire people would think 3 phase, neutral and a ground.
 

mbrooke

Senior Member
Even if the neutral is not used by a 3-wire load, there can still be current flowing across the neutral conductor that interconnects the two 4-wire sources if there's any imbalance between these sources. So in practice there will likely be some voltage drop and gradient across the neutral conductor. Therefore if the neutral conductor is not insulated and it get's connected to earth somewhere other than the desired connection to PE, unwanted current will flow through the earth and earthing system.
How is that possible when it carries no load?
 

synchro

Senior Member
How is that possible when it carries no load?

I'm assuming both 4-wire wye sources are active and parallel connected as indicated by the dashed lines on your diagram.
As an example, let there be perfect phase alignment between the two sources with no loads on the 3-wire line outputs. If the open circuit L-N RMS voltage is identical on all terminals of both sources, then no current will flow on any interconnecting conductors when connected.
Now if the L-N open circuit voltages are reduced on source 2 but they are still balanced, equal RMS currents will flow between the three phases (L1,L2,L3) of source 1 and those of source 2. However, the current on the neutral will still be zero because both sources are balanced.
Now consider the case where the open circuit voltage is reduced on only one of the L-N phases of source 2. In this case current will flow from source 1 to source 2 on that phase but not the other two, and therefore an identical return current will flow on the neutral conductor. So in a more general case with sources that are not perfectly balanced there can be current on the interconnecting neutral conductor, and therefore voltage drop across it..
 

kwired

Electron manager
4 th wire is the EGC, but if I wrote 3 phase 4 wire people would think 3 phase, neutral and a ground.
Seems you do have three phase 4 wire system, though you have no neutral loads.

Are these two sources connected in parallel to one another? You show no transfer equipment.

I could see any unbalance current that may occur if they are in parallel seeking path on the neutral and that may make the desire for one grounding point to eliminate stray current on non intended current carrying pathways.
 

mbrooke

Senior Member
Seems you do have three phase 4 wire system, though you have no neutral loads.

Are these two sources connected in parallel to one another? You show no transfer equipment.

I could see any unbalance current that may occur if they are in parallel seeking path on the neutral and that may make the desire for one grounding point to eliminate stray current on non intended current carrying pathways.
The thing is thats how the code wants it. Again, there is zero line to neutral connected loads.
 

mbrooke

Senior Member
Seems you do have three phase 4 wire system, though you have no neutral loads.

Are these two sources connected in parallel to one another? You show no transfer equipment.

I could see any unbalance current that may occur if they are in parallel seeking path on the neutral and that may make the desire for one grounding point to eliminate stray current on non intended current carrying pathways.
Here is the version with line to neutral loads- understandably you would want neutral grounding at one location and one location only due parallel current paths.
 

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