three phase load
- Thread starter hhsting
- Start date
- Status
That was my point. So neutral wouldn't be the path for motor fault current.
For us at least, a ground (earth) fault would go back, via the earth conductor, to the supply transformer.
Neutral does not play a part.
- Location
- Connecticut
- Occupation
- Engineer
For us, the path for a ground fault between the supply transformer and the service disconnecting means is the grounded circuit conductor.
For a wye system, the grounded circuit conductor is the neutral.
Most our LV supplies are wye systems.
Yes, it is usually the case here that the neutral is grounded at the supply transformer.
But the common 3-phase motor doesn't have a neutral so it plays no part in a ground fault.
The ground (earth) conductor which is a mandatory requirement does.
- Location
- Connecticut
- Occupation
- Engineer
And here, in this case, the ground (earth) conductor between the transformer and service disconnecting means is the neutral. So it does play a part in a ground fault.
I'm not sure what is difficult to understand about that.
Neutral and ground are two different things. Neutral is not connected to a three phase motor. Ground is.
- Location
- Connecticut
- Occupation
- Engineer
And no one here has suggested connecting a neutral to a three phase motor.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
We understand that 'neutral' and 'ground' are two different functions.
One requirement is that the neutral be tied to ground at a single point to provide the necessary fault current path back to the source.
I believe that in many parts of the world this connection is made _at the transformer_ and no where else.
In the US this connection is made _at the service point_, as well as at the transformer. If a single transformer serves multiple customers, then each service point will have its own connection between 'neutral' and 'ground'. The circuit conductor between the transformer and the service point is thus simultaneously the 'neutral' and also part of the 'ground' system.
So in the US a 'neutral' would be brought to the service point, then at the service point the ground wires would continue to the loads that don't require the neutral as a circuit conductor.
-Jon
- Location
- Illinois
- Occupation
- retired electrician
There are major differences between our systems any yours. We don't have a "ground (earth)" conductor on the line side of the service disconnect. For a grounded system, we have a neutral that functions as a circuit conductor for line to neutral loads and also functions as the fault return path back to the utility transformer. On the load side of the service disconnect, those two functions use two separate conductors. A neutral for the actual line to neutral load current and a grounding conductor for fault clearing. In the event of a "ground fault" at the motor in the system being discussed here, the fault return path would be via to Equipment Grounding Conductor, to the main bonding jumper (the point in the service equipment where the neutral and grounding conductors are connected together), to the neutral back to the supplying transformer.
The Overcurrent protection device will trip on a phase to phase fault or overload between the phases only, sure, but , it will not trip on a phase to case or (Otherwise known as a Ground Fault) unless there is a path for the current to return to the source (which is the XO terminal at the Utility Transformer) quickly.
Overcurrent protection, or Short Circuit Protection will not operate unless it sees enough current to trip during a fault condition.
The "Earth" or "Dirt " has too much resistance in it to be able to allow enough return current to trip an overcurrent device.
(That is unless you have extremely low impedance dirt where your located).
If we ran just (3) phase conductors to the motor (A,B and C with no EGC) , then, took a wire from the metal case of the motor and stuck it in the dirt next to the motor, and shorted one of the phase conductors to the metal case of the motor, we would never trip the Short Circuit Overcurrent Protection device ahead of it . There's simply too much resistance in the dirt to allow enough return amperage for that to happen.
Therefore we cant rely on that return path to facilitate the opening of the device quickly.
We use an Equipment Grounding Conductor (as we currently call it) to provide a positive return low impedance path from the motor, back to the Service Disconnect, through the bonding jumper installed in the disconnect, to the (Neutral or Grounded Conductor) installed from the Service Disconnect back to the Utility Transformer to clear the fault.
(Not the Dirt or Ground)
If a Neutral or Grounded Conductor was not installed and bonded at the Service Disconnect (on a grounded service) back to it's source at the Service Transformer, During a fault condition the equipment would simply remain energized with the fault current and create a shock or fire hazard.
Wheewwww.....
JAP>
ptonsparky
Tom
- Occupation
- EC - retired
Right. For the OPs situation the path back, from the motor, should be via the equipment ground or grounding conductor back to the Service, then via the Neutral or grounded conductor back to the transformer.
And, What do you call this solid conductor path from the motor back to the transformer "Star Point" or what we'd call a "Y". ?
We call it an Equipment Grounding Conductor.
Jap>
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
We should probably answer this in Esperanto, since the difference between American and English seems to be confusing the issue
In the US, the solid conductive path from motor frame to transformer star point is the 'EGC' up to the main service disconnect. In the main service disconnect the EGC is bonded to the neutral. From the main service disconnect back to the transformer, the _neutral_ is used as the solid conductive path for fault clearance.
-Jon
- Status