Ground Fault Protection in Corner Grounded Delta System

Hi Solar Eagle & Tonys,

I understand should a fault on "A" or "C" phases occur, the fault current flows through corner ground of the "B" Phase. But What happens if fault occurs on "B" phase. How the fault is identified, for example in solidly grounded systems current based fault detection is used and in ungrounded systems voltage based fault detection is applied. In this particular case how the fault is identified?

A break in conductor A, for example a fuse blowing, would require a trip on the basis of an earth fault, particularly if the fault circuit contained a human being in series with the supply. GA.

Update: the A phase conductor as shown can be considered to be a neutral conductor. Most national standards will not permit the additional grounding/earthing of a neutral downstream of the distribution point as it leads to non-fault currents in the ground/earth. A residual current device in the supply leads will disconnect in the event of the ground/earth current exceeding its trip setting, as has been correctly suggested elsewhere in this thread. One would not expect a circuit protective device to be placed in the neutral conductor as, were it to open, phase voltages would appear downstream of it; for this reason such an installation falls outside the content of most national standards.

67model,

Thank you!

But still I'm unable to grasp. As you can see in the below picture, the B phase is grounded at the service entry too. With current flowing through B phase to the motor, why doesn't the current circulates between the grounds?

By all means ground the outer casing of all field equipment that is not double-insulated.

It would be a mistake to ground the service conductor in the field, as to do so would ensure that operating current circulates via ground; such a situation would constitute a fault and the residual current equipment would operate to disconnect the supply.

Were such a connection to be made via a human being, and the residual current equipment were to be absent, it would not be reasonable to expect the human to survive the experience.

Your posted question was - how to protect motor by detecting "earth" fault! To protect motor pass three motor wires through CT at A.

To protect everything, pass 3 wires going into 3 pole isolator through CT at B.

Current will circulate between the grounds, if only because the load current through the ground/Bphase wire causes a volt drop, putting a voltage between the two grounds. One assumes earth fault protection (other than fuses/breaker) is not present upstream of incoming 3 pole isolator.

The motor windings are well insulated from its case, so only small leakage (mostly capacitance) currents will flow to the ground terminal of the motor casing.

Some street supply systems use cables with combined neutral & ground [maybe concentric with outer ground/neutral](and multiple grounds) up to the house incomer. At the house incomer, circuit protective conductors [CPC] from each outlet are connected to an "earth" ("ground") bar in the incoming distribution board/panel/box (DB) which is connected to earth/neutral at incoming cable end.

It would not matter if the DB was in a wooden "container" building sitting on top of a another wooden container. That's why our "wiring regulations" now write CPC except where it really is earth. The whole system including source generator could be on an insulated fibre-glass ship or aeroplane.

67model

GA

On a TT-earthed system, the CPC is wired to a local earth rod, and this is the route for the fault current - until the residual current device opens the circuit to disconnect the fault, that is.

If you want to detect a neutral to earth fault, which is really what you're saying, you need an earth leak trip device, because a neutral to earth fault also needs disconnection before you hurt or kill somebody. A fuse will disconnect a phase to earth fault.

If phase B is intentionally corner-grounded, then contact from phase B to ground is not a ground fault.

Ahem, it is if the circuit to ground contains a human being in series with the supply.

Friend/AP#1,

I would expect an imbalance in the current flow. The flow through the "B" phase would partly go through the insulated wires to the motor and partly through the earth/ground path.

Picture a circuit having a fusible disconnect switch with fuses on the A & C phases and no fuse on the B phase; have the B phase properly grounded on the line side of this disconnect switch. This is the normal way in which this type of system is wired. From the disconnect switch have the three wires then go to a motor starter and from there to the motor. Because the B phase has shorted to ground at the motor, instead of all its current flowing through its wire, some uses the alternate ground path and returns to the service ahead of this fusible disconnect switch.

Hence, if you are using an IEC-style overload heater, which senses and reacts to current imbalance (and not just a current overload), this current imbalance may be enough to trip the overload. If the motor currents are within the nameplate rating of the motor, you may not recognize the grounded B phase as the reason for this trip.

A large number of electricians have not seen a corner-grounded 3-phase service and will assume it is a typical single-phase service. However, when they read the voltages between all three wires and find they are equal they will scratch their heads and (hopefully) ask someone to explain what is happening.

--JMM