Switched neutral resistor

[wfowfo]

We (a utility) serve a plant that utilizes a switched neutral resistor in a 4w, 3 phase 277/480 volt wye. From reading the instruction book, apparently its' purpose is to switch a resistor into the netral of the circuit if it senses a phase fault. It also has the capability to switch a portion of this resistance in and out to "pulse" small fault currents into the line to facilitate locating the faulted phase.

The problem is that the transformer serving this unit has no ground or neutral reference connected to it. The three phases are connected to x1, x2, & x3, but no leads were even brought into the transformer for x0.

No one in the plant worked there when it was installed and no one knows exactly how it works. GE (who makes the switch) can find no information on it and, at one point, even denied it was their product.....so no help there (yet).

How can this device function with no refernce to x0?

 

[jschwei314]

It looks as if this has an ungrounded system. When one connects the switching resistor to the system, one end to X0 and the other end to ground, the system becomes a high restance grounded system.

The pulsing part on the switched resistor lets one find an unwanted ground. One uses a large jaw current probe around the 3 conducters of each feeder and the feeder with the pulsing current has the unwanted ground.

Hope this makes sense.

Please note the electrical system can still only supply three phase loads. One can not use 277 volt, single phase loads.

 

[PWR]

The only GE (or Siemens or others) high resistance grounded systems I have ever seen utilize a permanently connected resistor between XO and ground. After a ground fault is detected, it is possible to place the system in pulsing mode and intermittently short a portion of the resistor to vary the ground fault current so that a portable clamp on device as mentioned by sweitzereaton can trace the location of the fault.

The fact that no one in the plant knows how it is supposed to work is, unfortunately, all too common.

 

[wfowfo]

When we installed this years ago there was a lot of confusion. We were told to leave the system ungrounded. However,I gather from both responses that something needs to be attached to the X0 bushing.

If I understand correctly, if X0 is grounded at the transformer, the resistor would be bypassed when a phase faulted (to ground) and therefore useless. So to operate, the resistor would have to be between ground and X0. Trouble is, there is no lead pulled into the transformer for this purpose.

....so..... the installation has been wrong since day one?