Earth fault in motor on IT system

[davva]

Hi I put this thread on the elec power eng forum but thought I'd add it on this one as well.

We have an insulated earth (IT system) power system. This allows one phase to fail to earth and the system will continue to operate (albeit that the insulation is stressed higher).

So if a fault occurs at the terminal box of a motor or on the feeder cable to the motor should continue to operate as normal, right?

What happens if the earth fault occurs part way through the winding in the motor? Would the motor continue to operate as normal?

We have an earth fault on either the motor or the motor feeder cable. We are not able to prove whether the fault is witin the motor or on the cable at the present due to the installation.

The motor was stopped when it was reported to be running on lower than expected current (which is when the insulation checks were made and the fault to earth established). Would the low current be an indication that the fault is in the motor?

 

[7anoter4]

Usually an insulation fault-by short circuiting or interrupting one phase winding- will be followed by an increasing of the current in this phase or the remaining undamaged phases.
But, in some way, closing the circuit through supply line phases capacities with respect the ground it may improve the power factor [!!!!???] and reduce the reactive component .This scenario is not so verisimilar ,but theoretically possible.

 

[PHovnanian]

Faults are usually pretty complex. More often than not, you'll get a combination of some resistance to earth plus a higher series resistance due to thermal damage produced by arcing. Within a motor winding, there is also the possibility of a turn-to-turn short to be considered, adding yet more complexity to the analysis.

The end result is that the faulted phase current may go up, down or change it's phase relationship w.r.t. the other phases. The best approach is to start splitting the system up and test each component to isolate the location.