Grounding in IT-system 1

[fharry]

Why is there, on an 230 V IT-system (isolated neutral) on a ship, 127 V between ground and phase when there is no connection between tranformers neutral and earth?
Is this because of the capacitive currents in the long cables?

Thanx
Harry F

 

[jwerthman]

Yes. Ungrounded systems are really grounded through the distributed capacitance of the system to ground, mostly from the cable capacitance. In most systems, this capacitance is fairly balanced, so the voltage to ground ground from each phase is about equal. If the capacitances are not equal, then the voltages to ground can be very unbalanced.

 

[busbar]

Be well aware that on an ungrounded system (capacitance only ø-grd) transients may develop up to several times the ø-ø voltage, from intermittent ground faults and resonant effects.

 

[babu67]

some times high resistance earthing will be done for the detection of ground fauls in floating neutral systems. This can also result in the same observation.

 

[busbar]

Correctly sized high-resistance grounding can be desirable for damping transient and resonant overvoltages that inevitably occur on the inherent ø-grd capacitance.

Particles vary widely, but to get an idea of related problems experienced by some, read though thread238-3370 and thread238-6870. We’re approaching 50 years of warnings on ungrounded LV distribution.

 

[kitha]

In isolated neutral systems, the first ground fault is not detected. If a second ground fault occurs, think of the situation. It appears exactly like a ph-ph short circuit. therefore in any ungrounded systems an indication of three phase lamps along with leakage current sensing is highly desirable.

 

[jbartos]

Suggestion to fharry (Electrical) Oct 13, 2002 marked ///\\Why is there, on an 230 V IT-system (isolated neutral) on a ship, 127 V between ground and phase when there is no connection between tranformers neutral and earth?
///There just may be a fault, namely one line has become grounded.\\Is this because of the capacitive currents in the long cables?
///Alternately, yes.\\\

 

[kitha]

Yes JBartos is correct. 127V is a considerable voltage and possibly a fault coult exist. We can think that capacitive currents as an alternative.

 

[electricuwe]

jbartos,

if one line has become grounded, fharry would have measured 230V between the two other lines and ground, 127V could be caused by a grounded neutral.

fharry should check if the 127V he measures are able to supply some current (e.g. a small light bulb).If the volatge sgoes down in that case, its jsut capacitive coupling.