High Resistance Grounding vs Ungrounded LV systems

[lat321]

Hi- I am aware of the different types of grounding systems (solidly grounded, low/high resistance grounded and ungrounded). In particular relation to 3 phase LV (415/480V) systems, how does one select between High Resistance grounding vs ungrounded system? Is it down to regional practices or owner preferences or there are some technical considerations with regards to insulation monitoring or equipment costs or cable to consider.

Thanks
I

 

[waross]

Ungrounded 480 Volt systems went out of favour over 50 years ago.
Safety; an ungrounded 480 Volt motor will not trip if it develops a fault to the motor housing.
The motor will then have a potentially lethal touch voltage on the motor housing.
Unintended consequences. There is always the possibility for widespread motor and equipment failure due to excess high frequency, high voltage resulting from a "Discontinuous ground fault" (Arcing ground fault.)
It is possible for a motor winding or transformer winding to develop an internal arcing fault to ground. The fault current will discharging the system capacitance to ground and will be quite low, hence it may persist for a long time.
The winding may act as an auto-transformer and that, conbined with the induction of the faulted winding and the system capacitance to ground has been known to act as a spark gap transmitter.
The result may be widespread motor burnout throughout a plant.
When ungrounded systems were in use, they were typically 440 Volt delta systems.
Grounded and impedance grounded 480 Volt and 600 Volt wye systems have become the standard in North America.
Many code now require ground fault detection and tripping on ungrounded systems, so the ability to continue running the plant with a grounded phase (the main advantage of an ungrounded system) so any advantage no longer exists.
Note that wiring supplies have changed over the years.
70 years ago plastic insulation was becoming common. Before that, cotton covered rubber insulated wire was commonly used.
The insulation had a tendency to break down with age. The rubber would harden and crack.
The present plastic based insulations have reduced the probability of wiring faulting to ground by orders of magnitude.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Fischstabchen]

High resistance grounding gets you many of the benefits of ungrounded systems without all the draw back. You have low fault current, can maintain operations, easier fault location, and all without having to insulate the system for transient overvoltages.

 

[lat321]

Thanks for your replies. I don’t know how to “quote” a post but to the first post re touch potential. I didn’t know the touch potential is “lethal” with just one fault. I found some links below and they seem to suggest the touch potential is not lethal.

https://www.benderinc.com/know-how/...not perceptible will flow through the person.

Re 2nd post with requirement for “overvoltage”, can you pls expand on any specific requirement for low voltage systems?

Thank you

 

[PersonalProfile]

The following historical post may be of interest:

https://www.eng-tips.com/viewthread.cfm?qid=501819