LV system neutral grounding
LV system neutral grounding
(OP)
Hi!
In our power plant LV system (400V,50Hz) is solidly grounding system. We would like to propose high resistance grounding to reduce arc flash hazards and for more reliability. We know that there is problem of raising ground voltage in case of earth fault.
It would be appreciated if anyone discuss regarding cons and pros of LV system high resistance neutral grounding.
Thank you,
In our power plant LV system (400V,50Hz) is solidly grounding system. We would like to propose high resistance grounding to reduce arc flash hazards and for more reliability. We know that there is problem of raising ground voltage in case of earth fault.
It would be appreciated if anyone discuss regarding cons and pros of LV system high resistance neutral grounding.
Thank you,






RE: LV system neutral grounding
High resistance grounding introduces major safety hazards to all users on the LV system: in the event of a fault to earth the touch voltage between earth and a piece of 'earthed' equipment connected to the high resistance earth may be full line-neutral voltage, and thus have the potential to cause a fatal shock. Also in the event of an earth fault, the remaining live conductors will be at full line-line voltage relative to ground. The UK regs, and I think the European regs, allow a maximum touch voltage of 50V to ground, except in high risk environments where the limit is reduced to 25V in the UK. I'm not sure the reduction is a European requirement or UK-specific.
High resistance grounding could theoretically allow you run with a single ground fault, provided a second ground fault does not occur before the first is cleared. In that situation the fault essentially becomes line-line, which is likely to be spectacular in its effect.
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I don't suffer from insanity. I enjoy it...
RE: LV system neutral grounding
We are from UAE and it seems no such regulations imposed here. However we will check and confirm. Our power plant level of voltages:400kV,220kV,16.5kV and 400V, 50Hz.
As we are using the above voltage levels, still shall we need to recalculate step and touch potentials for 400V high resistance grounding?
RE: LV system neutral grounding
RE: LV system neutral grounding
Could any one please tell us that which regulations/rules are giving such type of grounding system (high resistance/solid grouning) should be used for LV system?
RE: LV system neutral grounding
The HRG systems are only applied to medium and low voltage systems catering 3 phase, 3 wire or line to line 1 phase loads. This system is having the capability to limit the ground fault current ot value slightly greater than the system charging current. Hence, it effectively controls transient over-voltages during fault conditions, minimizes flash hazards at the point of fault and allows continued operation in the event of a ground fault.
However, you have to bear some considerations to make the HRG system to meet its purpose:
1) Ensure that full time maintenance or operations personnel are available and capable of responding to, and isolating, a ground fault within one hour of an alarm. Quick response it is.
2) Equipment for rapid location of ground faults must be provided as part of the high resistance grounding system. Additionally, relays capable of quickly sensing and isolating a second ground fault shall be provided.
3) Do not apply this system to 3 phase 4 wire loads.
Check your operational requirements and see whether this type of system would serve you best.
GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata
RE: LV system neutral grounding
Sorry - I have gotten hold of the wrong end of the stick! I was thinking in terms of earth electrode resistance into the bulk of earth, not deliberate introduction of a resistance between neutral and earth electrode. Your comments are valid regarding touch voltage and that part of my post is nonsense.
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I don't suffer from insanity. I enjoy it...
RE: LV system neutral grounding
However, as mentioned, you still have to assume the possibility of a phase fault, so the required PPE protection would not change between high-res and solid grounding. (At least in the US).
RE: LV system neutral grounding
I don't mean to criticize in any way, but rather to accept reality.
If these faults are the result of mistakes on the part of the maintainance people while working on the system you may indeed benefit from a resistance grounded system.
If the ground faults are the result of insulation or equipment failure and are not the result of human mistakes you may cause more problems than you correct with resistance grounding.
The resistance grounded system will give you the best results when your crews are capable of locating and repairing faults quickly. The crews also need the support and direction of supervision and management. Management should be aware of the importance of locating and repairing grounds and refrain from exerting scheduling pressure that would divert the electricians from ground fault repairs.
respectfully
RE: LV system neutral grounding
Benefits : in the case of a phase to earth fault, the total impedance of the fault loop is very high, so no fault current flows (no damage to equipment), and there is no touch voltage on the exposed conductive parts, as there is virtually no fault current to create that touch potential.
Inconvenience : you need trained staff to intervene and fix the installation as mentionned by waross. Otherwise, another fualt (on another phase) wlll cause a high short circuit
This earthing system is legal, recognised by IEC, I suggest reading the requirement in the IEC wiring rules to implement such a system .. for example the need of Permanent Insulation monitors, etc...
A good source of documentation on the IT system (and other issues)is Merlin Gerin's Electrical Installation Guide document.
RE: LV system neutral grounding
RE: LV system neutral grounding
Avoidance of hull corrosion as well as safety means that the best system for a ship may not be the best system for shore based installations.
Another significant difference is that on a poorly designed shore based, the grounding system may be affected by earth voltage gradients caused by fault current on adjacent systems.
The ships hull forms an effective faraday shield and earth voltage gradients are not a factor.
Shore based grounding and ship based grounding are not apples and apples. Don't use ship grounding methods on shore unless you are willing to enclose your building including the foundations in a conductive system that has the same characteristics as a ships hull.
yours
RE: LV system neutral grounding
And, yes. It is referred to as an IT system From French "Isolée and "Terre" for isolated transformer star point and grounded equipmet (terre = earth).
You need a good monitor to be able to tell when an insulation fault has occured. And you need VERY GOOD electricians to be able to locate and eliminate ground faults. It may seem a simple enough task, but not when everything is up and running and your slightest mistake may cost USD 50 000 for an hour. So, most of these systems I have seen have lots of seemingly eternal little earth faults in them. The insulation monitors blinking at 3 or 4 kohms in some cases. Insulation was supposed to be in the 100 - 1000 kohms range from the beginning.
I know one steel work where the 500 V IT system has been turned into a 500 TN system (solidly grounded star point) just for that reason. Now, every real erth fault is cleared by the high fault current and there is no delay fixing that fault - no delay at all.
If you want to reduce damage caused by arcs, the ABB Arc Guard is a very good thing to have. As soon as there is an arc (or someone using a camera with a flash), the MV breaker trips and saves lives and property.
Gunnar Englund
www.gke.org
RE: LV system neutral grounding
Rugged
RE: LV system neutral grounding
RE: LV system neutral grounding
As a ference, the NEC allows the use of high resistance grounding on ac system of 480 volts to 1000 volts where all of the following conditions are met:
a. The conditions of maintenance and supervision assure that only qualified persons will service the installation.
b. Continuity of power is required.
c. Ground detectors are installed on the system.
d. Line-to-neutral loads are not served.
General recommendation for using high-resistance scheme :
* Do not use as substitute for proper system maintenance.
* Do not use on a four-wire systems.
* Do not applied when a large portion of the load (> 30%) is of the line-to-ground nature. Such loads must be isolated by separate transformers.
* When converting from a solidly grounded system HRG, attention should be placed on the checking the insulation of the different components.
RE: LV system neutral grounding
I pretty much agree with your chart. It gives a good framework for evaluation. Anyone who, like me, disagrees with a particular rating on the chart is free to change it to be in accord with his personal experiences. He can then still use the balance of the chart to help in his evaluation of a particular installation.
lps
respectfully
RE: LV system neutral grounding
RE: LV system neutral grounding
To do so would change the characteristics of the wiring system as seen by the electrician. The convention has been that the neutral is at ground potential. This convention has been in effect since before almost all the electricians working today were born. With resistance grounding the neutral will no longer be at ground potential in the event of a fault.
The magnitude of the safety issues involved is such that I don't forsee a change, nor do I wish to see a change.
respectfully