LV system neutral grounding 2

[power2020]

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,

 

[ScottyUK]

Depending on where you are, that is likely to be a breach of the wiring regulations. 400V/50Hz sounds European, and I'm not aware of anywhere in Europe which runs high resistance grounding on an LV network. The closest we have in the UK is either the TT system which should never become 'high' resistance grounding, or the ground free environment. I've never seen one of the latter, but the idea is that there is no ground reference to introduce a shock hazard, so contact with one live line at a time is 'safe'.

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...

 

[power2020]

Thank you Mr.ScottyUK for your reply.
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?

 

[davidbeach]

In the US, high resistance grounded 480V systems are often used in industrial areas. A single ground fault will draw 5-10A and raise the phase to ground voltage of the unfaulted lines to phase to phase voltage. No touch hazards to my knowledge. Clearing of the first phase to ground fault is absolutely critical to avoid a second fault, which becomes a phase to phase fault and can be a major problem. Arc flash hazard calculations are all based on 3-phase fault currents, so while reducing the single phase to ground fault currents, high resistance grounding would not reduce the calculated arc flash hazard.

 

[power2020]

We would like to reduce number of arc flash hazards as major faults are single phase earth faults in nature.

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?

 

[nightfox1925]

I agree with davidbeach. LV HRG is often applied to industrial facilities where process operations are required to be maintained and outages are not acceptable due to probable production losses that will occur. For 400V HRG system, the charging current is usually less than 2A.

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

 

[ScottyUK]

david, and all

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. I must have had my brain out of gear and my fingers in full ahead.


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I don't suffer from insanity. I enjoy it...

 

[dpc]

High-resistance grounding does greatly reduce the risk of arcing faults. Most faults (70-80%) start as ground faults. With high-res grounding, the maximum ground fault current is so low, that an arc cannot be sustained and there is little risk of this fault escalating to a phase-to-phase or three-phase fault (immediately anyway). So in practice, high-res grounding is much safer than solid grounding in terms of arc-flash hazard.

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).

 

[waross]

Hello power2020

We would like to reduce number of arc flash hazards as major faults are single phase earth faults in nature

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

 

[bfuchs]

The earthing system I think you are looking for is referred to in IEC60364 as the IT system. The star point is isolated (I) from the earth via a big impedance (a couple of kiloOhms or so), the neutral is connected to the star point of the supply (if you have a neutral, and a star point obviously), and the earth conductors are connected to an earth electrode (T) which does not connect to the star point.

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.

 

[davva]

There is quite a good "pros & cons" comparison of earthing systems within the IEE publication "Regulations for the Electrical and Electronic Equipment of Ships"

 

[waross]

Be aware that earthing systems for ships may not be applicable to shore based installations. If a ship with a grounded system is connected to shore power from a grounded system, it is possible for immediate and severe hull corrosion to result. Rapid degradation of protective zincs can also occur.
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

 

[Skogsgurra]

I see lots of high-resistance grounded 500 V systems in paper mills. The only reason they are used is that an insulation fault doesn't stop production. And that is thought to be good. I agree - it is a good thing to be able to continue producing wth a ground fault.

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

http://www.gke.org

 

[ruggedscot]

Ive seen someone use a flash camera near to a paper machine - I can tell you that it feels like the earth has ended !

Rugged

 

[power2020]

Thank you all for discussing pros and cons of LV HRG system.

 

[cuky2000]

Here is a summary for High Resistance Grounded application:
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.

 

[waross]

I love your graphics cucky2000.
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

 

[DogOne]

I wonder why we are not allowed to connect L-N loads to a system with high resistance grounding.

 

[waross]

"I wonder why we are not allowed to connect L-N loads to a system with high resistance 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