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#129737 - 07/29/05 09:58 AM Un-grounded system less than 1000V
vandax Offline
Junior Member

Registered: 07/29/05
Posts: 4
Loc: Sumatra, indonesia
My low voltage system is solidly neutral grounding, but due to some reason we need to change some system to be ungrounded, current practice is just disconnect the neutral point, with precaution, among of them is de-energizing the transformer when we intent to re-connecting the grounding lead. All downstream insulation such as cable, etc are already designed to meet ph-ph voltage insualtion. Later, we plan to change it permanently by using delta-delta tx. On top of this info, each transformer is dedicated to serve service single load, ie 3 phases motor-pump, using underground cable, min distance from transformer to the motor. All equipment panel etc is grounded/bonded to have min step/touch voltage.

Technically, we calculated that capacitive current may be appreared is very low, we may proceed it.

My quesition: what is safety issue on this ungrounded system? if the system is intended to serve/cover big load or area, yes .. we do need take care, but this is for a single load/motor. What is your opinion? pls
_________________________
gg

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#129738 - 07/29/05 12:12 PM Re: Un-grounded system less than 1000V
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
The subject of ungrounded electrical systems can get a little complicated, but let's see where this leads. I can't imagine why y'all need to do this, but I reckon it can be done.

First let me clarify something, that the conduit / enclusure / equipment grounding will remain firmly in place. I presume you intend to simply remove the neutral-to-ground bonding, leaving all the touchable metalic surfaces grounded, for obvious safety reasons.

When I was stationed on a submarine back in my Navy days, we had ungrounded 3-phase 450VAC & 120VAC systems for general power & lighting requirements, operating from delta transformers. Equipment enclosures were still "grounded" to the ship's hull for safety, but the electrical systems were not grounded at any point. This was for tactical reliability: the electrical system could become inadvertantly grounded at any one single point in the system with no effect on the operation of the system or the equipment. We were very aware that a second inadvertant ground at a different point would result in a short circuit. For that reason, we had permenant ground detection circuitry that monitored the systems, and any significant drop in insulation resistance was detected and corrected immediately.

As I understand things, there are a couple of reasons why land based electrical systems are grounded:
1. Provides a cap or limit on the max voltage to ground for your electrical system, and
2. Provides a low impedance ground path so any ground fault that might occur will draw a large enough current to ensure quick clearing by the upstream fuse or circuit breaker.

You naturally would loose these benefits if you ungrounded your system. Is that what you really want? Another option might be to check the possibility of setting up a hi-impedance ground. I'm sure there are others here more familiar with them than I am.

Good luck,
Radar
_________________________
There are 10 types of people. Those who know binary, and those who don't.

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#129739 - 07/29/05 08:48 PM Re: Un-grounded system less than 1000V
vandax Offline
Junior Member

Registered: 07/29/05
Posts: 4
Loc: Sumatra, indonesia
Thanks a lot Radar.
You right, touchable metalic surfaces is still grounded/bonded.
Understand that we may lost the benefit of grounding system, somehow, the benefit of become delta system is higher economically, due to operation continuity, and some device/instrument we use require delta connection system in the transformer secondary.
I repeat again, secondary system 480 V or 960 V is totally underground,
So, I presume if we can provide high level insulation class, fast enough phase to phase fault detection beside voltage unbalance detection for any single phase to ground fault, we may get a perfect system & not leaving a hazard for people/operator who come to this facility on weekly basis.
Is that other potential safety issue? or am I over looked?
_________________________
gg

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#129740 - 07/30/05 03:54 AM Re: Un-grounded system less than 1000V
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Not a direct safety issue as such, but wouldn't you have to re-identify any white/gray wires to be code-compliant? (Obviously not a problem if each single load is already just 3-wire delta connected).

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#129741 - 07/30/05 05:02 AM Re: Un-grounded system less than 1000V
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
My understanding is that there is a risk of extremely high voltages in ungrounded systems with some types of fault.

The thing to remember is that there is no such thing as a true 'ungrounded' system. Instead you have solidly grounded systems, resistance grounded systems, and _capacitively_ grounded systems. What is normally called ungrounded is in fact coupled by capacitance to ground.

If you have a single _solid_ phase to ground fault in an ungrounded system, then the voltage of the two other phases will immediately jump to the full voltage of the system. That part is easy to understand and expected, and the insulation system is supposed to be capable of dealing with the full voltage.

But if you have a 'restriking' fault, where you get conduction for only part of the AC cycle, then you can charge up the system capacitance relative to ground, and get a conductor to ground potential that is several times the normal system voltage. This can rapidly degrade insulation systems.

-Jon

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#129742 - 07/30/05 10:13 AM Re: Un-grounded system less than 1000V
vandax Offline
Junior Member

Registered: 07/29/05
Posts: 4
Loc: Sumatra, indonesia
Paul, thanks. That's really only single 3 phases load connected to the tarnsformer.

Winnie, thanks. I absolutely agree with your comment regarding grounded & ungrounded term.

We are aware with the disadvantage, derating cable/ insulation system. The disadvantage related to cost $$ may be ignored, as system continuity will overide that. I just need to makesure the safety concern. Someone ask me (with a reference/book) which mention ungrounded system is not safe. What kind of safety issue? We will try to eliminate.
Thanks all
_________________________
gg

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#129743 - 08/01/05 08:15 AM Re: Un-grounded system less than 1000V
Radar Offline
Member

Registered: 04/30/04
Posts: 349
Loc: Los Angeles, CA
The reference to an ungrounded system being unsafe probably is based on the fact that our solidly grounded systems are that way as a safety precaution: Limiting max voltage to ground; and prompt fault clearing. However, Jon's comment is well worth considering. Intentionally ungrounded electrical systems are still capacitivly coupled to ground, and I'm here to tell you that inadvertantly touching a live part even in an ungrounded system will still deliver a nasty shock.

I still think a high resistance ground might be worth looking into. Otherwise, there just isn't any way that I'm aware of to stabolize & limit the phase to ground voltages.

Lastly, imho, the most outstanding safety issue is clearing any ground faults that may develope. You need ground detection circuits and procedures. & vigilance.

Good luck,
Radar
_________________________
There are 10 types of people. Those who know binary, and those who don't.

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