AC from Hot and Neutral line simultaneously

Old 05-03-14, 09:14 AM
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AC from Hot and Neutral line simultaneously

I get a 3-Phase 240v line (Domestic Installation) here in INDIA. All connected load is single phase.My AC distribution board uses 4 pole ISOLATOR which serves to the input/output for 3Phase wires and One Neutral Wire.

Recently there was a situation where the AC was present in both the 'hot' and the 'neutral' line simultaneously. What I discovered was that the Isolator had malfunctioned and broke the continuity of the Neutral wire.

I lost around USD 2000 worth of electrical equipment and fixtures due to the above problem.

Could a senior explain to me what actually happened.

  1. I presume this to be due to what is called as 'open neutral', am I correct on that?
  2. I was under the impression that the Ac outlets would not work if there was Open neutral. Is this correct?
  3. Is my presumption correct that any voltage present on the neutral line will be dropped off if there is proper earthing?
  4. Finally, Could I have avoided the above situation by installation of additional devices in the distribution board.?
I shall be greatful if one could point me to "Best practices for a 240V Three phase(domestic) Distribution board design/basics in simple non technical language".

Thanks for reading.
Old 05-03-14, 09:35 AM
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This is primarily contributors from the US. Sounds like the system in India has many differences from the North American systems.

Hope someone can add to this.
Old 05-03-14, 03:36 PM
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Sorry for your loss.

Your "isolator" is what we would call a transformer.
This is a little confusing so let me see if I can illustrate it for you.

You have three phase service. In order to keep the load balanced on the service...... the loads (lights, receptacles, etc.) are split up so that some load appears on each one of the three legs of the service. So you would have loads connected to phase A and neutral, phase B and neutral and phase C and neutral. Each one of those pairs is 240vac.

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Higher voltage devices would be connected between A-B, B-C and A-C.
Each one of these pairs is 480vac.

Now....looking at the diagram.... remove the neutral connection. Now instead of your loads connected from a phase leg to neutral they are connected from a phase leg thru a central connection point (what was neutral) to another phase leg.

Depending on the actually load balance you could see anywhere up to 480v on a leg that should not normally be above 240vac.

Your connection to neutral and ground is of utmost priority. In this case.... there was nothing that could have helped save your equipment.

You can run into the same problem here as well as there if you employ a multiwire branch circuit system. The MWBC is two high voltage legs sharing a common neutral. The neutral must be connected carefully so as to not have a poor connection.
Old 05-05-14, 01:10 AM
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Thanks for the input. I hope someone could guide me to employ a branch circuit system (240V single phase) where such a situation could be avoided.
Old 05-05-14, 07:04 AM
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If you ask for a system to be built to "best" standards and practice, you will probably get what you have now. If something (the isolator in your case) malfunctions then all bets are off. There do exist yet superior and much more expensive standards used in military installations and in hospitals.

One expensive and also uncommon equipment addition is a simple (single phase) step down transformer connected to two of the phase legs and providing a 240 volt output. If the neutral was broken so as to cause incorrect phase to neutral voltages the phase to phase voltage stays the same so the 240 volt output also stays the same.

This is practical only fora small total load; you need three such transformers to balance a large load over all phase to phase pairs.

Check the master switch for the building. If it switches the neutral (has 4 poles for 3 phase power) then more reliability is had by installing a jumper for the neutral around the master switch. Note that such a jumper may or may not be appropriate for electrical systems that include a backup generator or solar panels.

It's still not foolproof because a similar malfunction could occur outside your property such as at a utility substation and higher voltages could come all the way down the street via the utility poles.

Occasionally a redundant neutral conductor is found. At least in the U.S., equipment grounding conductors or grounded metal objects may not be used as a redundant neutral or as a neutral where no neutral conductor was installed originally.

Although (at least in the U.S.) neutral refers to the conductors or points on the electrical system that are bonded to the grounding system (grounding electrode system), the important point (on the diagram above) is the center point of the isolator (typically a 3 phase transformer).

At least you need to be sure that all conductors that are supposed to be bonded to that point are.

Better earthing will not help in this situation.

Nitpicking: The "voltage" of a system states the highest voltage as measured between any two points,. For the 3 phase diagram above, it's actually 415 volts (with 240 volts phase to neutral). Common terminology is 240/415 volt 3 phase wye (or star). There exists 240/480 volt single phase systems with two hot legs with 480 volts between them, and 240 volts hot to neutral.

Last edited by AllanJ; 05-05-14 at 08:15 AM.

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