Power Line Phase to Neutral Contact No Fuse Blown, No Tripping

Unfortunately "touching" is not the same as "solidly grounded". I have seen relatively "poor" contact between current carrying lines and ground glow a dull blue while never causing a breaker to trip.

The reason has to do with the dynamic impedance of the arcing fault which can let many amperes of leakage current to flow but not enough to trip the ordinary upstream breaker. Worse yet, when the circuit is de-energized an ordinary VOM (Volt Ohm meter) may not indicate any contact on the lower ranges because the current carrying arc is extinguished and the contact resistance is very high when cold.

In exactly the same circumstances GFIs and ELCBs would have operated immediately because they sense the current unbalance between the line conductors.

Dear Mr.RAM Consult,

Thank you for your posting immediately.

Your points are valid. Kindly give your views how there were (or) can be - 2 different Voltage - upto 11 poles and remaining poles, there after, in the particular feeder.

This situation lasted for about 10 hours, during night time, being night time we did not want to take any chance.

One more point I should mention here is that, few days back - in the same system, the Voltage was as low as 120 to 130. There is no motor in the feeder except residential area. Any comment on this point please.?

Thanks,

DHAYANANDHAN.S

I am entirely agree with you, so no comment

bye

I have never come across this, but there sems to be some inductance effect on your line after the 11poles, that will increase the voltage , what you should find out is to compare the amount of current at both points, since some consumer equipment were destroyed by the current surge due to the L-N fault.

Please update the forum when you finalise your system diagnosis.

Dear Mr.John Ken,

Thank you for your immediate posting and your view has stimulated our thinking.

One more point I should mention here is that, few days back - in the same system, the Voltage was as low as 120 to 130. There is no motor in the feeder except residential area. Any comment on this please.?

Certainly I shall post the information, what we did and what was the Voltage level.

Thanks,

DHAYANANDHDAN.S

Not having a diagram of the system, I can only speculate.

1. Does system have neutral earthing at several points?
2. Or combined neutral/ earth, earthed at several points?
3. If neutral conductor were actually open circuit at some point, current might return via actual earth, with high resistance?
4. Displacement of neutral could cause high voltage in one phase and low in another.
5. Voltage upset would depend on consumer load with a high resistance connection, making different symptoms according to load.

I suggest you check the continuity of all the conductors in the LV system is as it ought to be.

67model

Dear Mr. 67model,

Thank you for your posting and giving some suggestions, which gives me a different way of thinking.

I furnish, the answer to your points in the same seriatim i.e.Sl. Nos.

1. Yes. There are several Neutral Points.

2. Combined Earth.

3. Open Circuit - this point was not reported to me, but I will verify and inform you by CR4 Personal Messaging, in addition to posting here.

4. Displacement of Neutral - - This I knew to some extent and its effect. But what I know is the Voltage will be Half of the Actual Voltage, and I am NOT aware of Increased Voltage, perhaps, VECTOR DIAGRAM under Neutral broken condition may explain. I request you to explain more on this.

I can mention another example for Half-of the Voltage - In a balanced load on a Transformer, if the H.G.Fuse blows out means, the voltage will be Half as the current takes a return path.

5. You are right. But it may be difficult to assess nature of Load - Resistive, since the Residential Quarters are in large Number.

DHAYANANDHAN.S

You should check the neutral wire connection at or near the pole, after which the voltage was ~ 300v phase to neutral. The neutral might be broken and it is making a partial conntact.(Neutral disconnected at worst for the rest of the circuit).

In such a case, depending on the existing loads being connected on the different phases, at any moment, you will have different voltage scenarios that can go from less than 240V to higher and near the maximum phase -phase voltage.

Approximate explanation: If 2 loads are connected between Phase 1 to Neutral and Phase 2 to Neutral, they will be in series between Phase 1 and Phase 2, thus splitting the voltage between them according to each load's impedence. (You can do some calculations with different impedence values to illustrate). One of the loads might have ~ 300V accross or higher, while the other will have less than the normal Pase-Neutral voltage. Some equipment will be damaged if the voltage is higher than required.

Your case will only be clear if we know the exact wiring and measured loads on each phase, when the 300 V is measued (Phase - neutral, right?).

NO FUSE is going to be involved here, because the currents are still withn the range of the fuses for the loads (No Short circuits here...)

Dear DHAYANANDHAN.S

Thank you for your reply, post #7, I suppose your system has combined neutral & earth - with an earth rod at each user's house - probably rods at each "distribution pillar/box". So multiple earthing of neutral.

I attach a sketch of a system with a 3 phase and neutral source G and two 3ph&N loads A and B. Both A and B have a local earth rod for the star point (and probably a rod at each single phase house point).

It is supposed that an L-N short is applied by the switch on phase 3, at distribution point B, also that the responsible fuse Fb for the load does not blow, causing fault current If to flow.

It is also supposed that the neutral return resistances Re and Ree, rather than being the intended low resistance cable, are via the earth rods and the earth, much higher resistances (I believe a good rod in normal soil is some tens of ohms) which mean current If is not enough to blow the fuse Fb.

At point B, it is supposed that loads on phases 1 and 2 are off or were lamps which would blow quicker on overvoltage than a fuse, giving no obvious fuse blowing to localise a fault. Obviously, phase 3 to neutral at B is zero.

However, at point A, the neutral is displaced, but phase 3 to neutral voltage is not zero.

I drew, with compass and ruler a vector diagram, with neutral displaced, which gives 300V phase to neutral you mentioned (on two phases). As you see, the neutral is displaced about 100V, with about 140V phase-neutral on the faulted phase. NOTE THAT IF SINGLE PHASE LOADS ARE UNBALANCED and vary in magnitude, a variety of different phase-neutral voltages can be got.

I guess a typical user supply fuse Fb would be 60 amp, while a 10 ohm neutral return would drop 100V at 10 amps across Re (remaining 140V dropped through Ree). Note that for a complete disconnection of neutral, the voltage balance would depend on the loads on each phase and could look OK for equal loads.

The words "metal thief" have occurred to me (probably because here in England there have been thefts of HT and control cables from rail lines etc recently, causing great disruption, even most of the lead sheet disappeared from the roof of a local bank one night!!).

Since there will not be much neutral displacement with low load currents, I suggest you will need to check line and neutral currents with a "clamp-on" ammeter - in addition to having enough insulated wire to run your voltmeter etc between neutral points houses apart to detect unusually high voltage drops.

I have used a 12V battery and resistor to give about 1 amp to measure (low) neutral-earth resistance on a live system. You could use same between two neutral points. For safety, check AC and DC voltages are low before applying the DC source and put a low current, high voltage, HBC fuse in the battery circuit! Make sure the resistor is enclosed, in case it blows. Also ensure voltmeters can survive 420 VAC, even on low DC range (fortunately modern DVMs, which auto-range, can usually survive the highest full-scale voltage).

Best wishes for locating your fault,

67model

From: dhayanandhan

I would guess the neutral was broken around pole 11, and the jumper was high impedance, so effectively the voltage was no longer a true L-N but rather L-L through the impedance.

Broken neutrals can give some rather peculiar voltages, often depending on the loads connected on the various phases.

My observation is beyond 11 th pole, the neutral is cut, therefore voltage is 300.

Fuse blowing depends on fuse rating and size. and also cable length, cable impedance If it is TNEB pillar box feeding then I have doubt because TNEB provides solid copper wire strand as fuse in the pillar box and it will blow only when current goes beyond 400 amps. When there is a short the cable impedance restricts the shorting current.