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Wednesday, 7 November 2012

Neutral Fail or Float in Electrical Power Supply


Neutral failure or floating

Neutral Floating condition does not impact on 3 Phase Load but It impacts only 1 Phase Load only. The Neutral Conductor is opened, broken or lost at either of its source side (Distribution Transformer, Generator or at Load side – Distribution Panel of Consumer), the distribution system’s neutral conductor will “float” or lose its reference ground Point. The floating neutral condition can cause voltages to float to a maximum of its Phase volts RMS relative to ground, subjecting to its unbalancing load Condition.
Floating Neutral conditions in the power network have different impact depending on the type of Supply, type of installation and Load balancing in the Distribution.
What is neutral break or Floating Neutral?
If the Star Point of Unbalanced Load is not joined to the Star Point of its  Power Source (Distribution Transformer or Generator) then Phase voltage do not remain same across each phase but its vary according to the Unbalanced of the load.
As the Potential of such an isolated Star Point or Neutral Point is always changing and not fixed so it’s called Floating Neutral.
Normal Power Condition
On 3-phase systems there is a tendency for the star-point and Phases to want to ‘balance out’ based on the ratio of leakage on each Phase to Earth. The star-point will remain close to 0V depending on the distribution of the load and subsequent leakage (higher load on a phase usually means higher leakage).
Three phase systems may or may not have a neutral wire. A neutral wire allows the three phase system to use a higher voltage while still supporting lower voltage single phase appliances. In high voltage distribution situations it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
                 
Three phases has properties that make it very desirable in electric power systems. Firstly the phase currents tend to cancel one another (summing to zero in the case of a linear balanced load). This makes it possible to eliminate the neutral conductor on some lines. Secondly power transfer into a linear balanced load is constant.

3 Phase 4 Wire Systems for Mix Load:
Most domestic loads are single phase. Generally three phase power either does not enter domestic houses or it is split out at the main distribution board.
As per Kirchhoff’s Current Law the sum of the currents entering a node is zero. If the neutral point is the node, then, in a balanced system, one phase matches the other two phases, resulting in no current through neutral. Any imbalance of Load will result in a current flow on neutral, so that the sum of zero is maintained.
The neutral should never be connected to a ground except at the point at the service where the neutral is initially grounded (At Distribution Transformer). This can set up the ground as a path for current to travel back to the service. Any break in the ground path would then expose a voltage potential.
Grounding the neutral in a 3 phase system helps stabilize phase voltages. A non-grounded neutral is sometimes referred to as a “floating neutral” and has a few limited applications.
Floating Neutral Condition:
Power flows in and out of customers’ premises from the distribution network, entering via the Phase and leaving via the neutral. If there is a break in the neutral return path electricity may then travel by a different path.
                                           
If you have flickering lights or tingly taps in your home, you may be at risk of serious injury or even death.

Voltage Measurement between Neutral to Ground
A thumb rule, Neutral to ground voltage of 2V or less is acceptable, while a few volts or more indicates overloading; 5V is seen as the upper limit.
Zero Reading:
A certain amount of Neutral to ground voltage is normal in a loaded circuit. If  the reading is stable at close to 0V. There is a suspect an illegal Neutral. Rectify it.
 Any Neutral to ground bonds other than those at the transformer source (and/or main panel) should be removed to prevent return currents flowing through the ground conductors.
Various Factors which cause Neutral fail or Floating
a) At The Three Phase Distribution Transformer
Neutral failure at transformer i.e. failure of Neutral bushing.
Poor workmanship of Installation and technical staff also one of the reasons of Neutral Failure. Some consumer will experience Overvoltage while some will experience Low voltage.
b) Broken Overhead Neutral conductor in LV Line
The impact of broken overhead neutral conductor at LV overhead distribution will be similar to the broken at transformer. Supply voltage floating up to Line voltage instead of phase voltage. This type of fault condition may damage consumer equipment connected to the supply.
c) Break of Service Neutral Conductor
A broken Neutral of service conductor will only result of loss of supply at the customer point. No any damages to customer equipments.
d) High Earthing Resistance of Neutral at Distribution Transformer:
 High Earthing Resistance may provide high resistance Path for grounding of Neutral at Distribution Transformer. Limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting. In domestic line  earth resistance value is below of 5 ohms.
e) Over Loading and Load Unbalancing
Distribution Network Overloading combined with poor load distribution is one of the most reason of Neutral failure. Neutral should be properly designed so that minimum current will  flow in to neutral conductor
In Overloaded Unbalancing Network; lot of current will flow in Neutral which break Neutral at its weakest point.
f) Shared neutrals
Some buildings are wired so that two or three phases share a single neutral.
From nonlinear loads, primarily third harmonic, will add up arithmetically and return on the neutral. In addition to being a potential safety problem because of overheating of an undersized neutral, the extra neutral current creates a higher Neutral to ground voltage.
g) Poor workmanship and Maintenance
Normally LV network are taken lightly for Maintenance. Loose or inadequate tightening of Neutral conductor will effect on continuity of Neutral which may cause floating of Neutral.
How to test neutral break or float?
Condition (I) – Neutral is not Floating
Where the Neutral is grounded
Phase & Neutral - 240V
Phase to phase -440V. The Neutral is not Floating.

Condition (II) – Neutral is floating
All Appliances are connected: If the Neutral wire for a circuit becomes disconnected from the household’s main power supply panel while the Phase wire for the circuit still remains connected to the panel and the circuit has appliances plugged into the socket outlets. In that situation, if you put a voltage Tester with a neon lamp onto the Neutral wire it will glow just as if it was Live, because it is being fed with a very small current coming from the Phase supply via the plugged-in appliance(s) to the Neutral wire.
All Appliances are Disconnected: If you unplug all appliances, lights and whatever else may be connected to the circuit, the Neutral will no longer seem to be Live because there is no longer any path from it to the Phase supply.
  • Phase to Phase Voltage: The meter indicates 440V AC. (No any Effect on 3 Phase Load)
  • Phase to Neutral Voltage: The meter indicates 110V AC to 330V AC.
  • Neutral to Ground Voltage: The meter indicates 110V.
  • Phase to Ground Voltage: The meter indicates 120V.
This is because the neutral is “floats” above ground potential (110V + 120V = 230VAC). As a result the output is isolated from system ground and the full output of 230V is referenced between line and neutral with no ground connection.
ATTENTION:  some consumer will have less than 240V and some will have higher up to near 415. All depends on the impedance of each connected item.
In an unbalance system, if the neutral is disconnected from the source, the neutral becomes floating neutral and it is shifted to a position so that it is closer to the phase with higher loads and away from the phase with smaller load.
Let us assume an unbalance 3 phase system has 3 KW load in R-phase, 2 KW load in Y-phase and 1 KW load in B-phase. If the neutral of this system is disconnected from the main, the floating neutral will be closer to R-phase and away from B-phase.
So, the loads with B-phase will experience more voltage than usual, while the loads in R-phase will experience less voltage. Loads in Y-phase will experience almost same voltage. The neutral disconnect for an unbalanced system is dangerous to the loads. Because of the higher or lower voltages, the equipment is most likely to be damaged.

How to rectify neutral break or float.
a) Use ELCB, RCBO or 4 Pole Circuit Breaker as incoming in the 3ph supply system e if neutral opens it will trip the complete supply without damaging to the system.
b) Use Voltage Stabilizer
 Neutral fail in single phase system, the connected loads will get connected between phases owing to floating neutral. Hence depending on load resistance across these phases, the voltage keeps varying between 230V to 400V. A suitable servo stabilizer with wide input voltage range with high and low cutoff may help in protecting the equipments.
c) Preventive Maintenance
 High Priority is required for Maintenance of Low Voltage network. Tight or apply adequate Torque for tightening of Neutral conductor in LV system
Conclusion
A Floating Neutral (Disconnected Neutral) fault condition is VERY UNSAFE because If appliance is not working and someone who does not know about the Neutral Floating could easily touch the Neutral wire to find out why appliances does not work when they are plugged into a circuit and get a bad shock.
It should be corrected at the earliest possible by troubleshooting.

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