Impact of Floating Neutral in Power Distribution
Introduction:
- If The Neutral
Conductor opens, Break or Loose at either 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. Broken Neutral or Loose Neutral would damage to the
connected Load or Create hazardous Touch Voltage at equipment body. Here We are trying to understand the Floating Neutral Condition in T-T distribution System.
What is 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 & Floating Neutral Condition
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).
- 3 Phase 3 Wire System:
- 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 System 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.
- Kirchhoff’s Current Law states that the
signed 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.
- For instance, in a balanced system,
current entering the neutral node from one Phase side is considered
positive, and the current entering (actually leaving) the neutral node
from the other side is considered negative.
- This gets more complicated in three
phase power, because now we have to consider phase angle, but the
concept is exactly the same. If we are connected in Star connection with
a neutral, then the neutral conductor will have zero current on it only
if the three phases have the same current on each. If we do vector
analysis on this, adding up sin(x), sin(x+120), and sin(x+240), we get
zero.
- The same thing happens when we are
delta connected, without a neutral, but then the imbalance occurs out in
the distribution system, beyond the service transformers, because the
distribution system is generally a Star Connected.
- 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. Power flow entering in
one Phase returns through remaining two phases. Neutral Point is not at
ground Level but it Float up to Line Voltage. This situation can be very
dangerous and customers may suffer serious electric shocks if they
touch something where electricity is present.
- Broken neutrals can be difficult to
detect and in some instances may not be easily identified. Sometimes
broken neutrals can be indicated by flickering lights or tingling taps.
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 rule-of-thumb used by many in the
industry is that Neutral to ground voltage of 2V or less at the
receptacle is okay, while a few volts or more indicates overloading; 5V
is seen as the upper limit.
- Low Reading: If
Neutral to ground voltage is low at the receptacle than system is
healthy, If It is high, then you still have to determine if the problem
is mainly at the branch circuit level, or mainly at the panel level.
- Neutral to ground voltage exists
because of the IR drop of the current traveling through the neutral back
to the Neutral to ground bond. If the system is correctly wired, there
should be no Neutral to Ground bond except at the source transformer (at
what the NEC calls the source of the Separately Derived System, or SDS,
which is usually a transformer). Under this situation, the ground
conductor should have virtually no current and therefore no IR drop on
it. In effect, the ground wire is available as a long test lead back to
the Neutral to ground bond.
- High Reading: A high
reading could indicate a shared branch neutral, i.e., a neutral shared
between more than one branch circuits. This shared neutral simply
increases the opportunities for overloading as well as for one circuit
to affect another.
- 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 to ground bond in the receptacle (often due to lose
strands of the neutral touching some ground point) or at the subpanel.
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 Floating:
- There are several factors which are
identifying as the cause of neutral floating. The impact of Floating
Neutral is depend on the position where Neutral is broken
1) At The Three Phase Distribution Transformer:
- Neutral failure at transformer is mostly failure of Neutral bushing.
- The use of Line Tap on transformer
bushing is identified as the main cause of Neutral conductor failure at
transformer bushing. The Nut on Line Tap gets loose with time due to
vibration and temperature difference resulting in hot connection. The
conductor start melting and resulting broke off Neutral.
- Poor workmanship of Installation and technical staff also one of the reasons of Neutral Failure.
- A broken Neutral on Three phases
Transformer will cause the voltage float up to line voltage depending
upon the load balancing of the system. This type of Neutral Floating may
damage the customer equipment connected to the Supply.
- Under normal condition current flow
from Phase to Load to Load to back to the source (Distribution
Transformer). When Neutral is broken current from Red Phase will go back
to Blue or Yellow phase resulting Line to Line voltage between Loads.
- Some customer will experience over voltage while some will experience Low voltage.
2) 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 customer equipment connected to the supply.
3) Broken 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.
4) High Earthing Resistance of Neutral at Distribution Transformer:
- Good Earthing Resistance of Earth Pit
of Neutral provide low resistance path for neutral current to drain in
earth. 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.
5) Over Loading & 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 be flow in to neutral conductor. Theoretically
the current flow in the Neutral is supposed to be zero because of
cancellation due to 120 degree phase displacement of phase current.
- IN= IR<0 + IY<120 + IB<-120.
- In Overloaded Unbalancing Network lot of current will flow in Neutral which break Neutral at its weakest Point.
6) Shared neutrals
- Some buildings are wired so that two or
three phases share a single neutral. The original idea was to duplicate
on the branch circuit level the four wire (three phases and a neutral)
wiring of panel boards. Theoretically, only the unbalanced current will
return on the neutral. This allows one neutral to do the work for three
phases. This wiring shortcut quickly became a dead-end with the growth
of single-phase non-linear loads. The problem is that zero sequence
current
- 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. This Neutral to ground voltage subtracts from
the Line to Neutral voltage available to the load. If you’re starting
to feel that shared neutrals are one of the worst ideas that ever got
translated to copper.
7) Poor workmanship & Maintenance :
- Normally LV network are mostly not
given attention by the Maintenance Staff. Loose or inadequate tightening
of Neutral conductor will effect on continuity of Neutral which may
cause floating of Neutral.
How to detect Floating Neutral Condition in Panel:
- Let us Take one Example to understand Neutral Floating Condition.We have a Transformer which Secondary is star connected, Phase to neutral = 240V and Phase to phase = 440V.
Condition (1): Neutral is not Floating
- Whether the Neutral is grounded the
voltages remain the same 240V between phase & Neutral and 440V
between phases. The Neutral is not Floating.
Condition (2): 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.
- If suddenly disconnect the Neutral from
the transformer Neutral but kept the loading circuits as they are, Then
Load side Neutral becomes Floating since the equipment that are
connected between Phase to Neutral will become between Phase to Phase ( R
to Y,Y to B), and since they are not of the same ratings, the
artificial resulting neutral will be floating, such that the voltages
present at the different equipments will no longer be 240V but somewhere
between 0 (not exactly) and the 440 V (also not exactly). Meaning that
on one line Phase to Phase, some 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.
- Here we observe that Neutral Floating condition does not impact on 3 Phase Load but It impacts only 1 Phase Load only
How to Eliminate Neutral Floating:
- There are Some Point needs to be consider to prevent of Neutral Floating.
a) Use 4 Pole Breaker/ELCB/RCBO in Distribution Panel:
- A floating neutral can be a serious
problem. Suppose we have a breaker panel with 3 Pole Breaker for Three
Phase and Bus bar for Neutral for 3 Phase inputs and a neutral (Here we
have not used 4 Pole Breaker). The voltage between each Phase is 440 and
the voltage between each Phase and the neutral is 230. We have single
breakers feeding loads that require 230Volts. These 230Volt loads have
one line fed by the breaker and a neutral.
- Now suppose the Neutral gets loose or
oxidized or somehow disconnected in the panel or maybe even out where
the power comes from. The 440Volt loads will be unaffected however the
230V loads can be in serious trouble. With this Floating neutral
condition you will discover that one of the two lines will go from
230Volts up to 340 or 350 and the other line will go down to 110 or 120
volts. Half of your 230Volt equipment will go up in high due to
overvoltage and the other half will not function due to a low voltage
condition. So, be careful with floating neutrals.
- Simply use ELCB, RCBO or 4 Pole Circuit
Breaker as income in the 3ph supply system since if neutral opens it
will trip the complete supply without damaging to the system.
b) Using Voltage Stabilizer:
- Whenever neutral fails in three 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 & low cutoff may
help in protecting the equipments.
c) Good workmanship & Maintenance :
- Give higher Priority on Maintenance of
LV 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. Single phase Appliances are design to work its
normal Phase Voltage when they get Line Voltage Appliances may Damage
.Disconnected Neutral fault is a very unsafe condition and should be
corrected at the earliest possible by troubleshooting of the exact wires
to check and then connect properly.
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