Introduction

If the Neutral Conductor is opened, broke 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.

Broken Neutral or Loose Neutral would damage to the connected load or create hazardousTouch 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 and 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), andsin(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.

Floating Neutral Impacts in Power Distribution

Posted AUG 27 2012 by JIGUPARMAR in ENERGY AND POWER with 3 COMMENTS

Floating Neutral Impacts in Power Distribution (photo by Mardix Limited; Fickr)

Introduction

If the Neutral Conductor is opened, broke 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.

Broken Neutral or Loose Neutral would damage to the connected load or create hazardousTouch 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 and 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).

Healthy power system scheme

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), andsin(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.

Floating neutral condition

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.

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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 Overvoltage 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 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 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 and Maintenance

Normally LV network are mostly not given attention by the Maintenance Staff. Loose orinadequate tightening of Neutral conductor will effect on continuity of Neutral which may cause floating of Neutral.

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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

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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 and low cutoff may help in protecting the equipments.

c) Good workmanship and 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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Vibration Damper in Transmission Line

Vibration Damper in Transmission Line:

Wind-induced vibration of overhead conductors is common worldwide and can cause conductor fatigue Near a hardware attachment.

As the need for transmission of communication signals increase, many Optical Ground Wires(OPWG) are replacing traditional ground wires.

In the last twenty years All Aluminum Alloy Conductors (AAAC) have been a popular choice for overhead conductors due to advantages in both electrical and mechanical characteristics. Unfortunately AAAC is known to be prone to Aeolian vibration.

Vibration dampers are widely used to control Aeolian vibration of the conductors and earth wires including Optical Ground Wires (OPGW).

In recent years, AAAC conductor has been a popular choice for transmission lines due to its high electrical carrying capacity and high mechanical tension to mass ratio. The high tension to mass ratio allows AAAC conductors to be strung at a higher tension and longer spans than traditional ACSR (Aluminum Conductor Steel Reinforced) conductors.

Unfortunately the self-damping of conductor decreases as tension increases. The wind power into the conductor increases with span length. Hence AAAC conductors are likely to experience more severe vibration than ACSR.

What is Aeolian Vibration?

• Wind-induced vibration or Aeolian vibration of transmission line conductors is a common phenomenon undersmooth wind conditions. The cause of vibration is that the vortexes shed alternatively from the top and bottom of the conductor at the leeward side of the conductor.
• The vortex shedding action creates an alternating pressure imbalance, inducing the conductor to move up and down at right angles to the direction of airflow.
• The conductor vibration results in cyclic bending of the conductor near hardware attachments, such as suspension clamps and consequently causes conductor fatigue and strand breakage.
• When a “smooth” stream of air passes across a cylindrical shape, such as a conductor or OHSW, vortices (eddies) are formed on the back side. These vortices alternate from the top and bottom surfaces, and create alternating pressures that tend to produce movement at right angles to the direction of the air flow. This is the mechanism that causes Aeolian vibration.
• The term “smooth” was used in the above description because unsmooth air (i.e., air with turbulence) will not generate the vortices and associated pressures. The degree of turbulence in the wind is affected both by the terrain over which it passes and the wind velocity itself.
• It is for these reasons that Aeolian vibration is generally produced by wind velocities below 15 miles per hour (MPH). Winds higher than 15 MPH usually contain a considerable amount of turbulence, except for special cases such as open bodies of water or canyons where the effect of the terrain is minimal.
• The frequency at which the vortices alternate from the top to bottom surfaces of conductors and shield wires can be closely approximated by the following relationship that is based on the Strouhal Number [2].
• Vortex Frequency (Hertz) = 3.26 V / d
• Where: V is the wind velocity component normal to the conductor or OHSW in miles per hour
• d is the conductor or OHSW diameter in inches
• 3.26 is an empirical aerodynamic constant.
• One thing that is clear from the above equation is that the frequency at which the vortices alternate is inversely proportional to the diameter of the conductor or OHSW.
• The self damping characteristics of a conductor or OHSW are basically related to the freedom of movement or “looseness” between the individual strands or layers of the overall construction.
• In standard conductors the freedom of movement (self damping) will be reduced as the tension is increased. It is for this reason that vibration activity is most severe in the coldest months of the year when the tensions are the highest.
• Aeolian vibrations mostly occur at steady wind velocities from 1 to 7 m/s. With increasing wind turbulence the wind power input to the conductor will decrease. The intensity to induce vibrations depends on several parameters such as type of conductors and clamps, tension, span length, topography in the surrounding, height and direction of the line as well as the frequency of occurrence of the vibration induced wind streams.
• Hence the smaller the conductor, the higher the frequency ranges of vibration of the conductor. The vibration damper should meet the requirement of frequency or wind velocity range and also have mechanical impedance closely matched to that of the conductor. The vibration dampers also need to be installed at suitable positions to ensure effectiveness across the frequency range.

Effect of Aeolian Vibration:

• It should be understood that the existence of Aeolian vibration on a transmission or distribution line doesn’t necessarily constitute a problem. However, if the magnitude of the vibration is high enough, damage in the form of abrasion or fatigue failures will generally occur over a period of time.
• Abrasion is the wearing away of the surface of a conductor or OHSW and is generally associated with loose connections between the conductor or OHSW and attachment hardware or other conductor fittings.
• Abrasion damage can occur within the span itself at spacers Fatigue failures are the direct result of bending a material back and forth a sufficient amount over a sufficient number of cycles.
• In the case of a conductor or OHSW being subjected to Aeolian vibration, the maximum bending stresses occur at locations where the conductor or OHSW is being restrained from movement. Such restraint can occur in the span at the edge of clamps of spacers, spacer dampers and Stock bridge type dampers.
• However, the level of restraint, and therefore the level of bending stresses, is generally highest at the supporting structures.                                       
• When the bending stresses in a conductor or OHSW due to Aeolian vibration exceed the endurance limit, fatigue failures will occur.
• In a circular cross-section, such as a conductor or OHSW, the bending stress is zero at the center and increases to the maximum at the top and bottom surfaces (assuming the bending is about the horizontal axis). This means that the strands in the outer layer will be subjected to the highest level of bending stress and will logically be the first to fail in fatigue.

working of Vibration Damper

• When the damper is placed on a vibrating conductor, movement of the weights will produce bending of the steel strand. The bending of the strand causes the individual wires of the strand to rub together, thus dissipating energy. The size and shape of the weights and the overall geometry of the damper influence the amount of energy that will be dissipated for specific vibration frequencies.
• Since, as presented earlier, a span of tensioned conductor will vibrate at a number of different resonant frequencies under the influence of a range of wind velocities, an effective damper design must have the proper response over the range of frequencies expected for a specific conductor and span parameters.

(1) VORTX/ Stock bridge Type:

• Some dampers, such as the VORTX Damper utilize two different weights and an asymmetric placement on the strand to provide the broadest effective frequency range possible.
  
• The “Stockbridge” type vibration damper is commonly used to control vibration of overhead conductors and OPGW. The vibration damper has a length of steel messenger cable. Two metallic weights are attached to the ends of the messenger cable. The centre clamp, which is attached to the messenger cable, is used to install the vibration damper onto the overhead conductor.
• Placement programs, such as those developed by PLP for the VORTX Damper, take into account span and terrain conditions, suspension types, conductor self-damping, and other factors to provide a specific location in the span where the damper or dampers will be most effective.
• The asymmetrical vibration damper is multi resonance system with inherent damping. The vibration energy is dissipated through inter-strand friction of the messenger cable around the resonance frequencies of the vibration damper. By increasing the number of resonances of the damper using asymmetrical design and increasing the damping capacity of the messenger cable the vibration damper is effective in reducing vibration over a wide frequency or wind velocity range.

(2) Spiral Vibration Damper:

• For smaller diameter conductors (< 0.75”), overhead shield wires, and optical ground wires (OPGW), a different type of damper is available that is generally more effective than a Stockbridge type damper.
• The Spiral Vibration Damper (Figure 15) has been used successfully for over 35 years to control Aeolian vibration on these smaller sizes of conductors and wires.
• The Spiral Vibration Damper is an “impact” type damper made of a rugged non-metallic material that has a tight helix on one end that grips the conductor or wire. The remaining helixes have an inner diameter that is larger than the conductor or wire, such that they impact during Aeolian vibration activity. The impact pulses from the damper disrupt and negate the motion produced by the wind.

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Explain thin film resistors and wire-wound resistors

Thin film resistors- It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.b. Wire wound resistors – length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

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