Isolator Interview Questions & Answers

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Isolator Interview Questions

    1. Question 1. What Is An Isolator And Why Is It Used In The Power System?

      Answer :

      An isolator is a disconnecting switch being operated under the condition of on load or negligible current. It is used in the power system for disconnecting the circuit breaker from the circuit at the time of repair and maintenance work.

    2. Question 2. Why Are Isolators Provided On Both Sides Of The Circuit Breakers?

      Answer :

      Isolators are provided on both sides of the circuit breakers for repairing work or replacement of breaker without any danger from the live circuit.

    3. Question 3. What Interlocking Arrangements Are Generally Provided In The Isolators?

      Answer :

      1. Interlocking with circuit breaker is that the isolator cannot be opened unless the circuit breaker is opened. 
      2. The three poles of the isolator should be interlocked for simultaneous operation.

    4. Question 4. What Are The Types Of O.h. Isolators Generally Used?

      Answer :

      The types of isolators are as follows:- 

      • Vertical break type single, double or triple pole isolator consisting mainly of fixed soiled core type insulator support with rigid contacts and moving copper blade supported by operating rod insulator. These are generally used in traction system of about 25 kV.
      • Horizontal two rotating post, center break type isolator consisting of two insulator stacks of post type insulator mounted on a bearing and also rotatable about its vertical axis through 90°. In this type one insulator stack swings in clockwise direction and other in opposite direction in such a way that the contact arms swings in horizontal plane isolating at the center. This is used at the voltage upto 220 kV.
      • Horizontal break center rotating double break isolator consisting of three insulator stacks per pole having two insulator stacks fixed on sides and center insulator stack rotating. Fixed contacts are mounted on the top of each of the insulator stacks on the side. It can be used in case of all outdoor isolation of circuit breaker, transformer banks, and lightning arrestors and also for line sectionalization.
      • Extra high voltage column isolator consisting of a two part pantograph on the top, revolving drive insulator, solid core type porcelain insulator acting as the main supporting unit, the operating cubical provided with all necessary auxiliary contacts & other contacts for supervision & control and a vertically erected steel tube support anchored to the concrete plinth.

    5. Question 5. What Are The Advantages Of Extra High Voltage Column Isolators?

      Answer :

      The advantages are as follows:- 

      1. It requires less ground area.
      2. It is clearly visible from a great distance.
      3. Keeping the fixed contacts alive the moving contacts can be inspected safely.

    6. Question 6. What Material Is Used For Making Pantograph Bar & Why?

      Answer :

      An aluminium magnesium alloy is used for making pantograph bar for its high mechanical strength, light weight, good conductivity and more flexibility due to its high elasticity characteristics.

    7. Question 7. In Which Arrangement Of Bus Bar The Bus Coupler Is Very Essential?

      Answer :

      Main and transfer bus arrangement.

    8. Question 8. What Is Meant By Insulation Co-ordination?

      Answer :

      Insulation co-ordination means the mutual relation between the insulation of the equipments and the overvoltage characteristics of the protective device the basis of which is that the withstand overvoltage characteristics of all the equipments should lie above the spar over characteristics of surge arrestor on worst on worst possible atmospheric condition.

    9. Question 9. What Is The Difference Between An Isolator And A Circuit Breaker?

      Answer :

      The difference between an isolator and a circuit breaker is that, an isolator is a mechanical device capable of opening or closing a circuit without any arc control device under conditions of no load or negligible current but a circuit breaker is a mechanical device for making and breaking a circuit with are control device under normal and abnormal conditions i.e. even when a heavy fault current is flowing.

    10. Question 10. Why Is An Isolator Installed?

      Answer :

      To isolate one portion of the circuit from another.

    11. Question 11. The Isolators Used In The Transmission Lines Are Capable Of Breaking?

      Answer :

      Charging current.

    12. Question 12. For Which Among The Following The Current Ratings Are Not Required?

      Answer :

      Isolators.

    13. Question 13. Why Are The Isolators Used?

      Answer :

      Breaking the circuit under no load condition.

    14. Question 14. It Is A Passive Device Which Allows The Flow Of Optical Signal Power In Only One Direction And Preventing Reflections In The Backward Direction?

      Answer :

      Ideally, an optical isolator transmits the signal power in the desired forward direction. Material imperfections in the isolator medium generate backward reflections. Optical isolators can be implemented by using FBG.

    15. Question 15. How Many Implementation Methods Are Available For Optical Isolators?

      Answer :

      Optical isolators can be implemented using three techniques. 

      These are as follows:

      • By using FBGs.
      • By using magnetic oxide materials.
      • By using semiconductor optical amplifiers (SOAs).

    16. Question 16. A Very Common Application Of Isolator Is To Provide Isolation Between A Low Power Source And The Load?

      Answer :

      Isolators have a wide variety of applications. The most common among them is the use of an isolator between a high-power source and a load to prevent the possible reflections from damaging the source. An isolator can be used in place of a matching network, but it should be realized that any power reflected from the source is absorbed by the isolator.

    17. Question 17. Forward Attenuation Provided By A Resonance Fer Rite Isolator Is?

      Answer :

      Zero forward attenuation cannot be obtained in resonance isolators because the internal magnetic field is not truly circularly polarized. Because of this, there is some amount of forward attenuation in the isolator.

    18. Question 18. An Isolator Has A Very Large Operating Bandwidth And Independent Of Any Isolator Parameter?

      Answer :

      The bandwidth of an isolator is relatively narrow, dictated essentially by the line width ∆H of the ferrite material.

    19. Question 19. The Length Of A Fer Rite Slab Required Operating With A Minimum Forward Insertion Loss And 30 Db Reverse Attenuation And The Reverse Attenuation At This Point Is?

      Answer :

      Length of the ferrite slab required is equal to the ratio of the minimum forward insertion loss to the reverse attenuation at the point. Substituting the given values in the above equation, length of the ferrite slab is 2.4 cm.

    20. Question 20. The Electric Field Distribution Of The Forward And Reverse Waves In A Fer Rite Slab-loaded Wave Guide Is Quite Different. This Property Is Used In?

      Answer :

      In a field displacement isolator, electric field distribution of the forward and reverse waves in a ferrite slab-loaded waveguide is different. The electric field for the forward wave can be made to vanish at the side of the ferrite slab.

    21. Question 21. Field Displacement Isolators Require Higher Bias Field Than Resonance Isolators?

      Answer :

      Field displacement isolators have much smaller bias field requirement since it operates well below gyro magnetic resonance. This property of field displacement isolator make it more preferred than resonance isolators.

    22. Question 22. Why Should I Use Isolators?

      Answer :

      Isolators offer many benefits that save money and time.

      1. Installation Savings – Machines are installed faster and easier than anchoring.
      2. Case Histories have shown that the precision leveling and alignment features of Micro/Level® Isolators reduces machine wear and tear; increases tool life; and increases productivity by reducing downtime and improving part quality and repeatability.
      3. Reduce vibration and shock levels for a better work environment.
      4. Re leveling and moving the machine is also much faster and easier.

    23. Question 23. How Are Vibro/dynamics Isolators Different From Others On The Market?

      Answer :

      Vibro/Dynamics doesn’t use a “one size fits all” approach to isolator selection. Every Micro/Level Isolator size has a wide variety of different elastomeric inserts to precisely meet the isolator characteristics required to solve a particular problem. 

      Our design is unique. Look at a cross-section drawing of a Vibro/Dynamics Micro/Level® Isolator. Its design is unlike anything on the market. Notice that the isolator has pins or “fingers” that extend down from the support housing, pass through the bearing plate, and then fit into “grippers” molded into a custom-engineered elastomeric. This feature, called “Glide/Damping™”, reduces the isolator’s horizontal stiffness and provides a greater degree of isolation.  It also helps keep machines from walking by decoupling the support housing from the elastomeric.

    24. Question 24. Why Don’t Presses “walk” On Isolators?

      Answer :

      Presses don’t walk on properly selected free-standing isolators due to friction between the isolator and the foundation. To keep a machine from walking, the static deflection (compression) of the isolator under load must be greater than any dynamic unloading of the isolator caused by the operation of the machine. Properly selected and applied isolators always carry load, so the friction of the elastomeric on the floor is usually sufficient to overcome any horizontal forces that cause a machine to walk.

    25. Question 25. My Press Is Rocking. Is This Normal?

      Answer :

      Yes, to an extent. A press with an unbalanced crankshaft or eccentric will generate a rocking force. Isolators compress and shear in reaction to these forces, so it is normal that a press will rock to some degree. The softer the isolator is, the more the press will rock. Depending on the press and isolators, one-quarter inch at the crown is not unusual. The human eye tends to magnify motion, so what looks like excessive motion may not be.

    26. Question 26. How Long Will Isolators Last?

      Answer :

      It is not uncommon for Vibro/Dynamics Isolators to last 30 years or more. When properly applied and installed, the isolators can last the life of a machine. 

      Chemicals can be harmful. Vibro/Dynamics has three alternate elastomeric compounds to handle most environments. Consult with us if your isolator installation will be subjected to a high degree of chemical exposure.

    27. Question 27. Why Are You Recommending A Larger Isolator Than Your Competitor?

      Answer :

      It could be due to two reasons. One, Vibro/Dynamics manufactures vibration isolators as opposed to simple machinery mounts. Our goal is to provide the best isolation possible while keeping machinery motion to an acceptable level.  A larger area isolator is usually a softer isolator. Due to the larger area, greater isolator deflection is possible without over stressing the isolator’s elastomeric element. Overstressing causes leveling instability (due to elastomeric creep) and the higher stress on the foundation. 

      Two, a larger isolator may have been recommended to provide better support coverage of the machine foot. The isolator support housing should span the foot gussets.

    28. Question 28. Why Does The Isolator Stick Out From Under The Machine Foot? Is This A Problem?

      Answer :

      The support housing of a Micro/Level® Isolator is designed to transfer the load on the isolator’s support housing to the leveling adjustment screw, which then distributes the load uniformly over a heavy-duty steel bearing plate. The isolator has a built-in swiveling capability that automatically compensates when the bottom of the machine foot is not parallel to the floor, assuring that the isolator’s elastomeric is uniformly loaded.

      Most machines have foot and leg designs strong enough to allow them to be supported at the mounting hole. Of course, there are exceptions and the Applications Engineering Staff at Vibro/Dynamics can offer assistance in this regard. Vibro/Dynamics has a complete line of vibration isolators designed for almost every situation. If the machine cannot be supported at the mounting hole, then Vibro/Dynamics will recommend a wedge-style isolator that can be placed anywhere under the machine foot.

    29. Question 29. How Do Isolators Reduce Noise?

      Answer :

      Isolators are very effective at reducing structural-borne noise caused by transmitted vibration. Vibration causes noise when it excites the natural frequencies of a structure. This “sounding board” effect is directly related to vibration in the supporting surface. Since isolators reduce vibration, structural-borne noise is reduced.

    30. Question 30. Is It Ok To Use Isolators On A Press With A Rolling Bolster Or Die Cart?

      Answer :

      Absolutely. This is very common. The alignment tolerance for most rolling bolster and die carts is sufficient so that it is not a problem.

    31. Question 31. What Is The Allowable Temperature Range For Your Elastomeric Products?

      Answer :

      Allowable temperature range for an elastomeric: 0°F to 100°F (-18°C to 38°C) with brief exposure up to 150°F (66°F).

    32. Question 32. Where Does The Vibration Go When I Mount My Press On Isolators?

      Answer :

      • Internal forces caused by the stamping operation occur whether the press is bolted to a foundation or installed on isolators. However, the magnitude of the vibration within the press is lower when using vibration isolators. 
      • It is a common misconception that bolting a press to a foundation somehow “sucks” the vibration out. Bolting a press to a foundation actually subjects it to more vibration and impact force. Vibration isolators are cushions that transform a sudden shock pulse into a decaying series of longer duration forces.  
      • Imagine hitting a brick wall with your fist. It would hurt! Now imagine hitting the wall with the same energy, but with your hand in a well padded boxing glove. It hurt less! Why? It’s because the impact duration is longer, effectively reducing the impact force. Short duration impacts mean higher force.
      • This is the same thing that happens on a hard-mounted press. The sudden release of potential energy at snap-through causes the press foot to slam against the foundation. This fast duration, high magnitude force hits the foundation and the foundation responds in kind, sending damaging forces back into the press.

    33. Question 33. Why Do I Still Feel Vibration In The Floor When My Press Is On Your Isolators?

      Answer :

      • Vibro/Dynamics rates isolation effectiveness as a percentage compared to the levels that would exist if the machine were anchored or “hard mounted”. It is not possible to obtain 100% with a passive isolation system, but you can come close. 
      • When selecting isolators, many factors are taken into account. But it usually comes down to two issues: How much isolation is required and how much motion can be tolerated? Isolators are usually selected to achieve the highest amount of vibration isolation possible while keeping motion to an acceptable level. These are conflicting goals. To reduce machine motion, the stiffness of the isolator is often increased, at the detriment of isolation, resulting in some vibration being felt in the floor. 
      • Inertia Force, which is the force generated by the motion of the press slide, is another possible cause of remaining vibration in the floor. Run the press without material to see if inertia is the cause of the vibration. If vibration is felt, then inertia is contributing to the vibration in the floor. 
      • Elastomeric isolators are not usually designed to isolate inertia force. These are high-tuned isolators, which mean the isolator’s natural frequency is higher than the operating speed (SPM) of the press, resulting in no inertia force isolation. In order to isolate inertia force, the natural frequency of the isolator must be lower than the operating speed of the press. Vibro/Dynamics VS Series Spring Isolators have natural frequencies as low as 2.5 Hz., which is ideal for machines running over 250 SPM.  Added weight is usually required to keep press motion to an acceptable level.

    34. Question 34. What Can I Do To Reduce The Vibration In My Press?

      Answer :

      Vibro/Dynamics isolators have been installed on tens of thousands of presses.  We have a lot of experience isolating machines with a wide variety of different press designs, jobs, and soil conditions.  Lowering the shock level within the machine will improve tooling and machine life and will further improve the work environment with less vibration in the plant.

      The following list of suggestions will help reduce vibration and save a lot of downtime and money. 

      1. Increase shear in the dies, flat punches will deliver very high snap-through loads. 
      2. Use an appropriately sized press for the job.  Most press manufacturers do not design their machines to accommodate overloads.  For long life, it is advisable to allow some safety factor in the job tonnage.  A hard or thick batch of steel or bad tooling can create an overload, causing damage to the machine and higher vibration levels.
      3. Avoid small dies in machines with a large die space.  Sometimes it has to be done, but small dies tend to load up and store energy by deflecting the slide like a bow.
      4. For High-Speed blanking in eccentric geared presses, Vibro/Dynamics has noticed that 500 ton and higher capacity eccentric geared presses running faster than 30 SPM tend to generate much more shock.  For these cases, coil spring isolators are strongly recommended.
      5. Set the air counter-balance properly.
      6. Make sure the press is level and not twisted.
      7. Reduce the moving weight or speed.  As press speed increases, so does inertia. In some cases, the reciprocating mass of the slide and upper die can generate enough force that it starts to approach the weight of a press, causing the press to “walk”.  The formula for determining the “inertia force” generated by the slide and upper die is: inertia force (Peak-Peak)= W/g*r*ω2    where W=moving weight, g=acceleration of gravity (9.8 m/s2 or 386 in/s2), r=stroke length, and ω=machine operating speed in rads/sec (1 SPM = 0.105 rad/sec).  Note that the machine speed is a dominant term, doubling the speed quadruples the inertia force. 
      8. To check for an inertia force vibration problem, run the machine without hitting material at the speed that normally causes problems.  If the vibration problem persists, then the inertia force generated by the unbalanced moving mass is the cause of the vibration.  
      9. Some machine builders reduce the generated inertia forces in their fast running machines by using a “dynamic balancer”, a slide that runs 180° out-of-phase (directly opposite) from the main slide and upper die.  The dynamic balancer is often 100% balanced for a “typical” upper die weight, but may be restricted due to clearances within the press crown structure.  Spring isolators can be used to install most dynamically balanced presses, but a motion analysis over the press’ speed range should be done first.  
      10. Make sure the tie rods are stretched correctly.
      11. Make sure there are no “short circuits.”  Avoid beams, deck plates, and feeds that are connected to both the press and the foundation. 
      12. Check and maintain press level.

    35. Question 35. My Machine Is Installed And The Vibrations Are Interfering With Nearby Offices Or Sensitive Equipment, What Can I Do To Fix This?

      Answer :

      Once machines are installed and a vibration problem is discovered, it is often very difficult and expensive to relocate the offending machine or sensitive area.  Surface vibration waves are typically the culprit.  The amplitude, or severity, of surface waves decreases by a function of at least (1/distance from vibration source)0.5.  High frequency waves will dissipate over shorter distances than longer wavelength, low frequency waves.  Vibration waves are further attenuated by damping losses characteristic of the materials they are travelling within.  Steel, concrete, and frozen water/soil is efficient conductors of vibration.  Surface waves can thus travel significant distance and still cause problems.  It is most effective to reduce the amplitude of the waves at the source and at the sensitive area.  The most cost efficient way to achieve isolation of the vibration waves is by using passive vibration isolators.

      Many other methods have been employed to attempt to reduce the surface wave vibrations, with generally limited success:

      1. Digging trench vibration barriers between the offending equipment and the sensitive area can reduce vibrations levels by 25% and sometimes more.  Hard barriers such as concrete are generally ineffective, although introducing any dissimilar material will result in some wave reflection.  The barrier method is very expensive and great care must be taken to design the trench.  If the trench is too shallow, the surface waves may pass below it.  If the trench does not completely encircle the source, the vibrations may bend and reflect and bypass the barrier on the sides.  There will be a shadow zone behind the trench where the vibration waves are reduced.  Trenches may also be ineffective if the offending vibrations are compression waves reflecting from deep rock.  Trench barriers will reflect some waves back towards the source resulting in increased vibration levels around the offending machine.
      2. Replacing conductive soil or structures with more highly damped or more flexible connections at uneven intervals can reduce vibration transmission.  This method is now common in pipe runs from vibration producing equipment within building structures.  Within industrial plants these measures are generally impractical and the vibration reduction usually small.  
      3. Separating the foundation from the surrounding soil and making the support area of the foundation as deep as possible will reduce surface waves.  Like trench barriers, this method is expensive and space inefficient.  This method works on the principle that surface waves do not travel efficiently at depth.
      4. Stiffening a vibrating structure that is in resonance can resolve problems in mezzanines and simple structures.  A common problem is a multi-story or temporary office floors have low natural frequency modes that can be excited efficiently by machinery shocks or, more commonly, periodic vibrations.  Machines like stamping presses and hammers introduce a high magnitude shock into the surroundings that cause a large transient wave to propagate.  Many machines with rotating components such as high speed presses, vibrators, flywheels, generators, and motors can produce steady periodic vibrations.  Stiffening the flooring of such structures to raise the natural frequency can alleviate the problem.  Stiffening an existing structure is expensive because often the stiffness needs to be at least doubled to achieve noticeable results.  When the problem is a machine with a very flexible arm such as on CMMs, EDMs, robotic welders or torches, and some mills, stiffening the arm can be a simple and effective solution.
      5. Adding weight to the sensitive equipment is somewhat effective with diminishing returns as weight is added.  Sometimes referred to as mass loading, this method increases the inertia of the equipment.  This method is most effective when employed with passive vibration isolators to create a very low natural frequency system.  In the form of a large inertia mass with isolators and a sub-foundation, the isolated system is termed an isolated foundation.  

      To avoid costly vibration mitigation measures it is best practice to be aware of vibration problems before a machine is installed.  It is important to understand the frequency content of the source and the sensitive frequencies of the surroundings.

    36. Question 36. Do I Still Need A Foundation If I Use Machinery Mounts And Isolators?

      Answer :

      Isolators do not eliminate the need for a well-designed foundation. The foundation must be strong enough to support the physical weight of the machinery, plus the dynamic forces generated by the machine. Dynamic forces vary with machine characteristics. Well-designed isolators will reduce the dynamic force transmitted into a foundation, but determining the amount going into a foundation is very difficult.

    37. Question 37. How Much Force Is Transmitted To The Foundation When A Machine Is Installed On Isolators?

      Answer :

      In order to determine the transmitted force, the frequency and forces being generated must be known as well as the characteristics of the soil, foundation, and machine construction.  Many of the variables influencing the magnitude of the input forces are not known or even measurable. 

      Factors that strongly influence the transmitted force:

      1. Machine construction
      2. Structural response of machine to shock
      3. Stamping operation (i.e., blanking, coining, drawing, etc.) 
      4. Soil conditions
      5. Foundation size and rigidity
      6. Die design
      7. Material properties
      8. Isolator properties

      Estimates of transmitted forces can be made, but they are only estimates. Vibration isolators are the most important factor in reducing transmitted vibration. However, the vibration isolators must be given the proper conditions to function at peak efficiency.

    38. Question 38. Is My Current Foundation Or Floor Sufficient?

      Answer :

      They can assess the local soil conditions, the status of the existing concrete, and the status of the existing reinforcing steel.  Areas with high water tables or that have been filled could potentially cause problems if the foundation design does not take them into consideration.

    39. Question 39. How Do I Measure The Foundation Stiffness (k) And Natural Frequency (fn)?

      Answer :

      The soil under the foundation will often be compacted during foundation construction and then further loaded and compacted as the installation ages and machines are run. So the finished installation is often very different from when the site was just a plant floor or an open field.  Foundation Natural Frequency (Fn) can be determined by dropping a mass and recording the ground vibrations. 

      Two types of tests can be done by Civil Engineers to determine the soils present at unimproved sites:

      1. The simplest is a penetration test where a pipe is driven into the ground and the number of blows and the given distance traveled are recorded.  More blows equal harder, more compact soil.  Core samples are retrieved and inspected to determine the type of soils present. The soil is analyzed and a chart of blows and soil type per depth is commonly supplied.
      2. Another common and fairly simple test is a cross-hole measurement of the soils.  Two holes are bored and a transducer is lowered into one and a seismometer in the other hole.  By measuring the velocity of the waves passed between the holes, an estimate of the soil stiffness (elastic modulus) is obtained.

      A Red Flag is when the soil is found to be very poor with high amount of fine clays and high water table.  The foundation will tend to have a lower natural frequency and lower bearing capacity if the soil is poor.  In these cases, Vibro/Dynamics advises a larger foundation and "foot print" with the softest isolators permissible to avoid settling and to improve isolator effectiveness.  If the foundation is under-sized, the isolators will have reduced effectiveness regardless of their stiffness.

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