Question 1
Electrode gap in electro chemical machining is generally ranged from
A. 0.5 mm to 0.9 mm
B. 1.1 mm to 1.2 mm
C. 0.1 mm to 0.2 mm
D. 3.1 mm to 4.2 mm
View Answer
Answer: Option C
Explanation:
Electrode gap in electro chemical machining is generally ranged from 0.1 mm to 0.2 mm.
Question 2
Electrolytes used in ECM must posses
A. Low electrical conductivity
B. Low chemical stability
C. High electrical conductivity
D. None of the mentioned
View Answer
Answer: Option C
Explanation:
Electrolytes used in ECM must have high electrical conductivity.
Question 3
In ECM, heavy electrical sparks are created.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
Sparks are generated in EDM, by using high voltage current, for proper erosion of material.
Question 4
In ECM, tool does not touch the work piece.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Tool is very near to work piece but does not touch it in ECM.
Question 5
Which of the following is conventional machining process?
A. Electro chemical machining
B. Milling
C. Electron discharge machining
D. None of the mentioned
View Answer
Answer: Option B
Explanation:
Milling is a conventional machining process used for material removal from the surface.
Question 6
Which of the following is correct about ECM?
A. Erosion of metal takes place as a reverse process of electroplating
B. Thermal stresses are induced
C. Mechanical stresses are induced
D. None of the mentioned
View Answer
Answer: Option A
Explanation:
In ECM, erosion of metal takes place as reverse process of electroplating.
Question 7
Which of the following is not a function of electrolyte in ECM?
A. It completes the circuit
B. It helps in electrochemical reaction
C. It carries away heat and waste product
D. It provide non reactive environment
View Answer
Answer: Option D
Explanation:
Electrolyte provides reactive environment for chemical reactions to takes place.
Question 8
Which of the following is un-conventional machining process?
A. Grinding
B. Milling
C. Turning
D. Electro chemical machining
View Answer
Answer: Option D
Explanation:
Electro chemical machining is an un-conventional machining process used for large material removal from the surface using electricity generated due to chemical reactions.
Question 9
Which of the following material can be machined using electro chemical machining?
A. Iron
B. Rubber
C. Plastic
D. Wood
View Answer
Answer: Option A
Explanation:
Iron is a good conductor of electricity and hence can be machined using electro chemical machining.
Question 10
_____ strength of stainless steel decreases when machined by ECM.
A. Yield
B. Tensile
C. Fatigue
D. Notched-tensile
View Answer
Answer: Option C
Explanation:
Fatigue strength of stainless steel is found to decrease by ECM. This can, however, be overcome by cold working the surface of the product after ECM.
Question 11
Commercial ECM is carried out at a combination of ___________
A. low voltage high current
B. low current low voltage
C. high current high voltage
D. high voltage only
View Answer
Answer: Option A
Explanation:
As we know, in ECM, MRR= IA/(Fρv), where I= current. Therefore, the amount of electrochemical dissolution or deposition is proportional to the amount of charge passed through the electrochemical cell, i.e. m∝Q, where m = mass of material dissolved or deposited Q = amount of charge passed.
Question 12
Conductivity of the electrolyte is unaffected by the loss of hydrogen during electrolysis.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
Loss of hydrogen reduces the electrical conductivity of the electrolyte. Also due to loss of water by evaporation or being carried off by hydrogen gas generated, concentration of the solution increases which affects viscosity and electrical conductivity.
Question 13
Conductivity of the solution can be increased by_____
A. increasing the gap between the tool and the workpiece
B. increasing the temperature of the solution
C. increasing the concentration of the solution
D. using conductive workpiece
View Answer
Answer: Option C
Explanation:
Electrolyte conductivity can be increased in following ways-• Increasing the concentration will cause conductivity to rise• Temperature increases of the electrolyte also increases conductivityLow concentration and low temperature will results in lower MRR. Electrolyte conductivity affects the resistance across the gap, but the gap doesn’t affect the conductivity of the electrolyte.
Question 14
Discrepancies are sometimes observed between theoretical and actual metal removal rates and electrode feed rates.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
In practise, metal removal rates are often higher than the estimated ones because;(1) The exact valency at which a metal behaves in the reaction is generally unknown.(2) ECM continuously exposes a new and clean surface to the electrolyte which is easily attacked chemically. It also varies with electrolyte used and metal being machined.
Question 15
ECM can also be called as un-controlled anodic dissolution.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
ECM can be thought of a controlled anodic dissolution at atomic level of the workpiece that is electrically conductive by a shaped tool due to flow of high current at relatively low potential difference through an electrolyte which is quite often water based neutral salt solution.
Question 16
ECM process is based on which of the following laws?
A. Coulomb’s law
B. Faraday’s law
C. Law of definite proportions
D. Law of chemical combination
View Answer
Answer: Option B
Explanation:
that if two electrodes are placed in a bath containing a conducting liquid and a DC potential is applied across them, then metal can be depleted from the anode and plated on the cathode – process universally used in electroplating by making the workpiece the cathode. In ECM, the material is removed and hence the workpiece is made the anode.
Question 17
Electro-chemical machining is opposite of electrochemical coating.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
In electrochemical or galvanic coating, some another material is coated on the workpiece metal; whereas in Electro-chemical machining, material is removed from the workpiece.
Question 18
Electrolyte flow plays an important role in ECM.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Need for sufficient electrolyte flow between the tool and the workpiece-• To carry away the heat and the products of machining• To assist the machining process at the required feed rate, producing a satisfactory surface finish, cavitation, stagnation and vortex formation should be avoided since these lead to bad surface finish• There should be no sharp corners in the flow path. All corners in the flow path should have a radius.
Question 19
Electrolyte should carry away the heat generated and products of the reaction.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Electrolyte used in ECM should-• Complete the electrical circuit between the tool and the workpiece• Allow the desirable machining reactions to take place• Carry away the heat generated during the operation• Carry away products of reaction from the zone of machining.
Question 20
Electrolyte used in ECM should have _____________
A. high specific heat
B. lower resistance to film formation on the workpiece
C. higher viscosity
D. corrosive nature
View Answer
Answer: Option A
Explanation:
Desirable properties of the electrolytes are as follows:• High electrical conductivity – for easy ionization• Low viscosity – for easy flow• High specific heat – to carry more heat• Chemical stability – to be chemically neutral ordoes not disintegrate during the reaction• Resistance to formation of passivating film onthe workpiece surface• Non corrosiveness and non-toxicity• Inexpensiveness and easy availability.
Question 21
For ECM of steel which is used as the electrolyte?
A. Kerosene
B. NaCl
C. Deionised water
D. HNO3
View Answer
Answer: Option B
Explanation:
During ECM, there will be reactions occurring at the electrodes i.e. at the anode or workpiece and at the cathode or the tool along with within the electrolyte. For electrochemical machining of steel, generally a neutral salt solution of sodium chloride (NaCl) is taken as the electrolyte.
Question 22
For ECM, tool dimensions are different from the actual mirror dimensions of the part to be machined.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
In ECN, tool dimensions must be slightly different from the nominal mirror dimensions of the completed part to allow for ECM overcut. Designer must also determine the nature and the extent of the required deviation or gap allowances from the mirror image configuration while providing a uniform and sufficiently high flow rate of electrolyte in the gap to allow a practical MRR.
Question 23
For which of the following ECM cannot be undertaken?
A. Steel
B. Nickel based superalloy
C. Aluminium oxide
D. Titanium alloy
View Answer
Answer: Option C
Explanation:
ECM can machine any electrically conductive work material irrespective of their hardness, strength or even thermal properties. In ECM, material is removed from the workpiece by oxidizing it. Aluminium oxide cannot be oxidized further; therefore it cannot be machined by ECM.
Question 24
For which of the following ECM cannot be used?
A. Tapering a predrilled hole in iron block
B. To make stepped hole is a nickel alloy
C. To generate ribs on metal surface
D. To machine Al2O3
View Answer
Answer: Option D
Explanation:
Open-flow cathode (tool) is used to generate the outside diameter wall and leave an embossment. Stepped tool is used for sinking a stepped through-hole. Cone shaped tool is to generate taper in a predrilled hole. Al2O3 cannot be machined by ECM as it is ceramic.
Question 25
Foreign particle in electrolyte or insulation failure within tool can cause_____
A. random inaccuracy in work
B. ripples on work
C. cavitation
D. spark damage to work
View Answer
Answer: Option D
Explanation:
Foreign particle in electrolyte or insulation failure within tool causes spark damage to the tool or work at the point near to electrolyte entry into machining gap. To avoid this, check internal filters, electrolyte ducts, seals and joints. Also check insulation of the tool frequently.
Question 26
In ECM of pure iron a material removal rate of 600 mm3/min is required. What will be the current requirement?
A. 157A
B. 183.6A
C. 247.8A
D. 268.8A
View Answer
Answer: Option D
Explanation:
MRR= IA/(Fρv)MRR = 600 mm3/min = 600/60 mm3/s = 10 mm3/s = 10×10-3cc/sA= 56,v= 2,F= 96500 coulomb, andρ = 7.8 gm/ccTherefore, I= (96500*10*10-3*7.8*2)/56I= 268.8A.
Question 27
In ECM operation of pure iron an equilibrium gap (h) of 2 mm is to be kept. What will be the supply voltage (v), if the total overvoltage is 2.5 Volts. The resistivity(r) of the electrolyte is 50 Ω-mm and the set feed rate (f) is 0.25 mm/min.
A. 5v
B. 7.8v
C. 11.3v
D. 13.2v
View Answer
Answer: Option D
Explanation:
h= c/fWhere, c= (v- overvoltage)*A/(F*ρ*r*v)Therefore, c= (v-2.5)*56/(96500*7.8*10-3*50*2) c= (v-2.5)/1344.1Now, h= c/f 2= [(v-2.5)/1344.1]/[0.25/60]Therefore, v= 13.2 volts.
Question 28
In ECM, accuracy of the product is independent of the accuracy of the cathode tool.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
The accuracy of the tool shape directly affects the work piece accuracy, since configuration of the cavity produced cannot be more accurate than the tool the produces it.
Question 29
In ECM, electrode or tooling cost is fixed because_____
A. there is little wear of the tool
B. tool can be used only once
C. tool can be regenerated
D. one tool can be used for any product
View Answer
Answer: Option A
Explanation:
Electrode or tooling cost is fixed because there is little wear of the ECM tool. There occurs, however, a negligible abrasion wear of electrode due to electrolyte flow across the gap.
Question 30
In ECM, material removal rate does not depend on chemical properties of the anode material.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
Only electrically conductive workpieces can be machined by ECM process. In this process workpiece material work as an anode, therefore the chemical properties of the anode material largely govern the MRR.
Question 31
In ECM, one needs to understand fluid flow while designing the tool for machining complex shapes.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Because of the interaction of working-tip shape and dimensions, location of insulation, current density and feed rate, the design of tools for machining complex shapes requires understanding of fluid flow, electrical and electrochemical principles as well as experience and ingenuity.
Question 32
In ECM, shape of the cathode does not affect the shape of the machined product.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
In ECM, workpiece is used as an anode and cathode is made up of some conducting materials like aluminium, copper, brass, titanium, etc. In this process, product gets the shape which is inverse as that of the tool (cathode).
Question 33
In ECM, workpiece acts as a cathode.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
In ECM, material is depleted from anode workpiece and transported to a cathode tool in an electrolyte bath. Electrolyte flows rapidly between two poles to carry off depleted material, so it does not plate onto tool.
Question 34
Increase in pressure of the electrolyte above atmospheric pressure is beneficial.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Increased pressure of the electrolyte above atmospheric pressure increases the boiling point of the electrolyte point of the electrolyte, reduces the over-voltage required at the tool surface and decreases the volume of generated hydrogen due to which it displaces less electrolyte permitting the use of higher current densities.
Question 35
MRR is ECM depends on _____________
A. hardness of work material
B. atomic weight of work material
C. thermal conductivity of work material
D. ductility of work material
View Answer
Answer: Option B
Explanation:
In ECM, material removal takes place due to an atomic dissolution of work material. Electrochemical dissolution is governed by Faraday’s laws. Also, for ECM, MRR= IA/(Fρv), where I= current, ρ= density of the material, A= atomic weight, v= valency, F= faraday’s constant.
Question 36
Precipitate formation reduces the conductivity of the electrolyte solution.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Precipitate formation increases the effective viscosity and interferes with the process in the work-electrode gap. Also, salt absorption by the precipitate reduces the concentration, thus reduces the conductivity.
Question 37
Solubility of the reaction products increases with_____
A. increase in pressure of the electrolyte
B. increase in temperature of the electrolyte
C. increase in work-tool gap
D. decrease in temperature of the electrolyte
View Answer
Answer: Option B
Explanation:
The solubility of the products of the reaction increases with rising electrolyte temperature. Also, increase in temperature of the electrolyte speeds up the electrode reaction and reduces the over-voltages required.
Question 38
Spraying or dipping method is used for applying insulation.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Generally the simplest method of applying insulation is by spraying or dipping. Teflon, urethane, phenolic, epoxy, powder coating and other materials are commonly used for insulation. Sprayed or dipped coatings of epoxy resins are among the most effective insulating materials. For optimum effectiveness, these coatings should be used with a protective lip on the tool to protect the edge of the insulation from the flow force of the electrolyte.
Question 39
Striation, ripples on work surface occur because of ____
A. differential machining of material phases
B. incorrect tool alignment
C. cavitation
D. electrical field concentration of machining current
View Answer
Answer: Option A
Explanation:
Differential machining of material phases causes striation and ripples on the work surface which leads to poor surface finish. It can be minimised by increasing electrolyte supply pressure, reducing tool feed rate, or by reducing the voltage to maintain the same gap.
Question 40
The insulation in ECM should have_____
A. adhesion to the tool
B. roughness
C. high water absorption tendency
D. chemically active
View Answer
Answer: Option A
Explanation:
Desirable qualities of insulation are as follows-• Adhesion to the tool: preformed insulation can be held to the tool by shrink fitting, adhesives or fasteners• Sealing without pores or leaks that could cause stray machining by current leakage• Adequate thickness• Smoothness to avoid disturbing the flow of electrolyte• Resistance to heat for continuous service at 200°C without breakdown• Durability to resist wear in guides and fixtures• Chemical resistance to the electrolyte• High electrical resistivity• Uniform application to minimize disturbance of the flow of electrolyte and to prevent interference• Low water absorption.
Question 41
Pick the incorrect one from the following options.
A. Voltage across the cutting gap influences the current and the MRR
B. Higher voltage decreases the equilibrium machining gap
C. Increased current leads to electrolyte heating
D. ECM can be used for facing and turning complex 3D surfaces
View Answer
Answer: Option B
Explanation:
Low voltage decreases the equilibrium machining gap and results in a better surface finish and finer tolerance control. Voltage across the cutting gap influences the current and the MRR and is controlled in most ECM operations. Increased current leads to electrolyte heating – low temperature of the electrolyte is conducive for a better surface finish and tolerances.
Question 42
The tool in ECM should have which of the following properties?
A. High machinability
B. Lower corrosion rate
C. High softness
D. High conductive resistance
View Answer
Answer: Option A
Explanation:
Tool material should have the following properties:• High electrical and thermal conductivity• Good stiffness• High machinability- particularly important if complex shaped tools are required• High corrosion resistance- to protect itself from corrosive electrolytes• High rigidity.
Question 43
The velocity and the electrolyte flow through the gap is also an important parameter affecting the surface finish and MRR.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
If the velocity is too low, the heat and by-products of the reaction build in the gap causing non-uniform material removal. A velocity that is too high will cause cavitation, also uneven material removal.
Question 44
There is a huge change in the mechanical properties of the material after ECM.
A. TRUE
B. FALSE
View Answer
Answer: Option B
Explanation:
There is no appreciable change in the mechanical properties, such as tensile strength, yield strength, hardness, ductility, etc. of the material due to ECM. Values of notch-tensile strength, notched-sensitivity, and the sustained-load characteristics of the ECM parts are comparable with those of conventionally machined parts.
Question 45
There is a limit to the minimum cross-section of the current carrying parts.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
To prevent over-heating, there is a limit to the minimum cross-section of the current carrying parts. For 1000 A, it is about 6 cm2 for copper, 25 cm2 bronze and brass, and 250 cm2 for stainless steel.
Question 46
In order to obtain good results in ECM, _____
A. maximise polarization
B. allow rise in temperature of the electrolyte
C. removal of used electrolyte form the working gap
D. decrease the concentration of the electrolyte
View Answer
Answer: Option C
Explanation:
In order to obtain good results, it is essential toi. remove the contaminated electrolyte from the working gap,ii. minimize polarization,iii. limit the rise in temperature of the electrolyte when working at higher current densities.
Question 47
Tool used in ECM should have higher rigidity.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
In ECM, rigidity of the tool construction and material is important because the high pressure can cause deflection of the tool. Therefore, materials like copper-tungsten, titanium, cupro-nickel, stainless steel, etc. are used for making tools.
Question 48
Tool with an electrolyte supply slot leaves small ridge on the work.
A. TRUE
B. FALSE
View Answer
Answer: Option A
Explanation:
Tool with an electrolyte supply slot is simple to manufacture, but such a slot leaves small ridges on the work. However, the ridges can be made very small by making the slot sufficiently narrow. The flow from a slot takes place in a direction perpendicular to the slot and the flow at the end is poor – therefore the slot is terminated near the corners of the workpiece surface.
Question 49
What does faraday’s second law electrolysis state?
A. W∝Q, where W= mass of substance deposited and Q= charge passed through the electrolyte
B. w1/w2 = E1/E2, where w1 and w2 = mass of different substances deposited, and E1 and E2 = their equivalent masses
C. Induced EMF = – dΦ / dt
D. Force between two charge particles q1 and q2, F= kq1q2/r2
View Answer
Answer: Option B
Explanation:
Faraday’s second law of electrolysis states that, when the same quantity of electricity is passed through several electrolytes, the mass of the substances deposited are proportional to their respective chemical equivalent or equivalent weight.
Question 50
What is faraday’s first law of electrolysis?
A. W∝T, where W= mass of substance deposited and T= temperature of the electrolyte
B. W∝K, where W= mass of substance deposited and K= ionisation constant of the electrolyte
C. W∝Q, where W= mass of substance deposited and Q= charge passed through the electrolyte
D. W∝ 1/Q, where W= mass of substance deposited and Q= charge passed through the electrolyte
View Answer
Answer: Option C
Explanation:
Faraday’s First Law of Electrolysis states that, the chemical deposition due to flow of current through an electrolyte is directly proportional to the quantity of electricity (coulombs) passed through it. i.e., mass of chemical deposition.
Question 51
Which of the following conditions are desirable for ECM?
A. High value DC and low value electric potential
B. Low value DC and high value electric potential
C. High value DC and high value electric potential
D. Low value DC and low value electric potential
View Answer
Answer: Option A
Explanation:
In ECM, high value DC (may be as high as 40,000A) and a low value of electric potential (in the range of 5-25V) across the IEG is desirable.
Question 52
Which of the following does not hold true about ECM?
A. ECM cell must avoid flutter and arcing
B. Part and the cathode must have adequate current-carrying capacity
C. Tool must have shape exactly same as that of the mirror image of the part
D. It gives burr free surface
View Answer
Answer: Option C
Explanation:
Tool dimensions must be slightly different from the nominal mirror dimensions of the completed part to allow for ECM overcut. ECM cell must have strength and rigidity to avoid flutter and arcing.
Question 53
Which of the following does not hold true about ECM?
A. Lower current density leads to poor surface finish
B. Small gap between tool and the workpiece can cause short circuit
C. Gap between the tool and the workpiece doesn’t affect the process parameters
D. MRR is dependent on feed rate and electrolyte composition
View Answer
Answer: Option C
Explanation:
Current density is not only controlled by the amount of current that the power supply is delivering, but also by the size of the gap. A small gap results in the highest current density. However, when it’s very small, there is a danger of sludge particles bridging the gap and causing a short circuit. When the gap is too large, current density is reduced, resulting in a poor surface finish and decreased MRR.
Question 54
Which of the following holds true about electro-chemical machining?
A. Material is removed from the cathode and deposited on the anode
B. Material is removed from the anode and carried away by the electrolyte
C. Major drawback is that the finished product has residual stresses
D. It can also be used for all non-metals
View Answer
Answer: Option A
Explanation:
In ECM, material is depleted from anode workpiece (positive pole) and transported to a cathode tool (negative pole) in an electrolyte bath, but instead of depositing it on cathode, it is carried away by the electrolyte which flows rapidly between two poles.
Question 55
Which of the following is a sludging type electrolyte?
A. NaOH solution
B. KOH solution
C. NaCl solution
D. CuSO4 solution
View Answer
Answer: Option C
Explanation:
There are two types of electrolytes-(1) Sludging type: These are solutions of salts which forms precipitate (sludge) after reaction with metals. For example: solutions of typical salts like NaCl.(2) Non-sludging type: These are solutions of salts which do not form heavy precipitate or form soluble compounds after reacting with metals. For example: strong alkali solutions.CuSO4 does not form any solution.
Question 56
Which of the following is not a desirable electrolyte property?
A. Low viscosity
B. High specific heat
C. Lower resistance to film formation on the workpiece
D. Non-corrosiveness
View Answer
Answer: Option C
Explanation:
Desirable electrolyte properties are as follows-• High electrical conductivity – ease in ionisation• Low viscosity – for easy flow• High specific heat – to carry more heat• Chemical stability – to be chemically neutral or does not disintegrate during the reaction• High resistance to formation of passivating film on the workpiece surface• Non corrosiveness and non-toxicity• Inexpensiveness and easy availability.
Question 57
Which of the following is suitable if work surface finish is important?
A. High machining voltage
B. High concentration
C. Larger gap
D. High current
View Answer
Answer: Option A
Explanation:
If work surface finish is important, high machining voltage, dilute electrolyte and a small gap between the workpiece and the tool should be employed instead of low voltage and concentrated electrolyte.
Question 58
Which of the following is true about ECM process?
A. Unable to machine high strength materials
B. Excessive tool wear
C. It gives burr free surface
D. Surface hardness of the workpiece gets reduced
View Answer
Answer: Option C
Explanation:
Following are the advantages of ECM process;• Can be used to machine complex cavities in high strength material• Applications in aerospace industry, jet engines parts and nozzles• ECM process gives a burr free surface• No thermal damage• Lack of tool forces prevents distortion of the part• No tool wear.
Question 59
Which of the following largely affects the MRR?
A. ECM cell size
B. Tool shape
C. Feed rate
D. Complexity of the product
View Answer
Answer: Option C
Explanation:
Feed rate determines the current passed between the tool and the work. High feed rate results in higher MRR. High feed rates also decrease the equilibrium machining gap resulting in improvements of the surface finish and tolerance control. At slower feed rates, the MRR decreases as the gap increases resulting in the rise of resistance and drop in the current.
Question 60
Which of the following manufacturing process is favourable for making tool for ECM?
A. Casting
B. Cold forging
C. Laser cutting
D. Shaping
View Answer
Answer: Option B
Explanation:
Shaping the tool is usually not a problem since all the materials are easy to machine. Cold forging and electroforming are favourable methods of tool making.
Question 61
Which of the following materials can be used for making work holding devices?
A. Steel
B. Rubber
C. Graphite fibres
D. Wood
View Answer
Answer: Option C
Explanation:
Work holding devices are made of electrically non-conductive materials having good thermal stability, and low moisture absorption properties. For example, graphite fibres-reinforced plastics, plastics, perspex, etc.