Archive for the ‘Knowledges’ Category

Ingress Protection (IP) rating

December 24, 2008
International standard IEC 60529 classifies the level of protection that electrical appliances provide against the intrusion of solid objects or dust, accidental contact, and water. The resulting ingress protection rating is identified by a code that consists of the letters IP followed by two digits and an optional letter. The digits (‘characteristic numerals’) indicate conformity with the conditions summarized in the tables below. Where there is no protection rating with regard to one of the criteria, the digit is replaced with the letter X.

For example, an electrical socket rated IP22 is protected against insertion of fingers and will not be damaged or become unsafe when exposed to vertically or nearly vertically dripping water. IP22 or IP2X are typical minimum requirements for the design of electrical accessories for in-door use.
One source reports that the Australian national standard AS 1939 adds to the international standard a third optional digit, which indicates protection against mechanical impact damage. It ranges from 0 for no protection to 9 for protection against 20 joule impacts (equivalent to 5 kg dropped from 40 cm).

First digit
The first digit indicates the level of protection that the enclosure provides against access to hazardous parts (e.g., electrical conductors, moving parts) and the ingress of solid foreign objects.

Second digit
Protection of the equipment inside the enclosure against harmful ingress of water.


Additional letters
The standard defines additional letters that can be appended to classify only the level of protection against access to hazardous parts by persons:

Further letters can be appended to provide additional information related to the protection of the device:

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Harmonic Distortion Accelerates Fuse Aging Failures

December 24, 2008
Often, in the wake of unexpected shutdowns due to costly equipment failures, superficial investigations result in plant engineers assigning fault to mechanical components. In many cases, however, thorough engineering analysis can delve deeper to reveal the causes for malfunctions and prescribe long-term, cost-effective solutions. Such was the case recently at an automotive assembly plant.

Premature failures of pulse-width modified (PWM) variable frequency drives (VFDs) serving supply and exhaust fan motors caused costly interruptions to the plant’s automated paint process. When the company started to investigate these intermittent drive shutdowns in the paint house, 70 VFDs ranging in capacity from 30-400 hp were in service at multiple 2000 kVA low-voltage substations.
Throughout the preceding year, the plant had reported multiple shutdowns of the paint house VFDs with each event costing approximately $300,000 in downtime and lost production. In each incident, maintenance technicians had performed field diagnostic procedures to determine what initiated the shutdowns. The technicians reported finding one or two blown fuses at the affected drive. They also tested and condemned one or more gate turn-off thyristors (GTOs) on the voltage-source inverters. The condemned GTOs and fuses were replaced and the drives were successfully restarted without further incident. Each event involved a different VFD and none of the drives had experienced more than one shutdown event. With plant maintenance technicians blaming the shutdowns on component failures within the PWM drives, they removed
the manufacturer from the plant’s acceptable bidders’ list.

Harmonic problems identified
Subsequent engineering analysis, however, proved that the shutdowns were due to accelerated aging and premature failure of PWM drive fuses. The fuse failures were linked to high voltage distortion levels on lowvoltage busses that resulted in erratic drive operation.
A Square D Engineering Services team, led by Blane Leuschner, P.E., identified harmonic problems through Harmonic Distortion Accelerates Fuse Aging Failures onsite measurements and harmonic modeling using the Alternative Transients Program (ATP). ATP is a shareware program developed in Canada and similar to one developed by the Electric Power Research Institute. Using ATP, the team resolved the problem by combining additional line impedance with the application of harmonic canceling techniques.
A Square D Powerlogic monitoring system comprising approximately 500 devices was already in place at the plant. That system was supplemented by temporary measurements with portable test equipment. Permanent electronic meters were located at main breakers in the plant’s 480 V system. In addition to measuring about 200 power system parameters, these meters were capable of capturing simultaneous waveform data on each phase of voltage and current, with sample-rate resolution high enough to detect suspected power quality problems. In addition, portable versions of the same meters were installed temporarily at the input terminals of several drives.
Onsite testing showed that harmonic distortion at the low-voltage bus increased with increasing numbers of VFDs in operation, as expected. “Measurements showed that voltage distortion at the main buses approached 10 percent when normal levels of drives were operated,” Leuschner said. “While this level of distortion exceeds the 5 percent total harmonic distortion (THD) level usually applied to low-voltage buses, it is below the THD level at which VFD problems are usually encountered.” Further onsite testing showed that VFDs currents measured at the drives were erratic and did not resemble the expected 2-pulses-per-half-cycle signature characteristic of PWM drives. The erratic current signature was unusual for a PWM signature and signaled an anomaly.
“The anomaly was high frequency harmonics, typically around 960 Hz, in the line currents,” Leuschner said.
“Analysis of computer simulations showed that the noncharacteristic current harmonics resulted from low circuit inductance and high voltage distortion due to the operation of VFDs on each bus.” The high voltage distortion resulted in severe flat topping of line-line voltage, which limited the ability of the dc bus capacitors to charge. Low circuit inductance compounded the problem by permitting a high-rate-of-change in the VFD line currents.
Harmonics modeling and analysis
In order to analyze the system mechanisms at work in producing the unusual current distortion, the engineering team created an ATP computer model of the VFDs and a low-voltage power system. The simulation included a complex PWM model consisting of about 1000 circuit elements. Actual measurements provided the basis by which the team verified the computer model in order to ensure accuracy of the simulations and effectiveness of the solution alternatives.
“Initially, we based our engineering analysis on the reports by plant technicians that GTOs had failed during each shutdown event,” Leuschner said. “The earlier conclusion reached by the plant was inconsistent with later onsite measurements and computer simulations. Those measurements and simulations didn’t expose a power system event anywhere close to damage levels on the GTOs in the inverter section.”
In addition, the drive engineers determined that drive design prevented GTO failure from drawing enough current to blow an ac fuse. The engineers asserted that the GTO location in the drive was not getting enough available fault current due to upstream circuit components such as diodes, dc bus, etc. None of the empirical evidence could explain why GTOs would fail unexpectedly.
Consequently, the team turned its attention to the GTOs themselves. They set out to determine how the GTOs had been damaged and if the damage was corroborated by laboratory testing under more controlled conditions than existed in the field under emergency circumstances. “We found out that none of the condemned GTOs had been re-tested under laboratory conditions,” Leuschner said. “Neither were the GTOs subjected to forensic investigation that might have revealed their mode of failure.”
He recommended that such additional testing be done. Why, the plant argued, would additional testing be performed, when the GTO/fuse replacements had apparently fixed the problem and returned the affected drive to service? The team replied that the replacements were only a stopgap measure and that, without testing to determine a long-term solution, degradation and premature fuse failure would continue.
Laboratory testing
The plant subsequently authorized more testing, which located only five failed GTOs. Tests were performed under laboratory conditions by the plant and by Square D to determine the mode of failure. Test results showed that none of the five GTOs was damaged, thus confirming Leuschner’s analysis and proving that the GTOs had been condemned by mistake.
The engineering analysis then turned to the drive fuses. These fuses had indeed opened during the shutdown events—there was no question about that conclusion. With the GTO failure theory freshly debunked, plant engineers and Square D investigators wondered if the drive fuses might have been damaged during prolonged operation under erratic harmonic currents.
“Our team searched published articles on the subject in search of a precedent, but found none,” Leuschner said. “Yet, evidence suggested that the substantial high frequency content and dramatic fluctuation in peak current magnitude of the drive currents subjected the fuse elements to abnormal stresses and resulted in accelerated aging.” That theory was supported when Square D measured other circuits in the plant and found that no fuse failures had occurred on a circuit where drive operation was stable. Investigators used two 400 hp drives already equipped with line reactors—which constituted the majority of the circuit load—to confirm stability.
Solution options
Following detailed computer simulations of the VFDs and power system, the team issued its prescription. The report indicated a combination of line reactors and phase-shifting isolation transformers would provide the most cost-effective solution to the problem.
Other typical solutions, such as harmonic filters and hybrid filters at the main 480 V buses, were considered and eventually discarded. “The harmonic filter option, a typical solution of choice in such a situation, encountered a common dilemma when harmonic filters are being considered for transformers with a high percentage of VFDs,” Leuschner said. He explained that conventional shunt filters contain capacitance that increases displacement power factor on the applied bus. VFDs, however, typically operate at high displacement power factor, while producing high levels of harmonic current. Many VFDs on a bus means high levels of capacitance that can result in leading displacement power factor and overvoltage.
Further, ATP modeling revealed that harmonic filters would not resolve the erratic drive current phenomenon. Low system inductance was the major factor contributing to that erratic drive current, yet harmonic filters would appreciably change system inductance. Low inductance allowed the dc filter capacitors inside the VFD to charge erratically, resulting in the noncharacteristic ac current.
Simulation identified the optimum simulated-voltage and current distortion reduction and proved that the best technical solution was to increase system inductance seen by the VFDs. While line reactors alone could provide this inductance, the investigators also modeled delta-wye transformers to assess additional benefits.
“We knew that even though harmonic current passes through line reactors and wye-wye or delta-delta transformers without appreciable phase shifting, delta-wye transformers have a different effect,” Leuschner said. “Fifth and seventh harmonic components, which comprise a significant portion of VFD currents, are phase shifted by 30 deg of the fundamental by delta-wye transformers.”
The resulting phase-shift of these two dominant harmonic components comprises currents that are 180 deg out of phase with fifth and seventh harmonics from nonphase-shifted drives. The combination of line reactors and delta-wye transformers contributed to significant cancellation of the aggregate fifth and seventh harmonic current contribution of all the drives.
Of the 66 drives in the paint house not already equipped with inductive isolation—four 400 hp drives had line reactors—only seven were equipped with delta-wye isolation transformers, while 50 received line reactors.
Delta-wye transformers were reserved for VFDs with ratings of 200 hp and above, while 100 and 125 hp drives were provided with open-style reactors in the existing drive enclosure. Inductive isolation was not required on drives under 100 hp. Fuses that had not failed and had been replaced were changed due to suspected deterioration.
Results and conclusions
During its Christmastime shutdown, the plant implemented the recommendations for line reactors and deltawye transformers. All equipment was installed and operating within a month. Bad, erratic voltage and waveforms were corrected into clean sinusoidal voltage and double-hump current waveforms. Harmonic voltage distortion was reduced to less than 5 percent. Drive currents returned to their normal signature.
“Our recommendations also identified the need for improved training of plant maintenance technicians,” Leuschner said. “Much of the early confusion about the cause of the shutdowns could have been avoided by more accurate assessment during in situ testing of the GTOs.” While the GTOs are difficult to test in situ, the plant implemented procedures to improve this testing, and to require that any electronic devices suspected of damage would be subjected to laboratory testing.
No further fuse failures have occurred since the modifications were completed, and the drivers’ manufacturer was returned to the plant’s acceptable bidders’ list.
Information supplied by Square D/Schneider Electric, Palatine, IL; telephone (800) 392-8781
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Overview Of Solutions for Static Electricity (3)

December 23, 2008
Operator/Personnel Shocks
This problem can occur in two ways and can be resolved in two ways.

1) Personnel can receive shocks from extremely highly charged material as it discharges through them to earth.
2) Operators can themselves become charged by handling or being in close proximity to highly charged material. When the operator is presented with a path to earth , usually metal objects or a machine, they discharge violently often believing they have had a mains shock.

The problem can be addressed by either neutralising the material with bars or blowers
or by continuously neutralising the operator with long range Pulsed DC bars. In extreme
cases both methods can be combined.

Electrostatic Discharge (ESD)
The electronics industry predominantly addresses this problem by the use of conductive or static dissipative materials in the work place and in packaging.
All of these materials are earth bonded to the same electrical potential thus eliminating the possibility of potential differences and subsequent discharges. However there are many areas in electronics where such materials cannot be used practically.

Clean Rooms The materials can cause contamination. Also the laminar air flows can generate static.
Powering-Up Testing PCB’s and components may short out and present an electric shock hazard.
Auto Assembly The movement and separation within the equipment generates its own static charges.
Product Assembly Where PCB’s are assembled into equipment made from ordinary static generative materials.
In all of these situations ionisation is recognised as a solution and sometimes as a
supplement to traditional methods.
Source: Meech Static Eliminators Ltd.

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Overview Of Solutions for Static Electricity (2)

December 23, 2008

Material Misbehaviour

A) Webs
Most commonly solved using AC ionising bars.

Bars can be positioned either side of a web to achieve the best results, usually at the point where the problem occurs, or just prior to this position. An example would be where a web exits a roller or at the exit rollers of a flat bag making machine.

B) Sheets
Two major problems exist with sheets – they can repel or stick together when collating. On sheet feeders, static attraction causes multiple sheet pick-up rather than the single sheets required.

C) Component Sticking And Repelling
On conveyors or bowl feeders, small plastic components can stick to the belt or inside the track of the bowl feeder: this leads to down time and ineffective production flow. Where components repel each other they can jump off or out of the delivery system. Deploying ionising bars , long range preferably (AC bars with air assist or Pulsed DC) can resolve this problem. Where larger components are concerned, repelling can cause components to fall over whilst in process transport e.g. blow moulded bottles on conveyors. The parrisons on blow moulding machines also can repel each other causing quality problems.

Overview Of Solutions for Static Electricity (1)

December 23, 2008
Electrostatic attraction (ESA)
A variety of equipment is available to resolve this problem in many different ways. Some examples follow :

A) Injection Moulding – Neutralisation
Bathe components in ionised air. Often using AC Ionised air blowers or Pulsed DC bars. This method will not remove attracted particles but will help prevent charging and thus prevent attraction of dust.

B) Injection Moulding – Particle Removal
Where particles have already been attracted they can be removed effectively by the use of high velocity ionised air. Note: The use of ordinary compressed air will often fail to remove dust. It can also worsen the problem by generating further static charges on the material.
C) Webs
Single or doubled sided web cleaners could be deployed in this situation or high velocity blow off systems, air curtains etc.,
D) Silk Screen Printing – Long Range
Long range Pulsed DC Bars can be used to eliminate charged screens without causing drying. Ionising guns or air curtains can remove attracted particles from substrates pre printing.
E) Semi Conductors – Long Range
Pulsed DC can be used for this situation – stand alone or using air assist from HEPA filters.

Addressing Common Electrostatic Problems

December 23, 2008
Identification of the problem
There are four main troublesome difficulties arising in industry, deriving from electrostatic charges, with a fifth affecting only the electronics industry but with very serious and costly consequences.

Electrostatic attraction (ESA)
Airborne particles are attracted to charged surfaces or indeed charged airborne particles are attracted to a surface which could be totally free of any charge. This problem effects most plastic based industries in one form or another, spoiling finishes of painted products and causing rejects in quality in the food, pharmaceutical and medical industries. In the printing industry dust attraction damages print finishes or indeed printing plates. The film industry also suffers with low quality prints and poor resolution projections in the cinemas. The microscopic nature of semiconductor manufacture can be effected by this problem.

Material Misbehaviour
This is another form of ESA. However, instead of the contamination of products, the problem manifests itself in the form of the product itself, usually webs, fibres or sheets, sticking to themselves or equipment, misrouting or repelling. Automated processes are particularly prone to this problem.
Operator/Personnel Shocks
This is becoming increasingly significant as companies look to improved safety standards. Whilst shocks can be painful the effects are usually quite safe and short lived. However, in extreme cases, the debilitating effects can cause personnel collision or entrapment with associated machinery or can even initiate a fire or an explosion in hazardous areas.
Electrostatic Discharge (ESD)
This problem is associated to electronics assembly, installation and field service and also electronic component manufacture.
Voltages as low as 5 volts which have no real meaning in other industries, can cause catastrophic failure of electronic components or much worse, latent damage which results in field failure, by far the most costly in terms of repair and manufacturers’ reputation.

Other Aspects of Static Electricity

December 23, 2008
Measurement
In order to select the correct equipment for a given application it is important to know the magnitude, and in some instances, the polarity of the charge to be neutralised. In cases where AC eliminators are to be used it is important to know the magnitude of the static charge so that the correct eliminator bar can be selected. In cases where Pulsed DC equipment is to be used, both the magnitude and the polarity of the static charge are important to allow the power and bias of the output to be set correctly. Whilst it is quite difficult to measure the actual amount of surface charge the surface voltage can be measured by looking at the electric field it generates.

High Accuracy Balanced Neutralisation
In the majority of applications the reduction of a static charge to a few hundred volts is normally sufficient to eliminate dust attraction and misbehaviour of the product. However in the electronics industry static charges of a few hundred volts can have disastrous consequences with modern micro-processor chips. In these instances it is important to ensure that the ionisation delivered by the neutralisation system is balanced.

Static Generation
As well as neutralising static it is also possible to generate static charges in materials by the use of high voltages. By passing materials through intense electric fields it is possible to charge materials either positively or negatively. This charge can be of assistance in a manufacturing process. A common usage of static generation is in high frequency welding applications where induced static charges are used to temporarily bond one item to another to hold them in place during the welding process.

Modes Of Operation Of Static Eliminators

December 23, 2008
Passive Eliminators
A charged object will generate an electric field between itself and any surrounding earthed object (or indeed any object of differing voltage).

In the case of passive eliminator the field is between the surface and the tips of the Carbon Fibre or Stainless Steel earthed brush. See Diagram 1. The fine point at the end of the individual bristles causes the electric field to be highly concentrated at this point. When the strength of this electric field reaches a sufficient value, ionisation of the air molecules surrounding the tip occurs.
In the example in Diagram 1 the positive charge on the surface of the material will cause electrons from the tip of the brush to jump to surrounding air molecules which will then have a net negative charge and are thus negative ions.


Diagram 1

From the simple rule that opposite charges attract, the negative ion will be drawn to the positively charged surface of the material. When the ion reaches the surface the extra electron the molecule is carrying is drawn from the molecule and delivered to the surface of the material. When this process has occurred in sufficient quantity the positive charge on the surface will be reduced. In this process there will come a time when the electric field generated between the surface and the tip of the brush is no longer sufficient to cause ionisation of the air and no further neutralisation takes place. In the instance of a negatively charged surface the opposite procedure takes place.
Passive eliminators are thus useful for reducing high levels of static charge, tens of kV’s down to levels of a few kV’s. However by their very nature they are not able to completely neutralise the surface charge.
Radioactive Eliminators
Radioactive eliminators employ polonium 210 or other low level radioactive source. In the process of radioactive decay alpha particles are emitted to the surrounding atmosphere. These high speed particles collide with the air molecules and in doing so cause the air to become ionised. This ionised air then neutralises nearby surfaces in similar fashion to the passive eliminators.
AC Eliminators
AC Eliminators operate at supply frequency. The mains voltage, 110 or 240 etc. is greatly increased using a ferro-resonating transformer to generate voltages of between 4.5 and 7 kV. This high voltage is fed to the ionising pins, whilst the casing of the bar is connected to earth. See Diagram 2.

Diagram 2

If we look at the positive cycle of the input waveform we will see that the electrode pin will be at a positive voltage compared to the casing. This generates a strong electric field between the two which is highly concentrated at the sharp point of the electrode pin. This in similar fashion to the passive bar generates positive ions at the pin point. These molecules are then repelled from the pin due to their like charge. On the negative half of the cycle the opposite occurs and negative ions are created at the pin point. These again are repelled from the electrode pin due to their like voltage. Thus around the ionising pin a cloud of positive and negative ions is produced. In the absence of outside influences the positive and negative ions are attracted to each other or to a nearby earth, such as the casing of the bar or nozzle, and would either neutralise each other or be dissipated to earth. However with the presence of a nearby static charge an ion will be attracted to on opposite charge on the surface of the material. At the surface of the material the electrons will be exchanged and the surface will be neutralised.
As the ionisation at the bar is not dependent upon the surface charge and ions are produced regardless of the proximity of a surface charge, complete neutralisation of a surface can be achieved. This is a significant advantage over the passive eliminators. The speed at which charges can be neutralised is dependent upon the rate of ion production and speed of repulsion of the ions from the emitter pins, which in turn is dependent on the voltage at the pin.
Pulsed DC Eliminators
Pulsed DC Eliminators like their AC counterparts produce ionised air by using high voltage. Whereas the AC units operate at supply frequency the Pulsed DC units operate at lower frequencies, between 0.5-20 Hz. The ionising bar consists of a series of emitters connected alternately to the negative and positive outputs. See diagram 3. The casing of the bar is made of plastic and hence there is no proximity earth. The output from the power supply is effectively a square wave switching from negative to positive at the chosen frequency.

Diagram 3

Looking at the positive half of the wave form the controller switches on the high output voltage connected to the positive emitters. This then sets up an electric field between the emitter and the surrounding earthed objects. At the sharp point of the emitter this field is extremely strong, and in similar fashion to the AC eliminators, positive ions are produced. The similar charge of the ion and the emitter drives the ions away from the bar.
On the negative half of the cycle the power supply delivers a high negative voltage to the alternate set of emitters. Again in similar fashion to the AC eliminators negative ions are produced at the emitter point.
A statically charged object in the vicinity of the ionising bar will attract or repel the ions, dependent upon their relative polarities. When the ions reach the statically charged surface, neutralisation takes place in a similar manner as described in the AC eliminators section.
The low frequency of operation lends Pulsed DC equipment to long range neutralisation. The relatively long duration of each half of the cycle cause large “clouds” of ions of alternating polarity to be emitted from the bar. This distance between the positive and negative ions close to the bar greatly reduces the rate of re-combination, (positive and negative ions coming together and cancelling each other out).
At long distances from the bar, less ions are deliverable to a statically charged surface and hence the speed of neutralisation is reduced. Hence when utilising Pulsed DC equipment on dynamic applications, such as moving webs, thought must be given to distance at which the bar will be mounted from the target surface.
An additional feature of the Pulsed DC system is that the output wave form can be altered and the duration of the negative and positive section of the wave form can be increased or decreased. For instance if the charge to be neutralised is known to be positive the duration of the negative part of the output can be increased and conversely the positive part of the wave form reduced.
This will increase the production of negative ions and decrease the production of positive ions, making the system more efficient at neutralising the positive charge. In similar fashion for a known negative charge the output can be biased towards positive ion production.

Methods of Elimination Static Electricity

December 23, 2008
The fundamental principle for neutralisation of static charges is the same whatever the technique used. Where a material has a positive surface charge electrons must be delivered to the surface to bring the charge back into balance. Where the surface charge is negative the excess electrons must be removed from the surface to neutralise the charge.

The delivery or removal of electrons can be done by one of the three following methods, either
1) Movement of electrons through the material itself
2) Movement of electrons through another material in contact with the surface
3) Movement of electrons through ionisation of the surrounding air

There is in actual fact a fourth method, sparking, which occurs when the surface voltage is sufficiently high to cause the air to become conductive. However the occurrence of sparking is normally due to the lack of application of other methods!
Humidity
As previously noted moisture on (or within) a material will tend to leach away static charges down to earth. For example paper generally has a relatively high moisture content and does not maintain particularly high levels of static. However if the paper is particularly dry static can become a severe problem.
Passive Ionisation
The close proximity of a conductor to a charged object will tend to discharge it. For example Meech Model 974 Carbon Fibre Brushes will reduce static charges in materials passed in close proximity to the brush.
Radioactive Ionisation
Radioactive sources such as polonium cause ionisation of the surrounding air which will neutralise surface static charges. Meech do not supply Radioactive Eliminators. A drawback of radioactive eliminators is the fact that they are only available on annual leases. The radioactive source loses its effectiveness over time and requires replacement on an annual basis.
Active Electrical Ionisation
By using high voltage AC or DC, ionised air can be produced which can then be used to neutralise surface charges. The use of AC or DC systems is application dependent.

What Factors Affect Static Electricity

December 23, 2008
Among the many factors that affect the generation and maintenance of a static charge are humidity, the type of material, repetition and change in temperature.

Type Of Material
Some materials are more readily charged than others. For example materials such as acetate will gain a charge very readily whilst glass will gain a charge less readily. Also the relative position of materials on the Triboelectric Series will determine whether a material charges positively or negatively dependent on the other material with which it has come into contact. For example hard rubber, when rubbed against nylon, will become negatively charged whilst when it is rubbed against polythene will become positively charged.

Humidity
Generally speaking the dryer the environment, the higher the level of static charge and conversely the higher the humidity, the lower the static charge. In relative terms water is a significantly better conductor of electricity than most plastics. Atmospheric humidity deposits small quantities of water on all surfaces in their environment and hence surface static charges on materials have a tendency to dissipate to earth by current flow through the surface moisture.
Repetition
Repeated actions such as friction or separation will increase the level of charge found on a material. For example a plastic web moving over a series of Teflon rollers will increase its surface charge after every roller.
Battery Effect
The combination of many charged items can lead to extremely high charges. For instance individual sheets of plastic with relatively low surface charges when stacked together can generate extremely high voltages.
Change In Temperature
As a material cools down it has a tendency to generate charge. The action of the cooling is to leave a net charge on the material throughout its entire volume. If the material is a very good insulator the internal (volumetric) static charge can be maintained for extremely long periods of time. However over time this charge normally migrates to the surface at which point it becomes a surface static charge. An example of this is an injection moulding which is seemingly neutral when hot but can subsequently be found to have a large surface charge once cool.