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Antony Anderson : copyright  Will F. Anderson 2001

I am listed on Chartered Consultant

Dr Antony Anderson C.Eng  FIEE/FIET

Electrical Fault & Failure Investigations

meandering interlaminar breakdown
enlarged picture
Paper on interlaminar breakdowns
in large electrical machine

armature fault large DC motor
motor armature failure

Examples of electrical fault and other  investigations carried out recently by Electrical Consultant Antony Anderson :

Winding failures on High Speed Magnetic Bearings [UK] The control systems for magnetic bearings have to act very fast. As a consequence, the bearing position control windings are repeatedly subject to rapid rates of rise of voltage, which, in turn tend, to cause partial discharges between turns in different winding layers, particularly if the windings are poorly wound and are not properly impregnated.  The cure in this case was detailed attention to uniformly rewinding the coils, the use of sufficient inter-layer insulation and thorough vacuum impregnation, so as to minimise the risk of localised high voltage stresses.

In-service stator winding failures on deep well pump induction motors [Libya]: Water was circulated round the pump motor by an elementary centrifugal pump - with water being drawn in along the axis of the rotor and pumped out towards one stator end winding by centrifugal action through  four holes placed around the rotor shaft. The spray action, rather similar to that of a garden water sprinkler, subjected the end windings to pulsating forces which in turn caused the end winding turns to vibrate. Because the windings were insufficiently restrained, both in the end winding and in the slots, the amplitude of winding vibration in the slots was high. Fretting against the stator core wore through the winding insulation, eventually causing winding shorts to ground. The failures occurred not in the end winding, but in the stator slots, some distance from the core end. Recommendations included stiffening up the stator end winding supports and protecting the winding from direct water spray impact by means of baffles.

Rotor winding failures on generators [EU and South America] the generators in question, built by different manufacturers,  were driven by reciprocating gas engines and were therefore subject to high levels of vibration. The rotor winding failures were progressive, with more and more turns short-circuiting over time. On examination windings showed multiple sites where micro-arcing had occurred. The damage was consistent with the initial inter-turn breakdown not resulting in a hard short circuit as might be expected normally but in an intermittent contact that could be made and broken by vibration. As a result of the making and breaking of current high transient inter-turn voltages would have been set up that could intiate further breakdowns. The remedy was to provide a higher degree of insulation on the rotor winding turns.

Unreliable hot drinks dispensing machine. [UK. France] The hot drinks dispensing machines worked satisfactorily in the UK, but when exported to France had a habit of tripping, as if on overload. The question was why.  It turned out that in the UK customers tended to want mainly coffee and tea dispensed, but in France  hot chocolate was more popular. The charge of chocolate was heavier than the charge of tea or coffee, with the result that when the machine was fully loaded with cups for dispensing hot chocolate, the carousel ran very slowly and the  timer ,  detecting  what it thought was a jammed cup, tripped out.  It turned out that some time previously the original motor - a substantial  single phase  ac induction motor suitably geared down - had been replaced by a much smaller geared  dc motor originally not designed for the purpose, but for use as a car seat adjuster.  Because the gear ratio was wrong for the new application, the motor had to be run significantly below its design voltage of 12V in order to get the carousel to run at the right speed. As a result, there was insufficient power to drive the fully loaded machine. The remedy was simple - replace the DC motor with the original AC motor. Why the original AC motor had been replaced by the DC motor in the first place was a bit of a mystery - The AC design was much simpler and had no drive electronics. It had more than adequate capacity to drive the heaviest load and never went wrong. The moral: if it ain't bust dont fix it.


Alleged electric shock [UK]from an electric lift call button. A hospital porter was wheeling a trolley down a corridor towards a lift so that he could take the trolley up to a higher floor. According to his story, he touched the lift button and received such a substantial shock that it threw him back through an open door into the laundry cupboard. There were no witnesses. Once in the laundry cupboard, after a while he recovered sufficiently to stagger out of the cupboard and call for help. He was rushed to A & E where he was medically examined, diagnosed as possibly suffering from post-traumatic shock and was kept in overnight for observation. He was later seen by medical experts who all agreed with the initial diagnosis. However, in the medical records there were no photographs and no observations of the physical signs of local burning that might be expected in the case of serious electric shock.  The Lift Maintenance Company was called in to examine the lift and found nothing wrong with it. In particular, the metal plate surrounding the lift button was found to be well earthed, as it should have been.  The lift was brought back into service without any modifications, none being required. In the records, there were several complaints of lift passengers getting minor electric shocks from time to time. The alleged victim of electric shock claimed a large sum of money from the hospital for post traumatic shock. A visit to the hospital was arranged for the electrical expert. It soon became apparent that by walking across the floor any individual became electrostatically charged and on touching either the lift button or the earthed surround resulted in them discharging themselves. The spark discharge was audible, just visible to the naked eye, and was detectable with a small radio receiver. It was estimated that the maximum electrostatic build up of charge - with the weather dry and the porter fully charging both himself and the trolley - was about one eightieth of that required to cause any kind of significant muscle contraction. In the opinion of the expert there was no way in which the electric charge discharged could possibly have caused sufficient current to flow to cause the claimed degree of muscular contractions that might have resulted in the claimant involuntarily ending up in the laundry cupboard. The case was settled on the basis of the expert's report without going to court.

Electrical Investigation of a Chicken Hatchery fire . The fire started in a room where the plastic trays used for the hatchlings were washed. The incident occurred early one morning just prior to the changeover from the night shift to the day shift. The initial report written by the insurance company's expert noted the presence of arc damage in the overhead wiring above a stack of fire-damaged trays but concluded that the arcing was the result of the fire not the cause of it.  The  expert's hypothesis was that someone had used a cigarette lighter to set fire to the plastic boxes and that these had set fire to the cable insulation and that the arcing in the overhead wiring was the result.  The only evidence cited was that the fire brigade had been able to set a stack of boxes on fire by holding a cigarette lighter to the side of one box for 40 to 50 seconds. An electrical expert was not called in at the time to give a second opinion on the significance, or otherwise of the arcing. Having fixed upon arson as the likely cause of the fire, the insurance expert then proceeded to identify the presumed arsonist, who was in due course arrested and charged. An independent electrical expert's report was commissioned by the Defence shortly before the trial which showed that the likeliest cause of the fire was a wiring fault, itself the result of poor maintenance, which caused arcing in the overhead ducting and a subsequent cable fire, which then spread to metal-clad polystyrene wall and ceiling panels and thence to the plastic trays. The presence of high levels of moisture, as a result of the use of high pressure sprays in the washroom, may have been an additional factor in initiating an electrical breakdown. As a result of the Defence electrical expert's report, the case was dismissed four working days before the trial.

Intermittent electronic malfunctions in automobiles [USA, UK, Sweden, France] - a number of investigations of  sudden uncommanded acceleration of vehicles in the USA, UK, Sweden and France. One case of suspected instability in a vehicle fitted with electronic stability control [UK].

Vehicle electrical fires - One battery fire [UK], one fire caused by a faulty cruise deactivation switch [USA], one fire where vibration fretted electrical insulation and caused a short circuit[UK].

Previous Investigations : Generator Core Failures

When Antony Anderson  was Group Leader of the Electromagnetics Group, R & D Laboratories NEI Parsons, he was responsible for managing an inter-disciplinary team that  investigated a number of generator core fault and core end heating problems. These investigations had to be carried out thoroughly but with minimum interference with repair programmes.  He also was responsible for coordinating the programme of instrumentation and measurement of flux, temperature, interlaminar voltage and eddy currents in generator stators. He therefore has considerable investigative experience to offer on problems associated with large electrical machines. For commercial reasons, only a small proportion of this work was ever published.  Further information related to generator core faults will be found under:


References on aspects of Generator Core Faults / Core Failures

Tavner, P.J., Anderson, A.F. :  Core faults in large generators  IEE Proceedings - Electric Power Applications -- November 2005 -- Volume 152, Issue 6, p. 1427-1439

Abstract. A core fault is a failure in the laminated core of a large electrical machine. Such faults are relatively rare but when they occur, the value of the machine and its importance mean that the investigation of the failure assumes a high priority. On many occasions the details of such failures assume major commercial significance, therefore failure investigations have, of necessity, to be handled in a confidential manner, touching as they do on the design, manufacture, operation and insurance of large electrical plant. There has therefore been no published literature on core faults. However, the scientific principles of the mechanisms at work have been studied in considerable detail and papers published on those principles in the international literature. This paper brings together that literature and those scientific principles, giving details of the underlying design, constructional and operational factors which affect these faults. New evidence is presented of the underlying factors which allow core faults to initiate and grow. The paper will allow engineers to analyse such faults, draw rational conclusions on the causes for each occasion and devise suitable repair/rebuild strategies applicable to that situation. [PDF copies of this paper can be purchased from the  IEE]  [In the event of any difficulties, contact Antony Anderson .]

Le Ny, R. Guile, A .E., Anderson, A. F. : Mechanism of "Meandering" Breakdown of Insulation in an Electrical Machine. Proceedings of First International Conference on Conduction and Breakdown in Solid Dielectrics. Université Paul Sabatier TOULOUSE -France July 4-8, 1983 Paper H9 p. 395-398
Meandering interlaminar breakdown
SEM photograph of the head of a meandering 
breakdown showing wavefront of transiently 
melted core varnish ahead of the meandering
Abstract: A Scanning Electron Microscope examination of the head of a meandering interlaminar breakdown suggests that a repetitive arcing process has been occurring between the laminations and that the cause may be a series of high voltage pulses caused by current chopping. 

Platt, R., Kerr, L.C., Anderson, A. F. :  Measuring Flux and Interlaminar Voltage in Turbine Generator End Regions. Int. Conf. Electrical Machines - Design and Applications IEE London 13-15 July 1982 p. 201-205 
Transient back of core leakage flux in a large turbogenerator
Back-of-core leakage flux
in large turbogenerator
Abstract: This paper describes the general approach to the measurement of flux and interlaminar voltages in turbogenerator end regions.Construction and locations of typical search coils are shown. Some selected results of measurements of interlaminar voltages in the end region are given which show typical values of 5v peak-to-peak at 50 Hz. The induced voltage in a back-of-core search coil at the core end is shown to rise sharply under short circuit conditions, whereas in a similar coil well away from the core end the voltage collapses. As far as is known, this paper records the first occasion on which attempts were made to measure transient back-of-core flux leakage phenomena on  a large electrical under sudden short circuit conditions.
Download PDF File of this paper

Anderson, A. F., Bedford, T., Craddock, A. F. Transient leakage flux in small universal motors. IEE PROC., Vol 128, Pt. B, No 5, September 1981 p. 254-254
Transient leakage flux measurements on a small universal motor
Back of core leakage flux 
in small universal motor
Abstract: This communication reports an investigation into the transient back-of-core flux that arises in a small universal motor during start-up. A substantial back-of-core leakage flux was  shown to exist under starting conditions (35 mT peak radial flux density on the direct axis and 14 mT on the quadrature axis) which was caused by excess field ampere turns saturating the core and forcing the flux into the back-of-core region. A standard portable electric drill demonstrated the back-of-core leakage phenomenon to a remarkable degree : attracting to itself under starting conditions a piece of thin sheet steel from a distance of about 20mm.
Comment : Whilst the conditions were admittedly very far from those existing in large AC machines, the experiment shows that transient leakage  flux cannot be ignored even in fractional HP motors. The transient leakage flux is quite sufficient to advance the movement of a quartz watch placed in close proximity to the drill casing by several seconds when the drill is switched on.
Download PDF File 550 kB

Anderson A. F., Steel, J.G, Reece, A. J. B., Carpenter C.J., Preston. T. W., Phemister, T.G., Smith, R., Hammond, P., Jackson, R. J., Tavner, P.J., Penman, J., Stoll, R.L., Lorch, H.O., Howe, D. : Contributions to 'Discussion on Interlamination voltages in large Turbine Generators and Influence of winding design on the axial flux in laminated-stator cores and Examination of flux distribution in segmented stator cores'  IEE PROC, Vol 127, Pt. C. No2 March 1980 p 114-115. 

Abstract : Wide-ranging discussion touching on interlaminar voltages and core failures in which substantial interlaminar voltages in the core end regions are agreed to exist. Spit marks on stator teeth under examination prove to have been micro spot welds requiring interlaminar voltages of at least the so-called melting voltage of iron of 0.6V to have been reached. Calculations by one manufacturer suggest interlaminar voltages in the end regions under steady state conditions of more than 1V. It is suggested that radial back-of-core fields and axial fields contribute approximately equally to interlaminar voltages in the end regions and that the interlaminar voltages will rise during pole slipping. Measured results of interlaminar voltage in the end half metre of a generator core give a mean interlaminar voltage of 500 mV, whereas towards the end of the machine individual interlaminar voltages as high as 4000 mV have been measured. 

Anderson, A. F., Guile, A. E. : An unusual type of interlaminar breakdown found in an electrical machine. Sixth International Conference on 'Gas Discharges and their Applications' Heriot-Watt University: 8-11 September 1980 
meandering interlaminar breakdown
Meandering breakdown
spot weld between generator laminations
Spot weld  
interlaminar  breakdown
Abstract: A specatacular example of a 'meandering breakdown' between laminations comprising fine filaments of melted steel is described and is contrasted with a much more typical example of an interlaminar breakdown that resulted in a circular spot weld. Metallurgical examination of the filaments showed that there had been very rapid cooling at perhaps 1000 degrees C per second between 1200 degrees C and 200 degrees C. Further evidence of the fast transient nature of the heating and cooling is that varnish can be seen to lie almost undamaged closely alongside these filaments. The authors conclude that the breakdown has some of the features of moving arcs, but some distinctive features of its own. Whilst the mechanism of formation remains obscure, interlaminar voltages of at least 18 volts would appear to be required. 


© Antony Anderson 2000
Updated Jun  2001, Dec 10th 2002, April 28th 2003, 10th August 2005, 11 Nov 2005, 16th January 2006, 8th January 2009