The forensic examination of fuses

SCIENTIFIC & TECHNICAL

The forensic examination of fuses JD TWIBELL and CC CHRISTIE* The Forensic Science Service, Huntingdon Laboratory, Hinchingbrooke Park, Huntingdon, Cambridgeshire, United Kingdom PE18 8NU Science & Justice 1995; 35: 141-149 Received 7 January 1994; accepted 25 August 1994

Plug top (mainly 13 amp rating), rewirable and larger cartridge fuses (20-32amp rating) were caused to operate (blown) under known conditions of overload up to full mains short circuit, and by overheating. All fuses were X-Ray photographed and the differences in internal appearance were recorded. Fuses which survived heavy overloads or short circuits showed internal features which might be a useful diagnostic aid in casework situations. The relative performance of fuses of different types and ratings was also studied in circuits protected by more than one fuse. In overload conditions the lowest rated fuse or the weakest fuse in the circuit was found to blow, but under full mains short circuit conditions the superior characteristics of modern consumer unit cartridge fuses could result in these fuses blowing before a 13 amp plug top fuse, even though they hzd a higher rating.

Die Funktion von Sicherungen in Steckern sowie von Sicherungen anderer Bauarten, Funktionsweisen und Amperwerte (13, 20-32) wurde durch Kurzschlurjbedingungen und Uberhitzungssituationen ausgelost und der Zustand im Inneren der Sicherungen mittels .Rontgenphotographie festgehalten. Exemplare die Uberlastung oder Kurzschlurj uberdauert hatten, zeigten interne Zustande, die fiir die Fallarbeit ein niitzliches diagnostisches Mittel sein konnen. Ferner ist die Funktionstuchtigkeit von Sicherungen verschiedener Typen und Leistung in Stromkreisen mit mehreren Sicherungen untersucht worden. Unter den Bedingungen einer Uberlastung des Stromkreises brannte stets die schwachste Sicherung durch. Unter Kurzschlul3bedingungen konnen jedoch Sicherungen mit hoheren Amperwerten eher durchbrennen als die Sicherungen in Steckern mit niedrigerem Amperwert.

Le coupe-circuit (principalement de 13 A) ainsi que les plus grands fusibles 21 cartouches rkenclenchables (20-32 A ) ont CtC prCvus pour agir (fusibles fondus) dans des conditions connues de surcharge, jusqu'au court-circuit d'alimentations, par ClCvation anormale de la tempkrature. Tous les fusibles ont CtC photographiCs aux RX et les differences d'aspect interne ont CtC enregistrkes. Les fusibles qui ont enregistre de fortes surcharges ou des court-circuits ont montrC des caractkistiques internes pouvant presenter une aide utile dans le diagnostic de cas reels. La performance relative de fusibles de diffkrents types et de diffirentes intensitis a Cgalement CtC CtudiCe sur des circuits protCgCs par plus d'un fusible. Lors d'une surcharge, le fusible de plus basse intensit6 ou le fusible le plus faible disposC dans le circuit a CtC dCcouvert fondu mais, B la suite de conditions de court-circuit sur l'ensemble de l'alimentation, les blocs fusibles d'intensitk supCrieure ont pu se dkclencher avant le fusible de 13 A bien que ce dernier ait une intensite de dkclenchement infirieure.

Se operaron (hasta fundirse) fusibles sustituibles de 13 A y mayores bobinados de 20 a 32 A , bajo condiciones de sobrecarga hasta cortocircuito a plena tensi6n y sobrecalentamiento. Se les hizo fotografias con rayos X y se registraron las diferencias en su aspect0 interno. Los fusibles que soportaron fuertes sobrecargas o cortocircuitos mostraron rasgos internos que pueden servir como medio diagndstico en casos semejantes. En 10s circuitos protegidos por mas de un fusible se estudi6 la relativa eficacia de 10s diferentes tipos de fusibles y sus rangos. En condiciones de sobrecarga se encontr6 que el fusible diseiiado mas bajo o el mas critic0 en el circuit0 era el que se fundia antes per0 en condiciones de cortocircuito a tensi6n de alimentaci6n maxima se encontrd que se fundian antes 10s m6s grandes (de caracteristicas superiores) a pesar de estar disefiados para mayor corriente.

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~

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--

Key Words: Fire investigation; Electrical circuits; Electrical fuses; Electrical overload; Radiography. *Present address: Geoffrey Hunt and Partners, 23 Berghern Mews, Blythe Road, London W14 OHN.

Science & Justice 1995; M (2): 143-149

Introduction In a fire investigation it is often important to determine which electrical circuits were live and which were not, prior to or during the fire. An examination of the fuses involved can often reveal useful information to assist the investigation. A fuse is designed to operate by 'blowing' to open an electrical circuit, and does not 'fail' when it blows. A failed fuse is one which does not blow under circumstances in which it ought to have operated.

Previous workers have differentiated modes of domestic plug top cartridge fuse operation by dismantling the fuses and describing their physical characteristics after overload, short circuit and external heating (Scott RJ and Strutt T, internal Home Office Forensic Science Service communication 1975). In the present work, X-Ray photography was used as a non-destructive method of revealing the internal appearance of plug top, rewirable, and larger cartridge fuses blown by short circuit, various overloads and external heating. All electrical tests were carried out at full mains potential however, as in the authors' view, unless the blowing fuse had to quench a resultant 240V AC arc, the results were likely to be unrepresentative of the casework situation. The present work also included a study of the 'discrimination' between different types and ratings of fuses, in order to check whether the operation of fuses in a circuit incorporating several fuses could be entirely predicted in all circumstances. Construction of fuses In its simplest form the fusible element is a thin metal wire placed in the circuit which it is designed to protect. As current is drawn through the fuse, some heat is generated in the element owing to its internal resistance. The heat generated is proportional to the square of the current being passed. At low currents this heat can be dissipated through the fuse body. As the current flow increases, the fuse body becomes hotter and a stage is reached at which equilibrium cannot be maintained. Increasing the current beyond this critical level causes more rapid heating of the fuse wire which, due to its own increase in internal resistance, causes further heat to be generated and results in cataclysmic thermal runaway. The midlength portion of the wire away from the end caps is rapidly driven to its melting point. As the resistance of the melt is higher than the solid, even more heat is generated, causing a rapid increase in melting, resulting in breakage of the electrical pathway. In mains circuits the breaking of the fuse conductor is usually followed by the generation of an arc, which

will persist until the mains potential across the gap falls to the null point of the cycle. The time taken for a fuse to start to open and reach the arcing stage is known as the pre-arcing time. It is clearly dependent on the degree of overload current being passed. Thus at about twice the nominal current rating a fuse may take up to half an hour before the fusewire reaches the critical stage of thermal runaway. With a full mains short circuit, however, the fuse may operate within milliseconds. The time (t) taken for a fuse to blow is inversely proportional to the square of the current (I), and the relationship 12t, sometimes referred to as the Joule Integral, is constant for a particular type of fuse. In order to improve the performance of modern fuses under overload conditions, the Metcalfe (or M) effect is used. This involves placing a blob of tin or solder on the wire or shaped fuselink. As the fuse conductor heats, it initially causes this low melting metal to melt. This then alloys with fuse link metal to form a low melting point alloy of higher resistance, thus increasing the heat generated and more rapidly bringing the conductor to the melted state. The result is a more definite and faster response at low overloads. Three types of fuses were used in this study. Plug top fuses (BS 1362) normally contain a silver wire (sometimes silver plated copper) of diameter appropriate to the nominal current rating, and an M effect bead of solder or tin in the centre portion. The fuse wire is encased in a sand filled ceramic cartridge (Figure 1).

Brass Terminal End Cap

Sand Solder Silver-coated Copper wire

FIGURE 1 Cross section of a plugtop fuse.

Rewirable (semi-enclosed) fuses are used in older domestic distribution units. They consist of a removable ceramic fuse carrier which plugs into connection sockets in a ceramic fuse base. The carrier is fitted with a replaceable single strand of fusewire (usually tinned copper wire). The fuse rating depends upon the diameter of the wire used. (Table 1). Science & Justice 1995; 35(2): 143-149

Cartridge fuses have largely replaced rewirable fuses in modern installations. Larger cartridge fuses for industrial or distribution unit mounting (BS88) contain a fuse element stamped from a metal strip (usually silver or silver-coated copper) in a sand-filled ceramic cartridge. The strip is cut to form constrictions which concentrate the blowing current at high overloads making for faster operation. A bead of solder or tin is deposited close to one constriction to utilise the M effect at low overcurrents (Figure 2). Fuses of higher nominal current rating are often made up of several such strips in parallel within a larger sand filled ceramic cartridge. These fuses are often known as HRC (High Rupturing Capacity) or HBC (High Breaking Capacity) fuses. TABLE 1 Rewirable (semi-enclosed) fuse wire dimensions (tinned copper) to BS 3036 Fuse rating (A)

Wire diameter rnrn ins

minutes before the fuse operates. For example, plug top fuses should meet the British Standard Institute specifications BS 1362, under which the fuse should carry a current of 1.6 times its rating for 30 minutes without blowing and must blow within 30 minutes if passing a current of 1.9 times its rating. Fuses are also designed for given conditions of voltage and for alternating or direct current applications (implicit in the standards criteria). Any fuse used for an application greatly different from its design criteria should not be expected to operate in a predictable manner. Fuses in distribution systems Electrical distribution systems incorporate a series of fuses, in a stepwise system. Successive tiers of fuses are normally incorporated at each stage when a larger circuit is subdivided into further sub circuits. Fuses at successive circuit divisions are normally separated by at least a factor of two in rated value, to allow for 'discrimination' between the successive tiers. If a fault occurs in a final circuit, only the final circuit (or plug top) fuse should blow, thus leaving all other circuits operating.

In many circumstances there is more than one fuse of the same rating in the final circuit. The flex to the appliance will be protected by a plug top fuse, which may itself be plugged into a fused adaptor or extension lead. Clearly, if a fault occurs in the appliance causing the final circuit to overload the plug top fuses, the only fuse to blow will be either the lowest rated one or the weakest.

Sand

I Silver-coated Copper strip

FIGURE 2 Cross section of an HRC fuse.

Fuse ratings Mains fuses are rated at the maximum current that they can carry indefinitely. The minimum current required to actually blow the fuse is a low multiple of this (within a factor of two), but it may be many Science & Justice 1995; 35(2): 143-149

Under severe short circuit conditions it may be expected that more than one fuse of the same rating would operate in the same circuit. Occasionally the results of a fire scene examination suggest that a short circuit has blown a consumer unit cartridge fuse in preference to a plug top fuse of lower nominal rating, which survived. In most experiments in the present study, different types and sizes of fuses were incorporated into the same circuit in order to test whether the expected discrimination still held under a range of conditions. Further information on fuses and on terminology used in fuse manufacture and operation is given in 'Electrical Fuses' [I] and 'Users Guide to Fuses' [2]. Experimental A selection of 36 Alert fuses was obtained from Beswick Ltd, (Dubilier PLC), Alert Works, Frome, Somerset. UK. These fuses had been blown at 240V

A C under known overcurrent situations during batch testing. Six fuses of each type, blown under each set of conditions, were supplied. Plug top cartridge fuses of 3, 5, 7, 10 and 13A ratings were obtained from RS Components Ltd, P O Box 99, Corby, Northants NN17 9RS. Unless otherwise stated, these fuses were held in RS Components polypropylene fused screw terminal connectors. Rewirable fuses were made using tinned copper wire of the appropriate diameter in a Wylex rewirable fuse consumer unit (ex-installation). All connections were made using 4 mm2 (710.85 mm) insulated cable, or by more substantial cables where possible. Larger, A2 type, HRC cartridge fuses of 20, 25 and 32A ratings, also obtained from RS Components Ltd, were held in A2 size fuseholders.

Fuse overloading experiments A number of fuse overloading experiments were carried out using the Europax loadbank. Further experiments were carried out at the laboratory or in a domestic installation using the test rig shown schematically in Figure 3, plugged into a 240V A C ring main fused at 32A. The fuse in the plug to the rig was thus incorporated as one of the fuses under test. Appropriate loads were applied by plugging electric kettles, fan heaters, etc., into the sockets. Load appliances were selected to be mainly resistive and to be relatively cool running so that the effective load would not decrease appreciably during the longer experiments. In all cases nominal circuit currents were deduced from the D C resistance of the complete circuit measured with an Avometer. The loads were adjusted to give nominal overcurrent factors of 2$x and 5x for 13A plug top fuses.

A number of 'short circuit' tests were carried out on this test rig using a plug shorted with 4 mm2 cable. In these experiments the fuse in this plug became one of the test fuses. At the start of each experiment the circuit was closed using a two pole 20A contactor (RS Components Ltd) operated from a remote location. The two poles of the contactor were connected in parallel for these tests. In appropriate cases the time taken for the circuit to be broken was timed using a low voltage transformer rectifier energised from the test rig. The unsmoothed direct current output was fed to a chart recorder with a fast chart drive.

High current short circuit testing A 240V A C supply fused at 100A and industrial loadbank facilities were used at Europax Ltd (Generator Testing Engineers) at R A F Alconbury, near Huntingdon. Short circuit tests were carried out with the appropriate fuses connected in series across the mains supply, the (short) circuit being made using the switch handle on the lOOA fused isolator unit. In order to simulate the effect of fire burning through live cables, similar experiments were carried out in which the circuit was completed by using a gas blowlamp to burn through a knotted 3A (1610.2 mm) twin flex or a 1.5 mm2 Twin and Earth cable, in which the neutral and earth leads were connected together.

Thermal fracture of fuses Fuses were either heated strongly in a sand bath using a bunsen burner or in a Carbolite Model ESF2 Muffle Furnace.

Plug incorporating Fuse 1

Contactor k Neutral

u Earth

~

e

~

~

Live

FIGURE 3 Schematic diagram of test rig used in laboratory experiments. The contactor was remotely switched.

FIGURE 4 Explosive blowing of centre bead region (in one fuse of three in series) at $x overcurrent.

Science & Justice 1995; 35(2): 143-149

X-ray photography The Alert fuses and all fuses used in the study were examined using a Hewlett Packard 43805N Faxitron X-Ray System and Polaroid 53 Coaterless film. Conditions used for plug top fuses were 75kV tube voltage and 45 seconds exposure. A2 fuses required 85kV for 45 seconds. Results and discussion

The Alert fuses The X-Ray photographs showed that in all but one of the fuses the fuse wire had sputtered on blowing, producing a series of small particles around the original fuse wire position (see Figures 4-6). This exploded appearance is what might be expected from a circuit cleared by the sudden vaporization of molten fuse link material. The observations listed in Table 2 show a progressive change in the internal appearance of the blown fuse as the overcurrent factor (i.e., the ratio of the blowing current to the nominal current rating) increased.

They are referred to as 'Short Circuit Survival' damage (denoted as 'SCS' in the Tables). At overcurrent factors in excess of lox, sputtering occurred on both sides of the centre bead. The bead and adjacent portions of the fusewire survived intact. It is clear that under such excessive conditions the thermal inertia of the bead is sufficient to prevent the rapid rise in temperature which occurs in other parts of the fusewire. Overload experiments Table 3 shows that plug top fuses rated at 13A, blown at an overcurrent factor of 2$x, showed good agreement with the Alert fuse results - all blown fuses showing explosive rupturing of the M effect bead (Figure 4). As expected, when a circuit was constructed where three fuses were wired in series, only one fuse blew. None of the surviving fuses, or the intact portions of blown fuses, showed Short Circuit Survival features (Figure 5). In all experiments the fuse ruptured in 6 to.20 seconds.

At relatively low overcurrent factors (around 4x) the fuse blew explosively in the M-effect bead region, the bead itself being totally or largely exploded. At 8-lox overcurrent factors, sputtering tended to take place to one side of the centre bead and at times included the bead itself. The surviving portions of fuse wire showed deformations suggestive of their having been close to blowing when the circuit opened. These deformations appeared as smooth rounded lumps in the fuse wire.

Table 4 shows that 13A fuses, blown at an overcurrent factor of 5x, also agreed with the Alert fuse results, rupturing explosively either at, or to one side of, the bead. Again as expected, where three fuses were wired in series, the tendency was for only one to blow (although two blew in one experiment). At this overcurrent factor, all intact fuses and portions of intact wire in ruptured fuses showed Short Circuit Survival damage (Figure 5). Rupturing was almost instantaneous, typically within one second.

FIGURE 5 Explosive blowing of fuse to one side of bead (in one fuse of three in series) at 5%overcurrent. SCS seen in intact portions of element.

FIGURE 6 Explosive blowing to each side of bead (in all three fuses in series) under short circuit conditions. Bead region remains intact.

Science & Justice 1995; 35(2): 143-149

High current short circuit testing In the short circuit experiments shown in Tables 5 and 6, the 13A plug top fuses blown under these conditions again showed results similar t o those of the Alert fuses for more than lox overload. Irrespective of whether the circuit was made (shorted) by means of the isolator switch o r by burning through a cable, the typical result was explosive rupturing of the fuse wire on both sides of the bead, leaving the bead area intact (Figure 6). In cases where the fuse wire ruptured on only one side of the bead, the remaining portion showed Short Circuit Survival features. Where three fuses were wired in series the tendency was for all three t o blow, but any survivors also showed features of Short Circuit Survival.

TABLE 3 Overload experiments on test rig (33A nominal) at an overcurrent factor of 2$x, with plug top, rewirable* and cartridge? fuses

Circuits containing BS88 cartridge, BS1362 plug top and rewirable fuses The results in Table 3 show that at 33A (nominal) there was no tendency for a 20A BS88 cartridge fuse to blow in preference to a 13A plug top fuse, although in one experiment a 15A rewirable fuse did show signs of overheating over the period leading to the rupturing of a 13A fuse. Even at 68.5A (nominal), 13A fuses operated in preference to 20A, 25A and 32A cartridge fuses and a 30A rewirable fuse (Table 4).

TABLE 4 Overload experiments (68.5A nominal) on load bank at an overcurrent factor of 5x, with plug top, rewirable* and cartridge? fuses Fuse (A)

1

2

3

4

32t

13

-

30*

13

13

30*

13

13

25t

13

13

20t

13

13

13A sputtered at blob. Showed SCS. 13 One sputtered at one side, blob intact. Others intact but showed SCS. 13 Two sputtered at blob. Both also showed SCS. Third intact showed SCS. 13 One sputtered at blob, showed SCS. Two intact but showed SCS. One sputtered at one side and including blob. Other side showed SCS. Two intact but showed SCS.

Fuse ( A )

1

2

3

20

13

-

20

13

13

20t 20t 20t

13 13 15*

13 13 13

4

Observations

13A fuse sputtered at blob. No SCS (7-11 secs) 13 One 13A sputtered at blob. Others intact. No SCS (8 secs) 13 As shown above (6 secs) 13 As above (instantaneous) 13A sputtered at blob (20 secs) Heat damage to rewirable -

Observations

-

SCS = short circuit survival damage.

SCS = short circuit survival damage.

TABLE 2 X-ray examination of 6 Alert Fuses in each case Fuse rating (A)

Nominal current (A)

Overcurrent factor

Observations

13

50

4x

13

100

8x

5

50

lox

3

50

16x

5 3

100 100

20x 33x

All fuses blown sputtered in centre bead region. Five fuses sputtered one side only, two fuses away from bead, three fuses including bead. Sixth fuse blown both sides but not explosively. Surviving portions of wire showed deformations in all cases. Five fuses sputtered both sides, large portion of wire remained either side of centre blob. Sixth fuse sputtered one side only and including centre bead. Sputtered both sides of centre bead which survived together with lengths of wire either side. As for 16x As for 16x except shorter lengths of wire either side

Science & Justice 1995; 35(2): 143-149

TABLE 5 Short circuits performed by closing isolator switch on outlet protected by lOOA fuse, with plugtop, rewirable* and cartridge? fuses Fuse ( A ) 1

2

3

4

Observations 13A fuse sputtered in three experiment both sides of bead

Two 13A fuses sputtered, one both sides of centre bead, one on one side of centre bead. Other side of bead and surviving fuse showed SCS. Three 13A fuses sputtered. Two blown on both sides of bead, one on one side only, SCS on surviving side. Three 13A fuses sputtered both sides of centre bead.

Three 13A fuses sputtered, one on both sides, two on one side. SCS noted. 3A sputtered both sides. No damage noticed on 13A or 7 fuses. 32A cartridge blown at the three short circuit positions and at overload position. 25A cartridge blown at two short circuit positions but not at overload. Two 13A fuses sputtered on one side of bead, third intact - all showed SCS. 20A cartridge blown at three short circuit positions and overload. All three 13A fuses intact and no signs of SCS. 32A cartridge intact. One 13A sputtered both sides, one 13A one side, ruptured other, third 13A ruptured both sides but not typically explosively. One large sputter particle displaced from original line of wire.

SCS = short circuit survival damage.

TABLE 6 Short circuits performed by burning through a cable on outlet protected by lOOA fuse, with plug top, rewirable* and cartridge? fuses Fuse (A) 1

2

3

4

30*

13

13

-

30*

13

13

13

30*

32t

-

-

Observations Burning through 3A cable Two 13A fuses blown, one sputtered both sides, other sputtered one side, SCS on other side Three 13A fuses sputtered, one blown both sides, others blown one side with SCS at other Burning through 1.5 mm2 T & E cable 32A cartridge blown at three short circuit positions, but not at overload position

SCS = short circuit survival damage.

Science & Justice 1995; 35(2): 143-149

Under short circuit conditions, however, the results were quite different. In one experiment a 20A cartridge fuse blew at all the constrictions including the M-effect position, providing complete protection to three 13A fuses. Not only were these three all intact but they showed no Short Circuit Survival features. A 25A cartridge fuse blew at two short circuit positions (i.e., non M-effect constrictions, Figure 7) affording some protection to the three 13A fuses in the circuit (Table 5). Two of these fuses were blown on one side of the bead but not explosively. The third fuse was intact but all three showed Short Circuit Survival features. In the four experiments listed, a 32A cartridge fuse survived in preference to the 13A plug top fuses. In one (unlisted) experiment, however, a 32A cartridge fuse of different manufacture, in a domestic ring main circuit, blew in addition to 13A plug top fuses being shorted. Although this fuse was not intended to be included in the study as its prior history was unknown, the fact that it blew is clearly significant to situations likely to be encountered in casework. In a single test, a 32A cartridge fuse blew in preference to a 30A rewirable fuse. These findings reflect the superior design and fast rupturing characteristics of larger modern cartridge fuses against simpler plug top and crude rewirable fuses.

preference to 7A fuses, the ruptured fuses showing explosive blowing on both sides of the bead. Two short circuit tests were carried out using a 13A, 7A and 3A fuse in series. In both cases the 3A fuse wire had blown explosively along most of its length and even at the bead position. In none of this range of experiments was Short Circuit Survival damage found in the surviving fuses. It is clear that the lower rated fuses gave adequate protection to those of higher rating at the approximate factor of two separation.

Thermal rupturing of fuses Heating 13A plug top fuses to 250°C for 1; hours did not rupture the fuse wire. After half an hour at 450°C a fuse, whilst still intact, showed apparent thinning of the fuse wire adjacent to one side of the solder bead. When three fuses had been heated to 700-750°C for twenty minutes two were found to have ruptured. In both cases the fuse wire had broken on each side adjacent to the bead and the wire ends were severely thinned into pointed spikes. The fuse wire of the intact fuse was severely thinned at one side of the bead. Whether the fuse was mounted in a horizontal or vertical orientation did not appear to affect the result. At 800°C, ten minutes was generally adequate to rupture the fuse.

Circuits containing multiple plug top fuses A limited number of experiments were carried out using different ratings of plug top fuses at a nominal current of 33A. A 7A-rated fuse was found to blow in preference to a 10A fuse, showing sputtering on one side of the bead. Similarly, 3A and 5A fuses blew in

In all experiments the fuse wire ruptured on one (or both) sides of the bead leaving a pointed wire and a rounded solder blob. In one fuse (mounted vertically) the fuse wire ruptured in three places; either side of the blob and also part way along the wire. In this case the wire ends had a rounded rather than a pointed appearance. In most of these tests the surviving portions of the fuse wire had a somewhat lumpy or irregular appearance, quite different from the smooth rounded swellings observed in 'Short Circuit Survival'

FIGURE 7 25 Amp HRC cartridge fuse blown under short circuit conditions.

FIGURE 8 Thermal rupturing of fuses after periods of up to 20 minutes at 800°C.

148

Science & Justice 1995; 35(2): 143-149

damage. Typical X-Ray photographs are shown in Figure 8. In these tests, the heating of fuses to such high temperatures caused changes in the external appearance of the fuse, such as by completely destroying the markings and by partial oxidation of the end caps. It is anticipated that in casework where fuses have been exposed to temperatures likely to cause internal rupturing, this should be evident from their external appearance. No significant changes were noticed in the internal appearance of thermally blown or electrically blown fuses after heating to 750°C for up to an hour. Differences between thermal and electrical rupturing With only one exception, all fuses blown electrically in this study showed some 'explosive' sputtering of the fuse wire or bead. The exceptional fuse had ruptured at two places remote from the bead and at one of these breaks a single large sputter particle had detached and moved away from the original fuse wire position.

In contrast the fuse wires in thermally blown fuses were found to have ruptured on at least one side of and adjacent to the solder bead. No detached fuse material was noticed in thermally ruptured fuses away from the original line of fuse wire, although spiky growths were often noticed on the side of the wire. It should therefore be possible to identify, with a high degree of certainty, the type of rupturing seen in modern M effect solder bead fuses. In some circumstances, difficulties may be experienced in interpreting the cause of rupturing of non solder bead fuses, or of fuses blown at low voltage. Two fuses blown by short circuiting at 12V A C did not show the explosive sputtering typical of the full mains potential fuse tests.

Science & Justice 1995; 35(2): 143-149

Conclusions The circumstances in which a fuse operated can normally be determined by X-Ray photography. In the ca:e of modern H R C fuses, the fuse blows electricall; at specially constructed short circuit or overload blocring points dependent upon the degree of electrical overloading. With plug top fuses of modern construction, blowing takes place at or to one or both sides of the centre solder blob depending upon the overcurrent factor. In almost all cases of electrical blowing at mains potential, the ruptured remains of the fuse wire has a sputtered appearance. The only exception to this in our study occurred where the fuse was given a measure of protection by the simultaneous blowing of a higher rated HRC fuse in the circuit.

Fuses which have survived large overloads or short circuits often show characteristic 'Short Circuit Survival' damage, indicating that they were about to blow at the time that another fuse opened the circuit. It is therefore recommended that X-Ray examination is carried out even on intact fuses in circuits of special interest at fire scenes, as this might reveal useful information. The results confirm that under overload conditions the lowest rated or the weakest fuse in a circuit will blow. Under full short circuit conditions, however, it is possible for modern H R C cartridge fuses to blow in preference to plug top fuses of lower nominal rating.

References 1. Wright A and Newbery PG. Electrical Fuses. Peter Peregrinus (IEE), 1982. 2. Turner HW, Turner C and Williams DJA. Users Guide to Fuses. ERA Report 87-0186R November 1987. Leatherhead: ERA Technology Ltd.