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8
Residual current
In the field of engineering "residual current" describes a form of current that occurs
due to an error in the device, the installation or the supply grid. The term is
probably common to the majority of men. For instance, modern installations are
equipped with a residual current protective device, which is fitted in the sub-
distribution system ("fuse box"). In the event of a short-circuit, the amperage of
residual currents may reach up to several hundreds of amps. On the other hand,
they may only slightly exceed the normal current rate, e.g. instead of 0.5 A, which
would be the case with a premium television set. Provided that this device
wouldn´t be equipped with a safety fuse, even the slightly increased current of said
premium television might set the device on fire. It´s simple as that: The melting
point of the copper in a trace is at 1083 °C.
The importance of fuses and the reason for its development can still be
comprehended by way of documented "accidents" resulting from current-related
errors.
"According to the fire service, most of the fires are the result of errors in electrical
plants and devices or careless handling of electrical household devices. Yet, it has
recently been shown that fires may as well be caused by new devices, i.e. such ones
equipped with electronic systems like e.g. it is the case with charging sets for
cellphones or energy-saving bulbs, PCs etc. (...).. Only too often, power lines become
overloaded and, thus too hot. An overload of the electrical power line may also
occur, because there are too many devices connected to one plug via several plug
connectors."
1
Most all of these devices are equipped with fuses. It´s likely that the number of
house coals would be other than cited in this paper which amounts up to 2000
per each year (which, in fact only applies to the city Düsseldorf) far higher, if the
electrical devices were not equipped with a safety fuse, however, the incidents of
fire would definitely be lower, if those safety fuses were applied in an appropriate
way and with the required expertise.
The two images below depict the consequences that might result from missing or
inappropriately fitted fuses by showing an overloaded electric control used for blind motors
whereas the other picture shows the experimental laboratory of the Wickmann plants:
1
Federal capital Düsseldorf (Hg.): Risk house fire" Recognizing hazards, Preventing fires, Düsseldorf 2006
(http://www.duesseldorf.de/feuerwehr/pdf/alle/risiko_wohnungsbrand.pdf <8.2.2012>), P. 4.
9
Consequences of a "residual current" in a device (Image: M. Rupalla/Wickmann-
Laboratory)
Already in 1906 the engineer Georg Isidor Meyer came to the realization of the following:
"The safety fuse can be considered as one of the most sophisticated inventions in the field of
electrical engineering. It provides a remedy by triggering the erroneous effect in an enhanced
degree. Actually, the reaction is always the same, however compared to the parts to be
protected, the effect is considerably increased, thus it can be regarded as an ideal protective
device.
2
2
Meyer, Georg Isidor: " On the concept of the safety fuse", Munich/Berlin 1906, cited as: Meyer, safety fuse.
10
1. ... until 1860:
The research into static electricity
The history of safety fuses dates back to the 19th century and was characterized by tragic
"accidents" that reflected the hazardousness and risks related to the largely unknown field of
electricity. Yet, people have known about the existence of static electricity since ancient times,
already. It was around 600 BC when the Greek savant Thalas von Milet discovered the static
electricity of amber. The attractive force of the charged amber was called "electron" (derived
from the Greek term "elkein" = attracting). For whatever reasons, that was it, already. Apart
from the "electrostatic generator", which was developed by William Gilbert (1544-1603), who
was also the personal physician of the British Queen Elizabeth I in the 16th century, it took
until the 18th century until the systematic investigation of the phenomenon commenced by
the Dutch physicist Andreas Cunaeus (17121788), who invented the "Leydener Flasche", thus
one of the first condensers in 1746. This "Flasche" (bottle) was used to collect electric charge,
concentrate it afterwards and triggered a heavy electric shock to everyone who got near that
point. This process was often compared to the lightning during thunderstorms. After these
findings became public, many scientists started to get involved in electric discharges with a
continuously growing quantity of the energy that was used for those investigations.
Already in 1774, the scientist Edward Narine used wires, which he aligned with the energy
charge in such a way that they were fusing when the energy charge was too high, which
resulted in a disruption of the discharge. It is owing to this "alignment" of the wire that laid the
foundation for the safety fuse.
This is, what Professor Georg Richmann (1711-1753) should have taken into account. On
August 6th, 1753 he was killed while trying to catch "lightnings" using his invented "weather
conductor", i.e. a lightning conductor in Petersburg. The investigations regarding the cause of
his death showed that the discharging lightning rod struck into his head, went through his
body and out of his left foot. Richmann was burned from the inside out. It turned out, that the
reason for this was a malfunction in the design of his weather conductor.
Scientists all over the world were shocked of this incident and started to figure out how to
avoid such electricity-related accidents in the future. At least since then, the driving force in
the development of continuous improvements in the field of safety fuses is the prevention
from accidents that might pose a risk to the plants, devices, and equipment, and, above all, to
the lives of the staff.
11
The duration of electrostatic discharges is relatively short, yet, they may be quite powerful and
therefore fatal, as the "accident" of Georg Richmann has shown. Besides the analyses
conducted with respect to electrostatics, scientists tried to find solutions to provide electricity,
which would be available at any:
3
o
1600 Gilbert (GB): Start of the first electrochemical investigations
o
1789 Galvani (Italy ): Discovery of electricity in experiments conducted on animals/frogs
o
1800 Volta (Italy): Discovery of the voltaic cell/"Voltasche Säule" (voltaic pile)
o
1802 Cruickshank (GB): First electric battery in serial production
o
1802 Ritter (Germany): First accumulator ,,Rittersche Säule" (Ritterian pile)
o
1820 Ampère (France): Electricity using magnetic fields
o
1833 Faraday (GB): Publication of the law by Faraday
o
1836 Daniell (GB): Discovery of the "Daniell-Element"/Galvanic cell
o
1859 Planté (France): Invention of the lead/acid battery
3
GRS Foundation The world of batteries. Function, systems, disposal, Hamburg o.J. (http://www.grs-
batterien.de/fileadmin/user_upload/Download/Wissenswertes/ Infomaterial_2010/-GRS_welt_der_batterien.pdf
<8.2.2012>), P. 4.
12
2. 1860 to 1900:
The golden years of the scientists and pioneers
Given those facts, the opportunity and thus the basic pillars for a functional and appropriate
use of electricity were already existent before the midst of the 19th century thanks to the
galvanic cell and the electromagnetically generated current (current generator). Yet, the
opportunity to provide for a permanent supply of electricity entailed the risk of line overloads
and short circuits.
Actually, the general use of such power sources was reserved to the field of electric machines
like those used for illumination and telegraphy.
All over the world, the electrical engineering was explored, filled with inventions, and put to
small and large experiments all the way through to the smallest sphere. Those days, the world
was literally overrun by researchers and technicians. Going more into details about these
persons would go beyond the constraints of this work.
The chart attached to this work provides a brief (but not complete overview) of the most
important names and years
The list contains also the names of German developers and scientists, as they made a
considerable contribution to the widespread use of electricity, actually. Examples are the
electric engineer Zènobe Thèophile Gramme (1826-1901) inventor of the direct current
generator, i.e. the "Gramme-Maschine" (Gramme machine), which was introduced to the
market in 1871 and the civil engineer Oskar von Miller (1855-1934).
Both set milestones in the field of heavy current engineering. Miller was part of the directive
board of the company AEG and was a highly regarded counsellor in the planning processes of
electric utilities. Besides that, the exhibitions and fairs whose realization was of vital
significance to facilitate the common use of electricity and which took place in 1882 in Munich
and 1891 in Frankfurt were organized by him.
4
Examples for the public use include with the illumination of Berlin´s downtown in 1881 and the
use of electric motors (navigation on the German Königssee in 1909) and of course the public
use of telegraphy.
There are plenty of other examples worth mentioning, however, it can be said that it is the
frequent occurrence of "accidents", which direct the focus of this trend towards the
indispensability of the development of a fuse system, so that the consequences arising from
4
Info board German Museum of Munich, 2011
13
overload currents in lines and devices or short circuits can be reduced.
A significant event that emphasizes the importance of fuses is the great fire of Saint Germain in
June 1846.
,,In June 1846, because of a fierce thunderstorm over Saint-Germain village, all the wires of
Le Vesinet-station were burnt and the apparatus were destroyed. This accident lets us
think that we had to protect the operators. We imagine to insert in the electrical circuit a
very small and resistant wire, which should burn before the copper wires of the electro-
magnets."
5
The first publication of a full summary on ,,the use of fuses" ("über den Gebrauch von
Sicherungen") dates back to 1874 and was written by Sir David Salomons (1851-1925). Around
that time, the development of safety fuses as a means to protect lines and devices.
In 1864, the engineer Sir William Henry Preece (1834-1913) who worked at the "British
General Post Office" started to elaborate the issues concerning the maximum capacity of
parallel lines. Even today, his findings with respect to the physical laws of the maximum
capacity of infinitely long wires are still applied in the development of safety fuses. Although
safety fuses were not the focal point of his research work, he was probably one of the first
scientists who examined the theoretical principles and requirements related to the processes
during the charge and discharge of wires. In collaboration with him, I.M. Onderdonk developed
a formula to calculate the shut-down intervals of copper wires. The calculation of these
intervals was, however, limited to a rather small amount of time so as to provide for sufficient
accuracy (10 s > t > 0.01 s), since the thermal output caused by radiation, lines, and convection
was not taken into account.
Later on, this formula provided the basis for the technical specification of the melting integral
of a fuse, also known as the I
2
t-Wert, which corresponds to the energy required for the fusion.
5
Cockburn, A.C.: On safety fuses for electric light circuits and on the behavior of the various metals usually
employed in their construction. J. Soc. Teleg. Eng. 16(1887)5, S. 650-665.
14
I
2
t-value and time-current characteristic
Every time electricity´s flowing through a fuse, which brings about the melting of the
fuse element, the interval starting with the inflow of the current through to its
disruption can be measured and depicted by means of a diagram. The connection line
of the measuring points are referred to as "Time-current-characteristic"(I = Current and
t = Time).
Source: "Wickmann Geräteschutz-Information") 1994
The diagram shows that the ,,shut-down time" decreases in accordance with the
increasing inflow of current. (starting at more than 1.000s in the overload range up to
0,01s in the short-circuit range). The shut-down time of a fuse in the overload range
(1) and the transition section (2) depends as well on the safety fuse´s capability of
conducting the heat to the connections and surroundings (see explanations on nominal
and limiting current). It is only the short-circuit range (3), where the shut-down time is
limited to such a short amount of time, that it leads to an immediate disruption of
heat conduction. The energy required for the melting process is defined using the I
2
t-
value. Th e I
2
t-value is of major significance for the technicians and engineers involved
in the development of new T.V.-sets, especially when it comes to the choice of the fuse.
15
Hence, the establishment of a basis for the research and development efforts, which were
required to facilitate the common use of safety fuses in times when people started to draw on
the application of electrical plants and devices more and more often. Technicians, engineers,
and scientists such as Silvanus Phillips Thompson (1851-1916), A.C. Cockburn, Thomas Alva
Edison (1847 1931) in the 19th century but also G. I. Meyer, E. Wintergerst, L. Vermij, and H.
Johann in the 20th century to mention a few, (L. Vermij listed the names of over 60 scientists
in his works) drew on the paradigms and principles created by W.H. Preece and I.M.
Onderdonk to implement their research- and development projects.
It was not until 1879 that electric circuits came with fitted "predetermined breaking points"
thus line sections with a smaller diameter. Considering the growing diversity of application
fields, Sir P. Thompson realized the necessity of improvements with respect to the safety fuses
that had been used until then. It turned out that it was no longer possible to cover the supply
of the devices which were supposed to be protected, as the branching of lines in the devices
was growing in complexity, i.e. the increasing number of lines resulted in an insufficient supply
so that the current flow rate diminished more and more. Hence it was obviously possible to
develop thinner lines, yet the risk of a potential overload could not be ruled out. Thompson
developed a safety fuse, which shut down the device at low flow rates already, and which were
therefore able to be customized to the individual needs of the respective consumer.
He connected to iron wires with a ball made of tin and lead. The resistance of the iron wires
generated the heat required to melt the tin-lead-ball, which resulted in a disruption of the line.
The resistance of the iron wires provided the basis to determine both the moment when the
melting point of the tin-lead-ball was reached and the kind of residual current that led to it:
6
6
Image source: Gelet, Jean-Louis: To the Origins of Fuses, 8
th
International Conference on Electric Fuses and Their
Application, Clermont-Ferrand 2007, S. 1-8, cited as: Gelet, Origins; here: P. 4.
16
Drawing of the safety fuse developed by Thompson
Not only, this safety fuse design turned out to be an appropriate means to avoid short-circuits
but also with respect to the prevention of undesired and hazardous excess currents.
Furthermore it opened up the opportunity to align the current intensity (nominal or limiting
current the difference will be explained in detail later on) of safety fuses with the demands of
the device or plant to be protected.
That way Thompson set the basic requirements for the safe operation of electrical plants: The
selectivity of the applied fuses. This necessity, i.e. the alignment of the nominal or limiting
current of a fuse with the lines´ cross-section to the conductor or conductors in the device is
still considered as the impetus for new developments and improvements of safety fuses.
17
Selectivity
Safety fuses with all kind of nominal currents are used in all house and apartment
buildings. Depending on the number of apartments, the supply line is usually fitted
with e.g. 63 A-fuses.
These are used to establish a connection by means of very strong cables (e.g.
cables with a diameter of 16 mm
2
per each line). Th is cable is fused with 32 A-
fuses which are installed in the sub-distribution system of the building. All kind of
devices (cooker, lamps, dish-washer, ...) or connection points for devices, installed
in the apartment (via the plugs) are now protected by means of a 16 A-fuse. Most
often, the 16 A-fuse comes in the form of an automatic fuse, which shuts off the
power supply in the event of a short circuit in a plug and in doing so, protects the
respective line in the apartment (usually those with a diameter of 1.5 or 2.5 mm
2
).
Provided that the above-mentioned 16 A-fuse wouldn´t exist, the upstream 32 A-
fuse would probably shut off or the line installed in the wall would melt, which
would eventually lead to a house fire.
The case is similar to the wirings of a plugged-in television. It´s very unlikely that the
generated flow rate of a defect component part is capable of generating such a high
flow rate, which would cause a shut-down through the 16 A-fuse. Yet, it can´t be
ruled out that the wirings of a television may cause a fire. That´s why (almost) all
electrical power sources are fitted with additional fuses, whose nominal current is
aligned with the failures that may potentially occur in the device. (e.g.. 1 A or 2 A
nominal current).
The selectivity principle can be described as follows: Large cable diameter = High
nominal current rate of the fuse; small cable or line diameter = low nominal current
rate of the fuse.
The 63 A-fuse used in the building connection line must be capable of bearing the
total current flow of all connected devices. Given this, it is rather unlikely that the
occurrence of a short in a television might cause a noticeable heating in them.
The development of Prof. Thompson was improved and patented by Charles Vernon Boys
(1855-1914) and
H.H. Cunyngham in 1883. Cunyngham connected two flat conductors with spring elements
using Sn/Pb-solder.
7
7
Gelet, Origins, S. 5.
18
Drawing of the safety fuse patented by von Boys and Cunyngham
Like it was the case with Thompson, the tin-lead solder was melted by heating the supply lines,
which resulted in a disconnection of them triggered by the spring force. The advantage of the
spring force effect was reflected in the operability of the fuse irrespective of its position. Any
"dripping"-process with respect to the liquefied solder by means of gravity wasn´t necessary
any longer.
The wide fields of applications of safety fuses in illumination systems inspired Thomas Alva
Edison (1847-1931) and Joseph Wilson Swan (1828-1914), owner of the company "The Edison
& Swan United Electric Light Co", to improve Cunyngham´s concept of the safety fuse by
applying plain wires or belts made of the low-fusing material tin. Later on, in 1881 that is,
Edison filed a patent application that contained the term "safety-guard" for the very first time.
Indeed, this term is still used today.
The realization of another significant development was achieved by W.M. Mordey. The safety
fuses back in those times were mounted in wooden boxes, or as it was patented by T.A. Edison
covered with glass. The same goes for the "Bates Fuse", which was fitted with a fuse conductor
that was guided through a tube (usually made of ceramics) and whose ends were open.
8
Not only T.A. Edison but also Mordey found out in 1890 that the molten conductor material of
the fuse in a device or switch (these days referred to as "hardware") might be interfering and
hazardous. Yet, what was even more important was the control or removal of the arc (also
known as "flashover"), which occurs during the melting process of the fuse conductor Due to
the increasing diversity of devices, which were connected to a current source and the growing
length of supply cables, it became necessary to provide for higher voltages that facilitated the
generation of arcs. On the other hand, an uncontrolled shut-down arc switch came along with
a higher risk for the environment than an overloaded supply cable, which shut itself off
uncontrollably. Mordey led the fuse conductor through a glass tube, which was
8
Andrews, Leonard: Electricity Control. A Treatise on Electronic Switchgear and Systems of Electric Transmission,
o.O. 1904.
19
closed by means of the metallic end caps. The glass tube was filled with sand, chalk, asbestos,
or other inflammable materials. That way, i.e. by means of cooling, it was possible to withdraw
energy from the shut-down arc and hence to eliminate it in time or to suppress it entirely.
Sand with a controlled grain size showed the best results.
Safety fuse by Mordey
9
Apart from that, Mordey combined several copper fuse elements with the tin foils used by
Edison. It´s not known, whether it was intended to establish the nowadays common use of the
"M-Effect".
10
The publication of Mordey's patent in 1890 came along with the introduction of the term
"Cartridge fuse". In the time afterwards, many other patents like those from Edison, Mordey
and many more were already based on the innovative developments and constructions that
were related to the "modern times" as is shown in the figure below:
11
9
Andrews, Leonard: Electricity Control. A Treatise on Electronic Switchgear and Systems of Electric Transmission,
o.O. 1904.
10
M-Effect = The capability of dissolving copper from liquid tin or lead. Mentioned in the BEAMA-Journal 1939: ,,A
New Fuse Phenomenon" von A.W. Metcalf.
11
US-Patent Nr. 622,511 April 1899.
20
Around 90 later, a safety fuse for the field of telecommunication produced by the company
Wickmann looked like as follows:
12
Obviously not much has changed. As is described above, these and other functional principles
and constructions of the past can be adopted and thus fit with almost all "developments" of
modern safety fuses .apart from minor exceptions. But more on that later.
In so doing the gap between the "predetermined breaking point" as a rejuvenation of the
conductor and the independent component part "safety fuse" was closed. Until the turn of the
19th century the functionality of all commonly used safety constructions and materials was
tested and approved.
In order to facilitate the further improvement of the safety fuses, whose functionality had to
be aligned with the growing diversity of devices and applications, it was necessary to elaborate
the functional principles and the description of the respective physical law with respect to the
processes before and after the melting of the fuse conductor.
12
Wickmann-Werke AG: Catalog, Witten 1980 (Copy in possession of the author.).
21
3. 1901 to 1920:
Systematization of theoretical principles
Until 1900 there were only a few theoretical papers, which aimed at conveying a better
understanding of the physical processes during the melting of a safety fuse conductor. Apart
from the above-mentioned publications by the engineers W.H. Preece and I.M. Onderdonk
back in 1864 and those by Herzogs and Feldmanns "Die Berechnung elektrischer Leitungsnetze
in Theorie und Praxis" (The calculation of electrical wiring system in theory and practice) from
1893, the important basic requirements to realize improvements with respect to the safety
fuse were missing. Although their publications were quite interesting, the renowned engineers
Forbes, Reinisch, Grassot, Skrischinski, Uppenborn and Oelschläger were not able to provide a
more revealing insight into the topic. As of 1900, the number of the scientists and engineers
who have been dealing with this topic started to push the boundaries; so let´s have a look at
the developments and trends in Germany. It should be mentioned that all developments
taking place over here were tightly connected to the research in other countries.
In 1906 Georg Isidor Meyer published his above-mentioned paper: "Zur Theorie der Ab-
schmelzsicherung".
13
(About the theoretical principal of the safety fuse) and did not only
investigate the processes taking place with respect to the heat distribution and heat
conduction during the heating of a wire, but also the compatibility of various materials and
their combinations by means of several different construction types. He analyzed the behavior
of parallel fuse elements, coiled wires and wires with constrictions. He was looking for
regularities in different kinds of impacts during the heating of all those designs and materials.
Besides numerous formulas for the calculation of fusion characteristics, Meyer discovered
constants, which are typical for every kind of material. In doing so, it was possible to calculate
the melt flow rate of high flows or very short intervals in a simple way. Even today, this
"Meyer-Constant" is applied by the developers of the safety fuse manufacturers to align them
with the existing applications or (primarily) standards. Despite all the ground-breaking and
innovative discoveries, the author Meyer described in his paper proved the voltage isn´t
important when it comes to the disconnection of the fuse. Most of the tests were conducted
using direct current voltage below 10 V. Not only this was technically incorrect but also it
considered merely a part of the applications.
13
Meyer, Safety fuse, P. 1.
22
Due to the strongly varying levels, the increase of the significance concerning the application of
higher operating voltages is rather unlikely, although the "Zed"-fuse (similar to today´s fuses
used in house wiring systems), which was developed and produced by Siemens for the purpose
of generating higher voltage levels back in 1912, was sold more than 250,000 times in the UK.
G.I. Meyer´s opinion concerning higher voltage levels:
"The very peculiar results of research works were published by W. Oelschläger (Elektrotechn.
Zeitschr. 1904 -- Journal for Electrical Engineering). He conducted research on the melting
process of a fuse in current- and voltage amplifications during short circuits using an
oscillograph. (...) The present paper discusses the principles of the safety fuse in general without
examining the effects after the melting process, so that the principles applying for the practical
operation, thus resistance, self-induction, and capacity with the voltage merely exerting a
relatively minor impact (...)."
14
Meyer continued, amended, and complemented the works of (e.g. Preece and I.M.
Onderdonk) in all areas. However, after all it´s only his findings in the field of the material
characteristics, which are still applied in our development departments and by the
manufacturers of safety fuses, although their theories and principles have fallen into oblivion,
already. Nevertheless, it is obvious that Meyer´s contemplations regarding the significance of
voltage were full of mistakes.
14
Meyer, Safety fuses, P. 4.
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