Tin alloy is sulpher so good
Hubble describes the potentially hazardous effects of sulphur laden atmospheres on control gear and the development of fuse switches that withstand sulphur contamination
Silver alloys and pure silver have long been used in the manufacture of industrial
control gear. Although silver surfaces will tarnish even under normal environmental
conditions, such deposits do not usually have an adverse effect on the performance
of electrical contacts or conductors. The deposit form is argentous oxide (Ag2O)
which, although resistive in nature does not usually form in sufficient quantities
to cause problems. The deposit is also relatively soft and is easily worn away
by continuous contact operation. This is particularly true of wiping type contacts,
which would normally be expected to break through the surface film completely
at the first operation.
Under these conditions, sulphur is normally present in the form of hydrogen
sulphide (H2S) or sulphur dioxide (SO2)
and a reaction takes place with the silver to produce a silver sulphide layer.
The coating can be recognised by its dull grey or blue/ grey colour. This layer
can become very heavy, has a higher electrical resistance than silver and usually
results in abnormal temperature rise, followed by ultimate failure of the device
due to thermal stress. In some cases, the layer can become so heavy that it
delaminates and flakes fall into the bottom of the switch or cubicle. Although
the sulphide layer is relatively high resistance compared with silver, it still
represents a short-circuit or earth path hazard if it accumulates in the wrong
An additional and probably even greater hazard is that of whisker growth, which
can take place over a period of months, days or even hours. Although somewhat
unpredictable, whisker growth generally initiates at corners or stress points
and is usually accelerated by higher temperatures. Whiskers can take the form
of straight or curled monofilaments or multiple whiskers growing in clumps with
a resemblance to steel wool. Monofilaments can grow to many centimetres in length
and as such, are able to easily bridge between phases or phase to earth. Although
the sulphide filament is many times the resistance of pure silver, it still
represents a potential conducting path.
Figure 1 illustrates an extremely long monofilament whisker found in a fuse-switch
in use in a paper mill. This whisker spans a gap of just over 2.5cm, but the
actual total length of the filament is in the order of 4cm. Finding examples
like this is a rarity as they are normally vaporised in the faults they initiate.
Figure 2 illustrates parts of the contact assembly in a fuse-switch removed
from the coking ovens in a steel mill. Multi-filament growths can be clearly
seen and, as previously mentioned, are reminiscent of steel wool both in texture
and colour. There is also
evidence of flaking of the surface deposit.
The basic problem can be solved by simply removing one of the basic elements
causing the reaction which creates silver sulphide, ie either the silver or
the sulphur. Removing the sulphur or preventing it from contaminating control
gear enclosures is easier said than done and is not normally an option. We therefore
need to find an acceptable substitute for silver. Gold would be a good option
as it does not readily react with sulphur, but its softness and prohibitively
high cost, renders it impractical for use in power switching devices.
Of the materials commercially available, tin is the most likely contender. Although
it does react with airborne sulphur, the reaction only takes place at elevated
temperatures approaching that of molten tin. However, it does have two problems.
Firstly, like silver, tin also produces whisker growths in the presence of
sulphur. However, the whiskers are generally single crystal growths rather than
multiple whiskers. They are also metallic in nature, which makes them seven
or eight times more conductive than silver whiskers. Hence they provide an even
greater risk of short-circuit.
Much research has been carried out aimed at overcoming this problem and a number
of non-whisker tin alloy plating solutions have been developed. There are two
main solutions available which provide non-whisker characteristics, these are
tin/lead and tin/copper. For power switching applications, the tin/copper process
would be the preferred alternative. Unfortunately, this process is extremely
difficult to control and is not offered on a commercial basis by any plater
at present. This leaves tin/lead as the best practical solution.
A second problem associated with tin and tin alloy plating is that a diffusion
process can take place between the copper and the tin. When this occurs, it
creates an inter-metallic layer between the copper substrate and the plating
layer. This intermediate layer has a resistance roughly six-times higher than
that of pure tin. Once again, this produces the probability of excessive temperature
rise and ultimately the probability of failure of the switching device. The
diffusion process can and does take place at normal room temperatures, but is
accelerated by high temperatures and hence can lead to thermal run-away.
Putting up barriers
The most effective way of inhibiting this diffusion process is by the introduction
of a thin barrier layer between the copper and tin, using a metal with which
tin or copper will not react.
Figure 3 is a final illustration, which shows the effects of both kinds of
sulphur contamination on the moving contact assembly from a fuse-switch. Here
the multiple whisker growths and flaking of the surface coating of silver sulphide
can be clearly seen.
After considerable investigation and research, the system introduced by Hubbell,
for use in sulphur laden atmospheres, is based on the use of a bright tin alloy
as the final plating layer, with a thin interposing plating barrier of dissimilar
metal. To be fully effective the alloy ration in the primary plating layer needs
to be carefully controlled along with the actual thickness of both primary
and barrier layers.
Hubbell fuse-switches using this construction have been in production since
1992 and to date there have been no reports of failures due to silver sulphide
surface contamination or whisker growth.
The entire range of Hubbell fuse switches are available in this form, from
the UFS32 up to and including the UFS800C3. The maximum current ratings are
generally reduced by 10% below that of the equivalent unit with silver-plated
contacts and conductors. To assist recognition, a suffix ‘T’ is added to the