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 place.

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.

Other options
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 product reference


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