Silver Whiskering in Electrical Systems

1. Management of Electrical Safety Silver Whiskering in Electrical Systems

2. What is it? • Silver alloys and pure silver have been used for many years in the manufacture of electrical control equipment. • When silver is exposed to airborne sulphur, (typically from hydrogen sulphide or sulphur dioxide) a reaction takes place to form a silver sulphide layer to affected parts. • Characteristically, the coating can be recognised by it’s dull grey or blue grey colour. • This layer can become very heavy, has a much 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, as stated, 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. Electrical Safety TRA-09 2.0

3. What is it? • 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. Electrical Safety TRA-09 2.0

4. Where’s the risk in Anglian Water • Any electrical panels with silver or silvered terminals which are exposed to: • hydrogen sulphide (mainly wastewater) • sulphur dioxide (mainly water and likely to be small numbers) • An explosion occurred at one of our terminal pumping station several years ago, when the isolator was operated – this caused silver whiskers which had previously formed to dislodge and bridge across the isolator terminals, resulting in a short circuit and explosion. Fortunately no one was seriously injured but the HSE did carry out a thorough investigation. Electrical Safety TRA-09 2.0

5. What does it look like? Figure 1 illustrates an extremely long monofilament whisker found in a fuse-switch. This whisker spans a gap of just over one inch (as the crow flies), but the actual total length of the filament is of the order of one and a half inches. Finding examples like this is a rarity due to the fact that they are normally vaporised in the faults that they initiate. Electrical Safety TRA-09 2.0

6. Electrical Safety TRA-09 2.0

7. Figure 3 - shows parts of the contact assembly in a fuse-switch.Here can be clearly seen multi filament growths which, are very reminiscent of steel wool, both in texture and colour. Electrical Safety TRA-09 2.0

8. Whiskers plus there is also some evidence of flaking of the surface deposit. Electrical Safety TRA-09 2.0

9. 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. Electrical Safety TRA-09 2.0

10. What to do if you find evidence of silver sulphide flaking or whiskers? • Let your line manager know • An assessment of the installation should then be carried out to determine if components require replacement and next inspection date etc. • Please also notify your local EaWR Engineer, so that periodic inspections frequencies can be amended accordingly. Electrical Safety TRA-09 2.0