wetted contacts?

in article bkhufe$7q0$ snipped-for-privacy@news8.svr.pol.co.uk, Spider at snipped-for-privacy@Hotmail.com wrote on 9/20/03 9:15 AM:

This may be refering to mercury wetted contacts.

Bill

Bill,

Thanks for the reply, the problem was to do with intruder alarm inertia switches that the internals stay in the exact same position for years, and therefore over time pit and so cause high resistances, a colleague mentioned that older equipment with higher detection loop voltages kept the device contacts clean by the application of this voltage, I at first thought the same as you, but the contacts are hermetically sealed and contain mercury or other conductive liquid.

snipped-for-privacy@Hotmail.com

Hello This is probably refierring to the wetting current of the contacts. The problem with contacts slowly oxidising and going electrically open over a period of time when they are physically closed can be prevented by ensuring that there is sufficient current flowing through them. This current is known as the wetting current. - the current that will ensure that the contacts will not go open circuit due to surface degradation. The wetting current is dependant on the contact material, surface area, pressure and voltage. Gold provides one of the best contact materials for low current, low voltage applications. Silver Cad Oxide contacts are good for switching high voltage high current, but have a high wetting current and cause problems in situations where they are maintained in a closed state for extended periods of time with low voltage low current circuits.

Ian, you've hit on one meaning of the term 'Wetted Contacts'. The other, which is the one I am more familiar with involves the contacts being wetted with mercury partially to reduce resistance but mostly to reduce contact bounce.

Harry C.

Many thanks for the informative replies

Regards

Wetted just means with voltage on them. No cleaning, no mercury.