What are mercury-wetted contacts?

Mercury-wetted contact relays consist of glass-encapsulated reed with its base immersed in a pool of mercury and the other end capable of moving between one or two stationary contact. The mercury films are reestablished at each contact closure and contact erosion is eliminated.

What is mercury-wetted?

For high-speed use, the mercury-wetted relay is used instead. This combines the speed of a low-mass relay, together with the fast wetting of mercury contacts. A relay, usually a reed relay, has its contacts coated with a small quantity of mercury.

What are the advantage of mercury relays?

For many years, mercury relays were considered a reliable solution. They are durable, inexpensive and capable of operating in demanding environments.

What does a reed relay do?

A reed relay is a type of relay that uses an electromagnet to control one or more reed switches. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion.

What is the difference between dry contact and wet contact?

A ‘wet’ contact is connecting a hose to a spigot that is flowing water. A ‘dry’ contact is connecting a hose to a spigot that is turned off. In electronics, electricity would be equivalent to water.

What replaced mercury switches?

The unidirectional tilt switch is a great alternative to older “mercury switches” in order to avoid the hazardous mercury in applications for safety and compliance standards. They also have much greater accuracy than most other non-mercury switch options.

How long do mercury relays last?

Because of this, mercury contacts cannot switch currents of more than a few amperes. The electrical life expectancy of general purpose and power relays is generally rated to be 100,000 operations minimum, while mechanical life expectancy may be one million, 10, or even 100 million operations.

What replaced the mercury switch?

What activates a reed switch?

Although a reed switch can be activated by placing it inside an electrical coil, many reed switches and reed sensors are used for proximity sensing and are activated by a magnet. As the magnet is brought into the proximity of the reed sensor/switch, the device activates.

Why is it called a reed switch?

A reed switch is an electromagnetic switch used to control the flow of electricity in a circuit. They are made from two or more ferrous reeds encased within a small glass tube-like envelope, which become magnetised and move together or separate when a magnetic field is moved towards the switch.

What is a wetted contact?

Wet contact is one in which the power is being supplied by the same power source that the control circuit is used to switch the contact. It can operate as an ordinary single-pole ON/OFF switch. It operates like a controlled switch. It can be referred to as a secondary set of contacts of the relay circuit.

What does volt free mean?

A thermostat or timer with ‘volt-free contacts’, or ‘volt-free switching’, operates a switch that opens and closes its contacts, but no voltage or current will flow.

Are mercury switches still made?

Mercury switches were widely used in the automotive industry for trunk lid lights, open door indicators, and anti-lock braking systems, but due to the harmful effects mercury has on the environment their use was discontinued in all American-made cars in 2003.

Do mercury switches fail?

Though mercury switches are known for their durability, they can still fail. When they do, the result can be messy. A switch that has overheated can crack or burst, potentially releasing mercury into the workspace.

Where are reed relays used?

Reed Relays are ideally used for switching applica- tions requiring low and stable contact resistance, low capacitance, high insulation resistance, long life and small size.

Do mercury switches wear out?

Contacts stay clean, and even if an internal arc occurs, the contact surfaces renew on every operation, so they don’t wear out. Even a small drop of mercury has low resistance, so switches can carry useful amounts of current in a small size.

How much mercury is in a light switch?

Each tilt switch has about 3 grams of mercury in a little sealed “button” that is very difficult to break. This type of switch is quiet and durable. Manufacturing was discontinued in the late 1980s because of concern over the health effects of mercury.

Why do reed switches fail?

Reed switches or relays eventually fail in one of three ways . They do not open when they should (usually called “sticking”), they fail to close when they should (“missing”), or their static contact resistance gradually drifts up to an unacceptable level .