Volt free contacts 1

[SAK9]

Could any one explain what is a volt free contact and how it works

 

[ScottyUK]

It's just a pair of contacts which aren't electrically linked to any source of supply.

 

[rbulsara]

To give an example for what scottyUK said, a regular relay with a coil and contacts have contacts which open and close when the coil is energized and/or deenergized. These contacts are 'dry' or volt free contacts, not connected to the supply that controls the relay coil. However, these contacts can be used to control (make or break) other circuits (such as a motor control circuit) which will have voltage.

Where as PLC's (unless specified otherwise) would have outputs which can have +/-24V to 48V ac/dc signal on them. They are not volt-free. Then you have to have a interveinig relay drive by PLC to get volt-free contacts.

 

[SAK9]

Rbulsara,

Many thanks for your post.When these volts 'free contacts"
are configured to communicate with a PLC or simply put in a motor control circuit, a control voltage(could be anything between 24V to 230V) is applied across them .Does it mean they are still volts free and no current flows through them?I would appreciate if you could enlighten me on this.

 

[jbartos]

Suggestion: The dry contacts are normally considered volt free and galvanically isolating downstream load or circuitry. However, there are or may be significant surface resistance or impedances depending of the environment and manufacturing. These high surface resistances or impedances are normally not considered unless there is some very low current application. Also, if improperly applied to higher voltages than designed for, they may malfunction and the surface resistances or impedances may lower or deteriorate.

 

[panelman]

in the world of control panels VF contacts are normally used for switching voltages that originate from a different source to the panel control voltage.

uses include telemetry and interlocking with other systems

 

[gsimson]

Every contact is potential free till the power is given.

In olden days "changeover" contact is used for interlocking purposes as the control scheme was simpler and operates in 'OR' scheme.

With larger systems and the sequencing & interlocking are to be taken to various levels & various voltages, "voltage free" contacts have become essential. These contacs will not have the influence of either coil voltage or the power voltage of the Contactor/ relay.

These "Voltage free" contacts are used in some other circuit and some other voltage for sequencing and interlocking.

 

[rbulsara]

SAK9:

The volt-free contacts do not remain volt-free 'after' the voltage (of the circuit it is controlling)is applied.

In otherwords, you can use them as you described in your second post. Volt-free does not mean they have to remain so when used in a circuit. Only that they have no voltage related to the logic or relay that is driving (actuating) them. The volt-free contacts are rated to be used on certain voltage and to carry certain current and should be used within their rating.

Hope this helps.

 

[jbartos]

Suggestion: There is no definition of "volt free contact" in IEEE Std 100-2000 "Dictionary"
Visit

http://www.a-tcontrols.com/pdf/MM/301-302-mm.PDF

for: volt free contact example at limit switch