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Volt Free Contacts /Dry Type Contacts/ Potential Free Contacts in Numerical Relay 1
- Thread starter TPEC
- Start date
??? Any old relay is a dry type. Use any one of the dozen depictions of run-of-the-mill relay contacts.
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are the two most common.
Keith Cress
kcress -
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are the two most common.
Keith Cress
kcress -
- Moderator
- #3
First, the name "Volts free contact" is very misleading.
Change that to "Volts free output".
Many PLC outputs source a voltage. Zero Volts out = off. 12 volts or 24 Volts or some other voltage = on. There are no mechanical contacts in these outputs.
Some PLC outputs sink a current from a solid state device. There are no mechanical contacts in these outputs.
Mechanical contacts are inherently "Volts free".
Possibly the term "Volts free contact" was originally "Volts free output" but was lost in translation.
Bill
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"Why not the best?"
Jimmy Carter
Change that to "Volts free output".
Many PLC outputs source a voltage. Zero Volts out = off. 12 volts or 24 Volts or some other voltage = on. There are no mechanical contacts in these outputs.
Some PLC outputs sink a current from a solid state device. There are no mechanical contacts in these outputs.
Mechanical contacts are inherently "Volts free".
Possibly the term "Volts free contact" was originally "Volts free output" but was lost in translation.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RRaghunath
Electrical
All are dry type contacts - numerical/electromechanical relays, contactors, auxiliary relays etc.
Apart, some of the relays have thristor outputs that can provide output in 4ms.
Wet contacts are of mercury in a glass bulb wit two electrodes and mercury closes the contact when the bulb is tilted. These contacts can be seen in Temperature indicators mounted in Transformer marshalling boxes as well as Transformer Buchholtz relays. These are not acceptable in earth quake prone regions.
All contacts, in general, are potential free or volts free.
Apart, some of the relays have thristor outputs that can provide output in 4ms.
Wet contacts are of mercury in a glass bulb wit two electrodes and mercury closes the contact when the bulb is tilted. These contacts can be seen in Temperature indicators mounted in Transformer marshalling boxes as well as Transformer Buchholtz relays. These are not acceptable in earth quake prone regions.
All contacts, in general, are potential free or volts free.
- Moderator
- #5
Another interesting variation of the mercury relay is the mercury displacement relay.
A solenoid pushes a slug into a pool of mercury, raising the level. The raised mercury level bridges the contacts.
Temperature measurement:
If contacts are directly moved by temperature changes as with a bi-metal strip, The slow opening and closing will cause arcing which will probably degrade the contacts or will possibly destroy the contacts.
The tilting mercury bulb is arranged to give a fast closing and opening of the contacts.
Another, more environmentally friendly solution is a small magnet close to the movable contact. This attracts the movable contact so that it closes abruptly, and opens abruptly.
Bill
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"Why not the best?"
Jimmy Carter
A solenoid pushes a slug into a pool of mercury, raising the level. The raised mercury level bridges the contacts.
Temperature measurement:
If contacts are directly moved by temperature changes as with a bi-metal strip, The slow opening and closing will cause arcing which will probably degrade the contacts or will possibly destroy the contacts.
The tilting mercury bulb is arranged to give a fast closing and opening of the contacts.
Another, more environmentally friendly solution is a small magnet close to the movable contact. This attracts the movable contact so that it closes abruptly, and opens abruptly.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Over here 'dry contacts' would indicate that there's no contact wetting voltage present, i.e. it's a volt-free contact. Mercury-wetted relays are pretty unusual these days, though they're still far superior to microswitches on applications like transformer oil and winding temperature instruments which change state one in a blue moon.
RRaghunath
Electrical
Scotty UK, You said Mercury switches are far superior to micro switches.... Appreciate if you could elaborate. Mercury switches close when the mercury moves to bridge electrodes as the switch tilts with movement of pointer in the temperature indicator.
How does the micro switch work. Does it use magnet??
Why do you say micro switch is inferior to mercury switch? Micro switches are recent entry in temperature indicators and I thought they may be superior.
Thanks.
How does the micro switch work. Does it use magnet??
Why do you say micro switch is inferior to mercury switch? Micro switches are recent entry in temperature indicators and I thought they may be superior.
Thanks.
flexoprinting
Electrical
Here is a good picture of typical Mercury switch, Link I kind of miss these thermostats as compared to the programmable ones which complicate things in an office filled with woman in their forties.
Chuck
Chuck
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1
- Moderator
- #9
The mercury switch is often pivoting on jeweled bearings. The micro switch often has a shaft passing through a bushing.
In applications where the switch may sit for years before operating to protect an expensive transformer the mercury switch is more dependable.
Over the years as the switch sits unattended, the shaft of the micro switch may become seized in the bushing due to corrosion or dirt build-up. The jeweled bearings of the mercury switch are much less likely to seize over time.
Atmospheric contaminants may build up on the contacts of a micro switch over the years so that when it is finally called upon to operate the contacts close, but do not pass current.
The sealed mercury contacts are immune to atmospheric contaminants.
Micro switches often have some type of over center spring arrangement to give a snap action to the contacts.
The thermostats with the magnet to give the snap action were typically seen in institutional grade wall mounted thermostats. This action is different from both the micro switch and the mercury switch.
Bill
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"Why not the best?"
Jimmy Carter
In applications where the switch may sit for years before operating to protect an expensive transformer the mercury switch is more dependable.
Over the years as the switch sits unattended, the shaft of the micro switch may become seized in the bushing due to corrosion or dirt build-up. The jeweled bearings of the mercury switch are much less likely to seize over time.
Atmospheric contaminants may build up on the contacts of a micro switch over the years so that when it is finally called upon to operate the contacts close, but do not pass current.
The sealed mercury contacts are immune to atmospheric contaminants.
Micro switches often have some type of over center spring arrangement to give a snap action to the contacts.
The thermostats with the magnet to give the snap action were typically seen in institutional grade wall mounted thermostats. This action is different from both the micro switch and the mercury switch.
Bill
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"Why not the best?"
Jimmy Carter
Thanks Bill.
Hi rraghunath - in my experience the mercury switches are pretty much impervious to the environment in which they're installed, where the microswitches typically fitted to WTI and OTI devices aren't IP67 / IP68 types and they either go high resistance due to contact oxidation or the internal leaf spring corrodes and fails.
The microswitches are typically cam-operated as the indicator needle moves on the dial.
Hi rraghunath - in my experience the mercury switches are pretty much impervious to the environment in which they're installed, where the microswitches typically fitted to WTI and OTI devices aren't IP67 / IP68 types and they either go high resistance due to contact oxidation or the internal leaf spring corrodes and fails.
The microswitches are typically cam-operated as the indicator needle moves on the dial.
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