View source for Physics:Wetting current

{{short description|Minimum electric current through a contact to break through the surface film resistance}}
{{redir|Contact fritting|the mechanical wear effect|fretting}}
{{use list-defined references|date=January 2022}}
In electrical and electronics engineering, '''wetting current''' is the minimum [[Physics:Electric current|electric current]] needing to flow through a contact to break through the surface film resistance at a contact.<ref name="McMillan_1999"/> It is typically far below the contact's nominal maximum current rating.<ref name="AAC"/>

A thin film of oxidation, or an otherwise passivated layer, tends to form in most environments, particularly those with high [[Physics:Humidity|humidity]], and, along with surface roughness, contributes to the [[Physics:Contact resistance|contact resistance]] at an interface.<ref name="Zhai_2015"/> Providing a sufficient amount of wetting current is a crucial step in designing systems that use delicate switches with small contact pressure as sensor inputs. Failing to do this might result in switches remaining electrically "open" when pressed, due to contact oxidation.<ref name="TE_2016_Relay"/><ref name="Mairs_2004_Contact"/>

==Capacitor discharge solution==
In some low voltage applications, where switching current is below the manufacturer's wetting current specification, a [[Engineering:Capacitor|capacitor]] discharge method may be employed by placing a small capacitor across the switch contacts to boost the current through contact surface upon contact closure.<ref name="TE_2016_Relay"/><ref name="Stewart_2011"/>

==Sealing current==
A related term '''sealing current''' (aka '''wetting current''' or '''fritting current''') is widely used in the telecommunication industry describing a small constant DC current (typically 1-20&nbsp;mA) in copper wire loops in order to avoid contact oxidation of contacts and [[Engineering:Electrical splice|splice]]s. It is defined in ITU-T G.992.3 for "all digital mode ADSL" as a current flowing from the ATU-C ([[ADSL]] Linecard) via the phone lines to the ATU-R (CPE).<ref name="ADSL2"/><ref name="Bennett_1996_ISDN"/> Carbon brushes develop high resistance glaze when they're used without current flow for an extended period. A special circuit is utilized for turbines and generators to introduce current through the brushes into the shaft to prevent this '''contact fritting'''.<ref name="Kiameh_2003"/>

==Contact cleaner==
[[Chemistry:Contact cleaner|Contact cleaner]] can be applied to the contact surfaces to inhibit the formation of resistive surface films and/or to ameliorate existing films.<ref name="Hack_2017"/>

==See also==
* {{annotated link|Chemistry:Anodic protection}}
* {{annotated link|Physics:Cathodic protection}}
* {{annotated link|Engineering:Coherer}}
* {{annotated link|Physics:Contact protection}}
* {{annotated link|Chemistry:Electromigration}}
* {{annotated link|Engineering:Electrowetting}}

==References==
{{reflist|refs=
<ref name="AAC">{{cite web |url=https://www.allaboutcircuits.com/textbook/digital/chpt-4/switch-contact-design/ |title=Switch Contact Design - Switches - Electronics Textbook}}</ref>
<ref name="Zhai_2015">{{cite journal |author-last1=Zhai |author-first1=C. |author-last2=Hanaor |author-first2=D. |author-last3=Proust |author-first3=G. |author-last4=Gan |author-first4=Y. |title=Stress-Dependent Electrical Contact Resistance at Fractal Rough Surfaces |journal=Journal of Engineering Mechanics |date=2015 |volume=143 |issue=3 |pages=B4015001 |url=https://www.researchgate.net/publication/277981414 |doi=10.1061/(ASCE)EM.1943-7889.0000967 |format=PDF}}</ref>
<ref name="Bennett_1996_ISDN">{{cite web |author-first=Brad |author-last=Bennett |date=1996-12-13 |title=Sealing current on ISDN loops |work=comp.dcom.isdn |url=https://groups.google.com/d/msg/comp.dcom.isdn/lXQx0lNpjR8/CSF3MgG4M1kJ |access-date=2018-05-19 |quote=[…] Wynn Quon: The Bellcore Layer 1 specs talk about "sealing current". This is a low current (1-20mA) DC signal applied to tip and ring. […] It is supposed to reduce oxidation at [[Engineering:Electrical splice|line splice]]s and it provides a troubleshooting aid in the field. […] Brad Bennett: I was personally the researcher that did the sealing current work while at Bellcore […] sealing current does effectively keep a copper loop intact (through a process called [[Chemistry:Electromigration|electromigration]] […] it does work on copper loops which have splices […] for direct copper loops (CO to customer sites) it is ALWAYS suppose to be applied (and is built into the [[Engineering:Line card|line card]]s). For other technologies (e.g. BRITE cards), which synthesize ISDN from 3 DS0 circuits at a subscriber loop cabinet (SLC), I am […] not sure it is a requirement. […] other interesting upshots of […] this work, which I am not certain have ever made it to the public. For example there are certain metal pairs [for] which you definitely do NOT want to use sealing current […] or […] run any continuous DC current (e.g. copper and precious metals). Such contacts (splices) are materially designed to fail if current continually flows in the wrong direction. […] on […] copper wires, sealing current helps to maintain good electrical connections […]}}</ref>
<ref name="Kiameh_2003">{{Cite book |author-last=Kiameh |author-first=Philip |url=https://books.google.com/books?id=7jdDzUZYhPYC |title=Electrical Equipment Handbook : Troubleshooting and Maintenance: Troubleshooting and Maintenance |date=2003-04-11 |publisher=McGraw Hill Professional |isbn=978-0-07-139603-5 |pages=13–9 |language=en}}</ref>
<ref name="Hack_2017">{{cite web |url=https://hackaday.com/2017/02/17/contact-cleaners/ |title=Down and Dirty with Contact Cleaners |date=2017-02-17}}</ref>
<ref name="McMillan_1999">{{cite book |editor-first=Gregory K. |editor-last=McMillan |title=Process/Industrial Instruments and Controls Handbook |edition=5th |publisher=McGraw Hill |date=1999 |isbn=0-07-012582-1 |page=7.26}}</ref>
<ref name="Stewart_2011">{{cite journal |title=The Case of the Intermittent Relay |journal=Design News |author-last=Stewart |author-first=Anthony |date=2011}}</ref>
<ref name="TE_2016_Relay">{{cite web |title=Relay contact life |publisher=Tyco Electronics Corporation (TEC), P&B Relays |location=Winston-Salem, NC, USA |id=13C3236, IH/12-00 |type=Application note |url=http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3236_AppNote&DocType=CS&DocLang=EN |access-date=2018-05-18 |url-status=live |archive-url=https://web.archive.org/web/20180519203336/http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=13C3236_AppNote&DocType=CS&DocLang=EN |archive-date=2018-05-19}}</ref>
<ref name="Mairs_2004_Contact">{{cite journal |author-first=William |author-last=Mairs |publisher=NHP Electrical Engineering Products Pty. |journal=NHP Technical News |title=Keeping in contact |issue=42 |date=September 2004 |id=TNL-42 10/04 14M |url=http://www.nhp.com.au/files/editor_upload/File/TNL/TNL-42.pdf |access-date=2018-05-19 |url-status=live |archive-url=https://web.archive.org/web/20180519204000/http://www.nhp.com.au/files/editor_upload/File/TNL/TNL-42.pdf |archive-date=2018-05-19}}</ref>
<ref name="ADSL2">{{cite book |title=Recommendation G.992.3: Asymmetric digital subscriber line transceivers 2 (ADSL2) |publisher=ITU-T |url=http://www.itu.int/rec/T-REC-G.992.3 |access-date=2018-05-19 |url-status=live |archive-url=https://web.archive.org/web/20180519204439/http://www.itu.int/rec/T-REC-G.992.3-200904-I/en |archive-date=2018-05-19}} [https://web.archive.org/web/20160508062533/https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-G.992.3-200904-I!!ZPF-E&type=items]</ref>
}}

==Further reading==
* {{cite book |title=Ney Contact Manual - Electrical Contacts for Low Energy Uses |author-first=Kenneth E. |author-last=Pitney |edition=reprint of 1st |date=2014 |orig-date=1973 |publisher=Deringer-Ney, originally JM Ney Co. |asin=B0006CB8BC |url=http://www.deringerney.com/techpapers/abstract/?DocumentId=pnk8SYvTdl5PBEYgylFWFjQO5IfWmXnrKiYltdIYKjM%3d}} (NB. Free download after registration.)
* {{cite book |title=Electrical Contacts: Principles and Applications |author-first=Paul G. |author-last=Slade |edition=2 |date=2014-02-12 |orig-date=1999 |publisher=CRC Press, Taylor & Francis, Inc. |isbn=978-1-43988130-9 |work=Electrical and Computer Engineering |series=Electrical engineering and electronics |volume=105}}
* {{cite book |title=Electric Contacts: Theory and Application |author-first1=Ragnar |author-last1=Holm |author-first2=Else |author-last2=Holm |author-link2=Else Holm |editor-first=J. B. P. |editor-last=Williamson |edition=reprint of 4th revised |date=2013-06-29 |orig-date=1967 |publisher=Springer Science & Business Media |isbn=978-3-540-03875-7}} (NB. A rewrite of the earlier "''Electric Contacts Handbook''".)
* {{cite book |title=Electric Contacts Handbook |author-first1=Ragnar |author-last1=Holm |author-first2=Else |author-last2=Holm |author-link2=Else Holm |edition=3rd completely rewritten |date=1958 |publisher=[[Physics:Springer-Verlag|Springer-Verlag]] |location=Berlin / Göttingen / Heidelberg, Germany |isbn=978-3-662-23790-8}} [http://www.gbv.de/dms/ilmenau/toc/176968075.PDF<!-- https://web.archive.org/web/20180520151548/http://www.gbv.de/dms/ilmenau/toc/176968075.PDF -->] (NB. A rewrite and translation of the earlier "''Die technische Physik der elektrischen Kontakte''" (1941) in German language, which is available as reprint under {{ISBN|978-3-662-42222-9}}.)
* {{cite book |editor-first1=Eduard |editor-last1=Vinaricky |editor-first2=Karl-Heinz |editor-last2=Schröder |editor-first3=Josef |editor-last3=Weiser |editor-first4=Albert |editor-last4=Keil |editor-first5=Wilhelm A. |editor-last5=Merl |editor-first6=Carl-Ludwig |editor-last6=Meyer |author-first1=Manfred |author-last1=Huck |author-first2=Eugeniucz |author-last2=Walczuk |author-first3=Isabell |author-last3=Buresch |author-first4=Josef |author-last4=Weiser |author-first5=Lothar |author-last5=Borchert |author-first6=Manfred |author-last6=Faber |author-first7=Willy |author-last7=Bahrs |author-first8=Karl E. |author-last8=Saeger |author-first9=Reinhard |author-last9=Imm |author-first10=Volker |author-last10=Behrens |author-first11=Jochen |author-last11=Heber |author-first12=Hermann |author-last12=Großmann |author-first13=Max |author-last13=Streuli |author-first14=Peter |author-last14=Schuler |author-first15=Helmut |author-last15=Heinzel |author-first16=Ulf |author-last16=Harmsen |author-first17=Imre |author-last17=Györy |author-first18=Joachim |author-last18=Ganz |author-first19=Jochen |author-last19=Horn |author-first20=Franz |author-last20=Kaspar |author-first21=Manfred |author-last21=Lindmayer |author-first22=Frank |author-last22=Berger |author-first23=Guenter |author-last23=Baujan |author-first24=Ralph |author-last24=Kriechel |author-first25=Johann |author-last25=Wolf |author-first26=Günter |author-last26=Schreiner |author-first27=Gerhard |author-last27=Schröther |author-first28=Uwe |author-last28=Maute |author-first29=Hartmut |author-last29=Linnemann |author-first30=Ralph |author-last30=Thar |author-first31=Wolfgang |author-last31=Möller |author-first32=Werner |author-last32=Rieder |author-first33=Jan |author-last33=Kaminski |author-first34=Heinz-Erich |author-last34=Popa |author-first35=Karl-Heinz |author-last35=Schneider |author-first36=Jakob |author-last36=Bolz |author-first37=L. |author-last37=Vermij |author-first38=Ursula |author-last38=Mayer |title=Elektrische Kontakte, Werkstoffe und Anwendungen: Grundlagen, Technologien, Prüfverfahren |language=de |edition=3 |location=Berlin / Heidelberg / New York / Tokyo |publisher=[[Physics:Springer-Verlag|Springer-Verlag]] |date=2016 |orig-date=1984 |isbn=978-3-642-45426-4}}
* {{cite web |title=Overcome Wetting Voltage and Current Limitations With an External Wetting Power Supply |author-first=Martin |author-last=Lee |publisher=Schweitzer Engineering Laboratories, Inc. (SEL) |type=Application note |id=AN20 16-01 |date=2016-01-11 |url=https://cdn.selinc.com/assets/Literature/Publications/Application%20Notes/AN2016-01_20160111.pdf?v=20160406-152853 |access-date=2018-05-21 }}
* {{cite book |title=The Test, Usage and Maintenance of Power Switching Subsystems |author-first=Kevin |author-last=Paton |publisher=Teradyne, Inc. / IEEE |location=North Reading, MA, USA |isbn=978-1-4244-9363-0 |date=2011 |url=http://www.teradyne.com/File%20Library/Defense-Aero/Technical%20Papers/2011/Paton_The-Test--Usage-and-Maintenance-of-Power-Switching-Subsystems_2011.pdf |access-date=2018-05-21 |url-status=live |archive-url=https://web.archive.org/web/20170916192848/http://www.teradyne.com/File%20Library/Defense-Aero/Technical%20Papers/2011/Paton_The-Test--Usage-and-Maintenance-of-Power-Switching-Subsystems_2011.pdf |archive-date=2017-09-16}}
* {{citation |title=Fretting corrosion in power contacts: Electrical and thermal analysis |author-first1=S. |author-last1=El Mossouess |author-first2=N. |author-last2=Benjemâa |author-first3=E. |author-last3=Carvou |author-first4=R. |author-last4=El Abdi |author-first5=L. |author-last5=Benmamas |author-first6=L. |author-last6=Doublet |date=2014 |url=https://hal.archives-ouvertes.fr/hal-01104802/file/Holm%202014%20fin%20(1).pdf |access-date=2018-05-21 |url-status=live |archive-url=https://web.archive.org/web/20180521175550/https://hal.archives-ouvertes.fr/hal-01104802/file/Holm%25202014%2520fin%2520(1).pdf |archive-date=2018-05-21}}

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[[Category:Electrical engineering]]
[[Category:Electrical parameters]]


{{Sourceattribution|Wetting current}}