View source for 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 dmy dates|date=January 2022|cs1-dates=y}}
{{use list-defined references|date=January 2022}}
In [[electrical and electronics engineering]], '''wetting current''' is the minimum [[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 [[humidity]], and, along with surface roughness, contributes to the [[contact resistance]] at an interface.<ref name="Zhai_2015"/> Providing a sufficient amount of wetting current is a crucial step in designing [[Control engineering|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 [[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 [[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]] ([[Customer premises equipment|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==
[[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|Anodic protection}}
* {{annotated link|Cathodic protection}}
* {{annotated link|Coherer}}
* {{annotated link|Contact protection}}
* {{annotated link|Electromigration}}
* {{annotated link|Electrowetting}}
* {{annotated link|Switches#Wetting current|Wetting current in switches}}

==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 [[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 [[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 [[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-link1=Ragnar 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-link1=Ragnar Holm |author-first2=Else |author-last2=Holm |author-link2=Else Holm |edition=3rd completely rewritten |date=1958 |publisher=[[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=[[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]]
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