Wetting current

Key takeaways

Wetting current belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Wetting current to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wetting current from memory before moving on to harder problems.

Reference excerpt

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. It is typically far below the contact's nominal maximum current rating. 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. Providing a sufficient amount of wetting current is a crucial step in designing circuits that use switches with low contact pressure. Failing to do this might result in switches remaining electrically "open" when pressed, due to contact oxidation.

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. While this can work, one must also keep the discharge current on contact closure well within the max current rating for the switch. Just putting a cap across a switch without current limiting resistor will result in cumulative contact damage.

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 mA) in copper wire loops in order to avoid contact oxidation of contacts and splices. 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). 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.

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.

See also Coherer – Early radio wave detector Contact protection – Concept in switch design Electromigration – Movement of ions in an electrical field

References

Further reading Pitney, Kenneth E. (2014) [1973]. Ney Contact Manual - Electrical Contacts for Low Energy Uses (reprint of 1st ed.). Deringer-Ney, originally JM Ney Co. ASIN B0006CB8BC. (NB. Free download after registration.) Slade, Paul G. (2014-02-12) [1999]. Electrical Contacts: Principles and Applications. Electrical engineering and electronics. Vol. 105 (2 ed.). CRC Press, Taylor & Francis, Inc. ISBN 978-1-43988130-9. Holm, Ragnar; Holm, Else (2013-06-29) [1967]. Williamson, J. B. P. (ed.). Electric Contacts: Theory and Application (reprint of 4th revised ed.). Springer Science & Business Media. ISBN 978-3-540-03875-7. (NB. A rewrite of the earlier "Electric Contacts Handbook".) Holm, Ragnar; Holm, Else (1958). Electric Contacts Handbook (3rd completely rewritten ed.). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag. ISBN 978-3-662-23790-8. {{cite book}}: ISBN / Date incompatibility (help) [2] (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.) Huck, Manfred; Walczuk, Eugeniucz; Buresch, Isabell; Weiser, Josef; Borchert, Lothar; Faber, Manfred; Bahrs, Willy; Saeger, Karl E.; Imm, Reinhard; Behrens, Volker; Heber, Jochen; Großmann, Hermann; Streuli, Max; Schuler, Peter; Heinzel, Helmut; Harmsen, Ulf; Györy, Imre; Ganz, Joachim; Horn, Jochen; Kaspar, Franz; Lindmayer, Manfred; Berger, Frank; Baujan, Guenter; Kriechel, Ralph; Wolf, Johann; Schreiner, Günter; Schröther, Gerhard; Maute, Uwe; Linnemann, Hartmut; Thar, Ralph; Möller, Wolfgang; Rieder, Werner; Kaminski, Jan; Popa, Heinz-Erich; Schneider, Karl-Heinz; Bolz, Jakob; Vermij, L.; Mayer, Ursula (2016) [1984]. Vinaricky, Eduard; Schröder, Karl-Heinz; Weiser, Josef; Keil, Albert; Merl, Wilhelm A.; Meyer, Carl-Ludwig (eds.). Elektrische Kontakte, Werkstoffe und Anwendungen: Grundlagen, Technologien, Prüfverfahren (in German) (3 ed.). Berlin / Heidelberg / New York / Tokyo: Springer-Verlag. ISBN 978-3-642-45426-4. Lee, Martin (2016-01-11). "Overcome Wetting Voltage and Current Limitations With an External Wetting Power Supply" (PDF) (Application note). Schweitzer Engineering Laboratories, Inc. (SEL). AN20 16-01. Retrieved 2018-05-21. Paton, Kevin (2011). The Test, Usage and Maintenance of Power Switching Subsystems (PDF). North Reading, MA, USA: Teradyne, Inc. / IEEE. ISBN 978-1-4244-9363-0. Archived (PDF) from the original on 2017-09-16. Retrieved 2018-05-21. El Mossouess, S.; Benjemâa, N.; Carvou, E.; El Abdi, R.; Benmamas, L.; Doublet, L. (2014), Fretting corrosion in power contacts: Electrical and thermal analysis (PDF), archived (PDF) from the original on 2018-05-21, retrieved 2018-05-21

Worked examples

Example 1 — a first encounter with Wetting current

Start with the simplest possible case. Write down what Wetting current claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wetting current before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wetting current ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wetting current

In research: Wetting current appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wetting current in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wetting current is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical engineering, Electrical parameters, so understanding it makes those chapters shorter.
In everyday life: Look for Wetting current outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Wetting current in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Wetting current means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Wetting current out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wetting current in simple terms?

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. It is typically far below the contact's nominal maximum current rating.

Why does Wetting current matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wetting current?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wetting current.