Merely touching two pieces of metal together does not provide a reliable electrical connection. Oxides, dust, films, pitting, and other debris can increase contact resistance. Contact resistance in relays and switches is improved by fritting, which is defined as breaking through the contaminants. The literature associates fritting with the concept of wetting current. In practice, wetting current is an important consideration for long-term reliability as it drives the level of fritting.
Reliability requires breaking through the accumulating crud on the contacts.
This engineering brief demonstrates the principle using a 23-year-old relay. It then classifies the problem as a programming consideration for modern industrial automation digital inputs (Figure 1). This old part represents the relays and switches in your facility. We explore how to design in reliability for those low-power dry contacts that are expected to operate flawlessly after sitting for many years.
Key Takeaways
- Stop thinking about switches and relays as ideal elements. You should care because wetting current is a part of long-term equipment reliability.
- Modern industrial components have turned wetting into a programming decision.
- The 2 mA current did not materially improve measured contact resistance.
- The 24 mA wetting reduced measured contact resistance by nearly three orders of magnitude.
- Initially tapping the relay would cause observable changes. After fritting, the changes disappeared.
- The effects of wetting are not easy to measure as old corroded parts are required for an effective demonstration. Also, once they are “improved” they stay improved for a period of time.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Understand It → Applied Theory
Difficulty: 
Technician — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey, Author bio.
Last update: 22 Sep 2026
Figure 1: Image of the Analog Devices AD74413R installed on a MIKROE-3861 Click board.
Contact Resistance Demonstration
We begin with a 23-year-old relay. I will not divulge the brand other than to say that it is a well-known relay that would be immediately recognizable if a picture were included. I do not want you to leave thinking that brand X relays are unreliable. Instead, the elements of age are working against all contacts. Some are better than others. In fact, this is one of the reasons for golden contacts.
Initial Trial
The initial experiment was conducted by measuring the voltage drop across a set of relay contacts. The circuit is shown in Figure 2. In this case, the power supply was set to 2 VDC.
Observations:
- Gently tapping the relay with a screwdriver would cause the voltage to change.
- When toggled, the measured voltage would change as shown in Figure 3.
In both cases, the voltage was constantly changing and not appreciably trending to the desirable near-zero VDC (zero resistance). Stated another way, the contact quality was not improving.
Observe that the applied voltage is significantly greater than the voltage drop across the contact. Consequently, we can simplify the contact resistance calculation by assuming current is approximated as 2 VDC / 1 kΩ ≈ 2 mA. We can then calculate the contact resistance as:
R_{Contact} = \dfrac{V_{Contact}}{I_{Nominal}}
The starting contact resistance is approximated as
R_{Contact_Initial} = \dfrac{0.152 V}{2 mA} = 76\ Ω
Figure 2: Wiring diagram for the contact wetting experiment.
Figure 3: Initial results for the relay contacts showing inconsistent contact results.
Contact Wetting
The experiment continued by setting the load-side power supply to 24 VDC to yield a 24 mA current. The results are shown in Figure 4 where we see the voltage drop trend toward the ideal 0 VDC condition. A clean trendline is shown from about 0.1 VDC to 0.001 VDC. Also note that the screwdriver-tap-induced fluctuations essentially disappeared.
Note that the relay had been toggled dozens of times by this point of the experiment. This strongly suggests that the observed improvement was not merely the result of operating the relay.
Tech Tip: Tapping things with a screwdriver is called percussive maintenance. Also known as “bang on it till it works.” If this describes your equipment, you have some work to do. Unfortunately, we have all been there. These types of problems may go away for a few days, months, or even years. However, it’s been my experience that they come back with a timeline determined by Murphy. Tap, tap, tap…
The contact wetting described in this note is baked into most PLCs. This is a surprising discovery as many PLCs have an unexpectedly low input resistance. Be sure to research the IEC 61131-2 standard. For example, a PLC’s input screw terminal may be rated as “IEC Type 1 sink.” As a quick assignment, research the distinction between a Type 1 and a Type 3 IEC input. Hint: one implies gold.
R_{Contact_Final} = \dfrac{0.001 V}{24 mA} = 0.04 Ω
Figure 4: Improved relay contact performance with a wetting current of 24 mA .
Programmable Wetting
Contact wetting current is a programmable setting. As an example, consider the Analog Devices AD74413RBCPZ-RL7. This is a multipurpose programmable interface that can operate as a variety of analog and digital I/O configurations. The DIGITAL INPUT, LOOP POWERED mode is directly related to our conversation about contact wetting.
We normally think about industrial automation inputs in terms of 24 VDC. Figure 5 from the AD74413R suggests otherwise. Instead of 24 VDC from the primary supply, the loop power (blue dashed) passes through the AD74413R and through the externally mounted 100 Ω sense resistor. The state of the switch contact (far right) is determined by the voltage drop (monitored current) measured across the sense resistor.
This relates back to our contact wetting discussion as the current is programmable. It may be set anywhere from 0.5 to 24.5 mA with a 4.6 VDC minimum headroom at 25 mA.
Figure 5: Block diagram of the AD74413R with highlights for the digital input, loop powered mode.
Technician Complications
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This is unexpected for a technician trained to think in terms of 24 VDC good, 0 VDC bad. The measured voltage will be about 5 VDC when the contact of interest is open and about 0 VDC when the contact is closed. This is certainly unexpected for the technician looking for a 24 VDC signal.
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As suggested by the opening experiment, higher contact currents are generally more desirable. Our experiment strongly suggests that 2 mA is insufficient while 24 mA showed a measurable improvement.
Tech Tip: A review of PLC specifications shows that wetting current conditions are baked into the screw-terminal inputs. For example, a modern name-brand PLC has 6 mA (nominal) input current for a high-speed input and a 4 mA (nominal) input for a standard screw-terminal input connection.
I trust that the PLC manufacturers have done their homework and carefully considered the wetting current. To do otherwise is to risk unreliable long-term equipment operation. At the same time, the current should be set as low as possible to reduce energy consumption. Like many engineering decisions, these statements are held in tension. We must balance fritting against energy consumption.
On a closely related note, refer to my previous article to explore the screw-terminal input resistance associated with a two-wire proximity sensor. Once again, the PLC’s non-zero input resistance is a stumbling block for the novice.
Next Steps
To learn more about input switching, take a look at the datasheet for the Analog Devices ADE1201ACCZ. Initially, this looks like an isolated input stage for an industrial automation block. Look closer. Instead of providing a steady-state contact wetting current, this device may be programmed to produce a pulse up to 205 mA before returning to a lower current such as 6.3 mA (programmable max). This gives the initial fritting jolt but then settles to a steady-state energy-saving current. This pulsed operation may be especially suited for a low-power dry-contact relay that spends most of its life waiting for something to happen.
I would be remiss if I didn’t mention the importance of contact material. Gold plating resists oxidation and therefore does not require as much fritting effort to provide a low resistance contact.
Tech Tip: Don’t be swapping relays in your control panel. The gold layer is thin and may be vaporize if switching a high energy inductive load. A careless relay swap could put a relay with pitted contacts into a slot where only pristine gold-plated contacts belong.
Equipment reliability will suffer.
About this Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.