Dry Contact vs Wet Contact: Reading Japanese Specs
A Japanese supplier's interface spec lists a signal as 無電圧a接点出力 (mudenatsu a-setten shutsuryoku). Another row says 有電圧 (yudenatsu) 100 V AC. Before anyone lands those wires on your PLC input card, you need to know who supplies the voltage, and whether your input can take what arrives.
The short answer: a mudenatsu setten is a dry contact. It is also called a voltage-free or potential-free contact. The contact carries no voltage of its own and only tells the other side whether it is open or closed, so the receiving side supplies the power. A yudenatsu setten is a wet contact: the sending side supplies the power, and a voltage appears on the wires when the contact closes. "無電圧a接点出力" combines that distinction with the a-contact, or normally open (NO), type: "voltage-free NO contact output."
This guide compares the two, explains how they combine with a- and b-contacts, shows how a control panel typically receives each one (24 V DC PLC input or an interposing relay), and ends with a checklist and questions you can send to the Japanese side. The first thing to pin down is always the input spec on the receiving end.
Contents
- Dry Contact vs. Wet Contact: Which Side Supplies the Power
- Comparison Table: Dry vs. Wet Contacts
- How Dry/Wet Combines With a- and b-Contacts
- Deciding at the Interface: Three Questions in Order
- Wiring Diagrams: Receiving a Dry Contact
- The Easy-to-Miss Spec: Contacts for Very Small Currents
- Checklist: Four Items to Agree Before Receiving a Dry Contact
- Japanese Terms You Will See on Interface Specs
- Questions to Send the Japanese Supplier
- Limits of This Guide
- FAQ
- Sources
Dry Contact vs. Wet Contact: Which Side Supplies the Power
A dry contact (無電圧接点, mudenatsu setten, literally "no-voltage contact"; also ドライ接点 or 乾接点, kan-setten) is a contact that carries no voltage even when it closes. The contacts of an electromechanical relay and a plain switch are typical examples. The sending equipment only decides whether the circuit conducts; the power comes from the receiving equipment.
A wet contact (有電圧接点, yudenatsu setten, "voltage contact"; also ウェット接点) also closes a circuit, but the sending side puts its own voltage on that circuit. The receiving side gets a signal at a fixed voltage, so both sides have to agree on what voltage the receiver needs.
Both signals mean "ON when the contact closes." What makes one dry and the other wet is which side supplies the power.
The battery icon is the power supply. The square icon is the input that receives the signal (a PLC, timer, lamp, etc.).
Picture a relay X1 in the sending panel and a lamp in the receiving panel. Pressing a switch on the sending side energizes X1 and closes its contact. With a dry contact, the receiving panel has its own 24 V supply, so current flows from that supply through the X1 contact and lights the lamp. With a wet contact, the 24 V supply sits in the sending panel, in series with X1, and the receiving panel only has the lamp. The circuit looks almost the same; the only difference is which side the supply is on.
A relay's coil and its contacts are electrically isolated. As Omron puts it, a relay contact simply conducts when ON and does not output a voltage. That is why relay contacts, limit switches, pushbuttons, and the auxiliary contacts of magnetic contactors are normally handed over as dry contacts.
Seen from the receiving side, a wet contact is a signal you read as "voltage present or not." In Japanese control panels, these signals are usually 100 V AC or 24 V DC.
Related: Japan Industrial Voltage: 200V 3-Phase and 50/60 Hz Guide
On a Japanese signal list (I/O list), record the contact type on each DI/DO row. For how DI/DO rows are laid out, and for the IEC 61131-2 input types, see DI/DO, AI/AO, PI/PO meaning: reading a Japanese I/O list.
Comparison Table: Dry vs. Wet Contacts
Everything else follows from that one difference: where the power comes from. It decides what the receiving input looks for and what you need to check before connecting.
Which side has the power supply decides what you check on the receiving side.
無電圧接点
有電圧接点
Check contact ratings and input ON/OFF conditions in the datasheet for each device.
How Dry/Wet Combines With a- and b-Contacts
Dry vs. wet answers "does the contact carry voltage?" The a-contact / b-contact split answers a different question: "is the contact normally open or normally closed?" Japanese specs put the two words side by side, which gives four combinations.
A 無電圧a接点 (mudenatsu a-setten) carries no voltage, is open at rest, and closes when the device operates. In English: a voltage-free NO contact. A 無電圧b接点 (mudenatsu b-setten) is closed at rest and opens when the device operates: a voltage-free NC contact.
Japanese specs write the two terms together, e.g., 無電圧a接点 = voltage-free NO contact.
An a-contact is also called NO (normally open) or a make contact; a b-contact is NC (normally closed) or a break contact (Omron FAQ).
Fault signals often use b-contacts because a broken wire then looks like "open = fault," so you notice it. With an a-contact, a broken wire can look like "normal."
Related: Interlock Circuit Diagrams: How to Read Japanese Drawings
Related: How to Read Japanese PLC Programs: Mitsubishi X/Y/M/D
Related: How to Read Ladder Diagrams: Mitsubishi and Omron Basics
Deciding at the Interface: Three Questions in Order
In practice, the receiving input decides what the sending side provides. If the receiver has a no-voltage input (it supplies its own power), send a dry contact. If it has a voltage input (it looks for a voltage), send a wet contact. At the 取合い (toriai, the interface point between two parties' equipment), settle these questions in this order:
- Is the receiving input a no-voltage input or a voltage input?
- If it is a voltage input, what voltage does it need, and is it AC or DC?
- Will the sending contact be passed on as is, or received by a relay and converted?
A voltage input accepts only a defined voltage range as ON. For example, Omron's H5CX-A-N timer (now discontinued) treats 4.5–30 V DC as high level and 0–2 V DC as low level. If the voltage type or level doesn't match, the input either won't respond or the device can be damaged.
The advantage of a dry contact is that it keeps the two sides' power supplies separate. Because a relay's coil and contacts are isolated, a relay can switch a circuit at a different voltage from its coil voltage.
Wiring Diagrams: Receiving a Dry Contact
A control panel usually receives a field signal in one of two ways: directly on a 24 V DC PLC input, or through a relay whose coil matches the other side's voltage.
On a 24 V DC PLC Input
This is the most common arrangement. The panel's own 24 V DC supply powers the other side's dry contact. Current from the 24 V supply runs through that contact and into the input terminal, so the input turns ON when the contact closes.
Current leaves the + terminal of the panel's 24 V DC supply, passes through the other side's contact, the input terminal, the input circuit, and COM, and returns to 0 V.
Simplified for illustration, with COM connected to the 0 V side (Mitsubishi Electric calls this a source input). Whether COM goes to the 24 V side or the 0 V side (sink vs. source) depends on the model and settings, so follow the wiring diagram in the module manual.
The current through the contact is only a few milliamps. On Mitsubishi Electric's RX40C7 DC input module, for example, the rated input current is 7.0 mA (typical, at 24 V DC). Under IEC 61131-2, a Type 1 24 V DC input reads 15 V or more as ON and 5 V or less as OFF; the DI/DO guide covers Types 2 and 3.
Inputs are called sink or source depending on whether COM goes to + or −.
Related: What Does COM Mean? Common Terminals on Japanese Equipment
Through an Interposing Relay (Wet Signals)
When the other side sends a wet signal such as 100 V AC, you cannot connect it straight to a 24 V DC PLC input. Receive it with a relay whose coil matches that voltage, and pass the relay's contact to the input. In English this is usually called an interposing relay.
The 100 V AC signal drives the relay coil. Only the isolated relay contact goes into the 24 V DC input circuit.
Choose the relay coil voltage to match the incoming signal (e.g., a 100 V AC coil for a 100 V AC signal). The PLC side is the same 24 V DC input as in Figure 4; the panel's 24 V DC supply and COM are omitted here.
Omron's FAQ gives the same advice for feeding a 100 V AC signal into a timer: go through a relay. For a no-voltage input, wire the relay contact as is. For a voltage input, put a DC supply in series with the contact. The second case means you add your own supply and turn the contact into a wet signal before passing it on. Either way, the relay keeps the other side's power system separate from yours.
When you pick the relay, check more than the coil voltage: compare the contact rating and the receiving input type against the datasheets as well.
The Easy-to-Miss Spec: Contacts for Very Small Currents
A PLC or counter input draws only a few milliamps, so for the contact it is a micro load. Omron notes that on general-purpose silver contacts, repeated switching builds up an oxide film that can cause poor contact at very small currents. For PLC inputs, Omron recommends contacts designed for micro loads, which use gold.
The number to look for in the datasheet is the minimum applicable load (最小適用負荷, saisho tekiyo fuka). Omron's WL-N limit switch, for example, lists 5 V DC, 1 mA. Check that your input's voltage and current do not fall below it.
Also avoid reusing a contact for signal duty once it has switched heavy current, since that roughens its surface. A rating of several amps is an upper limit on what the contact can carry. It does not guarantee reliable conduction at small currents.
Checklist: Four Items to Agree Before Receiving a Dry Contact
Agree on these four items at the interface before you take a signal from a dry contact, and you will have fewer surprises at commissioning. Write them into the signal list or the interface table so both sides work from the same values.
Recording these four items on the signal list or interface table prevents mismatches.
These are the same items a DI/DO signal list records: dry or wet, voltage and current, a- or b-contact, maintained or pulse.
Maintained (保持, hoji) means the signal stays ON as long as the condition lasts; pulse (パルス, parusu) means it turns ON only briefly. Field wiring from the equipment to the panel may also pass through a junction box (JB) on the way.
Related: Junction Box (JB) on Japanese Drawings: Terminal Box Guide
Related: Pulse vs Status Signals: Momentary vs Maintained Commands
Japanese Terms You Will See on Interface Specs
| Japanese | Reading | English |
|---|---|---|
| 無電圧接点 | mudenatsu setten | Dry contact (voltage-free, potential-free) |
| 有電圧接点 | yudenatsu setten | Wet contact |
| 無電圧a接点出力 | mudenatsu a-setten shutsuryoku | Voltage-free NO contact output |
| a接点 / b接点 | a-setten / b-setten | NO (make) contact / NC (break) contact |
| 無電圧入力 | mudenatsu nyuryoku | No-voltage input (receiver supplies the power) |
| 電圧入力 | denatsu nyuryoku | Voltage input |
| 最小適用負荷 | saisho tekiyo fuka | Minimum applicable load |
| 取合い | toriai | Interface point between two parties' equipment |
Questions to Send the Japanese Supplier
If the spec or signal list leaves any of these open, ask before wiring starts. You can paste the questions into an email as is.
- For each contact signal, is it a dry contact (無電圧接点) or a wet contact (有電圧接点)?
- For wet signals, what is the voltage, and is it AC or DC? What current should we expect?
- For our inputs, we plan to supply [24 V DC] from our panel. Please confirm that your contact's minimum applicable load is at or below this voltage and a current of [a few mA].
- Is each contact an a-contact (NO) or a b-contact (NC)? For fault signals, please confirm which state means "fault."
- Is each signal maintained or pulse? If pulse, what is the pulse width?
- For signals we send to you, does your input expect a dry contact or a voltage? If a voltage, what level, and AC or DC?
Limits of This Guide
- Model numbers and values (Omron H5CX-A-N, H7EC-N, WL-N; Mitsubishi Electric RX40C7) are examples. Other makers and models use different thresholds and ratings, so check the datasheet for the device you actually use.
- Sink/source wiring and COM connections vary by model and settings. Always follow the wiring diagram in the manual.
- Which side supplies the voltage, and whether 100 V AC signals are used, depends on the project and the designers on each side.
- The circuit diagrams are simplified for explanation and are not wiring instructions. Wiring and testing on live equipment must follow your site's electrical safety rules and be done by qualified personnel.
FAQ
What Is a Voltage-Free a-Contact?
It is a contact with no voltage on it that is open at rest and closes when the device operates, i.e., a dry NO contact. The receiving side supplies the power and reads whether the contact is open or closed (Figure 3).
What Is a No-Voltage Input?
It is an input that supplies its own power and reads whether the connected contact is closed or open. Omron's counters, for example, judge ON when the resistance between the terminals is low enough and OFF when it is high enough. The H7EC-N reads 10 kΩ or less as ON and 750 kΩ or more as OFF. A sensor's transistor output (open collector) can also connect to a no-voltage input if it meets these conditions.
Can I Connect a Wet Signal to a No-Voltage Input?
No. A no-voltage input uses its own supply to sense the contact, so applying an outside voltage can cause malfunctions or damage. Receive the wet signal with a relay first and feed the relay's dry contact to the input (Figure 5).
Can I Check a Dry Contact With a Multimeter?
Yes, for open/closed status. Because a dry contact does not output any voltage, you check continuity, not voltage: with the circuit de-energized, use the resistance (continuity) range and see whether the reading changes as the contact opens and closes. Whether the receiving side's supply is present is a separate check, done on the voltage range.
Related: How to Measure Current with a Multimeter (Japanese Testers)
Sources
- Dry contact (無電圧接点) and wet contact (有電圧接点): Panasonic TERASU dictionary, "No-voltage contact" (Japanese)
- Relay contacts are voltage-free and isolated from the coil: Omron FAQ06085 (Japanese), Omron FAQ06042 (Japanese)
- a/b/c contacts and NO/NC/COM terminals: Omron FAQ04785 (Japanese)
- Receiving 100 V AC signals with a relay; voltage input levels: Omron FAQ05645 (Japanese); Omron H5CX-A-N datasheet
- No-voltage input ON/OFF conditions: Omron FAQ00898; Omron H7EC-N datasheet
- Micro loads and contacts: Omron FAQ04783, FAQ02810, FAQ05347 (Japanese); Omron WL-N specifications
- DC input module rated input current: Mitsubishi Electric, MELSEC iQ-R I/O module specifications
- 24 V DC input ON/OFF voltages (IEC 61131-2): Beckhoff Application Note DK9222-0909-0008
Original article in JapaneseThis guide is adapted from a MONOSCOPE article written for readers in Japan: 無電圧接点とは？有電圧接点との違い・a接点との関係と回路図
About the editor
MONOSCOPE EditorFOUNDER & EDITORSpent about ten years at a plant equipment maker and a plant engineering company, proposing, designing, and commissioning equipment, then moved into web marketing. Now a web consultant for B2B manufacturers, helping them explain their products and technology. Licensed in Japan as an Energy Manager and a Third-Class Chief Electrical Engineer.
Has run an engineering blog since 2017 (500+ articles) and a Japanese YouTube channel with about 20,000 subscribers. Each English guide is written from the Japanese shop-floor point of view and checked against the cited standards.
