Product Description

Siemens 3TX7004-1LB00 — SIRIUS Output Interface Coupling Relay, 24V AC/DC, 1 C/O, 6.2mm

Every control panel has transitions — points where the logic level from a PLC output, a safety relay, or a sensor signal needs to drive a load that sits on a different voltage, at a different current level, or that must be electrically isolated from the originating circuit. That transition is exactly what an interface relay handles. Done right with a well-specified component, it's invisible to the system. Done with a marginal part or a misapplied specification, it's a source of field faults that can take hours to trace.

The 3TX7004-1LB00 is a Siemens SIRIUS series output coupling relay, purpose-designed for that transition duty. One changeover contact, 24V AC/DC coil, 6A thermal current rating, 250V maximum switching voltage, screw terminals, and a 6.2mm-wide housing that makes it one of the most space-efficient relay modules in its class. Non-pluggable two-tier construction, monostable switching behaviour, IP20, and a –25°C to +60°C operating range round out the key parameters.

Genuine Siemens manufacture. New original. In stock. Available for immediate worldwide dispatch.

Technical Specifications

Parameter	Value
Siemens Part Number	3TX7004-1LB00
Product Series	SIRIUS 3TX7
Device Type	Output Interface Coupling Relay
Design	Two-tier, non-pluggable (nicht steckbar)
Contact Configuration	1 CO (SPDT changeover)
Contact Guided	No
Switching Behaviour	Monostable
Relay Mechanism	Poled
Coil Voltage	24V AC/DC
Thermal Current	6A
Max Switching Voltage	250V AC/DC
AC-15 Switching Current @ 230V	3A
DC Switching Current @ 24V	1A
DC Switching Current @ 110V	0.2A
Terminal Type	Screw-type
Jumper Socket	No
Width	6.2mm
Height	78mm
Depth	81mm
Weight	~30g
Mounting	DIN rail TH35
IP Rating	IP20
Operating Temperature	–25°C to +60°C (–13°F to +140°F)
Country of Origin	Czech Republic
RoHS	Compliant
Successor Model	3RQ3018-1AB00 (Siemens 3RQ3 series)

What Is an Output Interface Relay — and Why Does It Matter?

The term "output interface relay" has a specific meaning in Siemens' product nomenclature, and it describes something more precise than the general term "relay" suggests.

In a typical PLC output circuit, the output card drives a load through one of its transistor, triac, or relay outputs. But PLC output modules have limits: current per channel is restricted, the number of outputs per card is finite, and the output circuit shares a common reference with the PLC's logic supply. For loads that need more current than the output card can supply, for circuits that need to be isolated from the PLC's logic ground, or for signals that need to be split and routed to multiple downstream circuits — direct connection from the PLC output to the load is either not possible or not safe.

An output interface relay sits between the PLC output and the load. The PLC drives the relay coil at its rated control voltage — in this case 24V AC/DC, which is the standard control voltage in most European and international industrial panel designs. The relay's contact then switches the load circuit independently, providing the isolation, current amplification, and voltage translation the application requires. The PLC output current is limited to what's needed to energise the coil; the load current is handled entirely by the relay contact circuit, rated at 6A thermal current and up to 250V switching voltage.

This architecture has remained the dominant approach in industrial panel wiring for decades because it is simple, transparent in operation, easily diagnosed, and repairable at component level.

The 6.2mm Housing: Why Width Matters in Panel Design

Cabinet space is a real engineering constraint. DIN rail length in a panel is finite, and the terminals, relays, circuit breakers, and other components mounted on it must fit within the available space while leaving room for wiring, cable ties, and maintenance access.

At 6.2mm wide, the 3TX7004-1LB00 occupies less than two-thirds the width of a conventional 11.5mm relay. On a 35cm DIN rail section, the difference between 6.2mm and 11.5mm relays translates to fitting roughly 56 units versus 30 — a significant difference when a control system calls for 20 or 30 interface relay points in a single cabinet section.

The 6.2mm format was Siemens' answer to the need for higher I/O density in relay-per-output architectures, particularly in PLC output expansion applications where each digital output channel gets its own interface relay. For panels with 16, 32, or 64 interface relay points, the difference in total rail width between 6.2mm and standard-width units is immediately visible in the panel layout drawing — and often the difference between needing one DIN rail section and two.

The two-tier design also contributes to density: with control and load wiring on separate tiers, the unit remains accessible for wiring and inspection without the wiring congestion that can occur with single-tier relay terminals in dense installations.

24V AC/DC: One Coil Voltage, Two Source Types

The 24V AC/DC coil specification is worth unpacking, because it represents a design choice with practical installation consequences. Many relay coils are rated for either AC or DC — using a DC-rated coil on an AC supply or vice versa can cause operational problems including relay chatter, overheating, or incorrect switching timing.

The 3TX7004-1LB00's coil accepts both 24V AC and 24V DC without requiring the installer to specify or stock separate versions for different panel types. In a DC-supply panel — the standard for PLC I/O circuits, where 24V DC is generated by the panel's SITOP or equivalent power supply — the coil energises directly from the DC bus. In a panel section where 24V AC is available, the same relay can be used without substitution.

This flexibility reduces the number of relay variants that need to be stocked in a spare parts inventory, and eliminates a class of wiring errors that occur when an AC-only coil relay is inadvertently installed in a DC circuit or vice versa.

Contact Configuration: 1 C/O (SPDT), Monostable, Non-Guided

The single changeover contact (1 CO / SPDT) gives the relay three connection points per contact: a common (COM), a normally open (NO), and a normally closed (NC). In most output interfacing applications, only the NO path is used — connecting the load to the control supply when the relay is energised. The NC path is available without additional hardware for applications needing an indicator when the output is de-energised, for interlocking circuits, or for fail-safe monitoring.

Monostable switching means the relay is spring-return: de-energise the coil and the contact returns to its resting position. This is the standard behaviour for interface relays — bistable latching relays are a separate product category used for different applications.

Non-guided contacts mean the contact set does not mechanically guarantee that NO and NC cannot be simultaneously closed (force-guided, or mirror-contact relays are used where IEC 60947-5-1 Annex L safety relay requirements apply). The 3TX7004-1LB00 is a standard signal-duty interface relay — not a safety relay. For safety function applications requiring force-guided contacts to meet SIL or PL requirements, a different relay type is required.

Non-pluggable construction means the relay body is a single-piece assembly permanently wired via its screw terminals. There is no separate socket — control and load circuits wire directly to the relay terminals. This simplifies the component count and reduces connection resistance, but means the relay cannot be swapped without disconnecting both circuits. For applications where fast field replacement is a priority, pluggable relay modules with separate sockets are available in the 3TX7 range.

3TX7004-1LB00 and Its Successor, the 3RQ3018-1AB00

Siemens has discontinued the 3TX7004-1LB00 as a current-production product, with the 3RQ3018-1AB00 designated as the replacement in the current SIRIUS 3RQ3 relay series. The 3RQ3 series offers similar coupling relay functionality in an updated housing, with the same basic electrical parameters.

Genuine new 3TX7004-1LB00 units remain available from stock. For facilities with existing installations using the 3TX7004-1LB00 throughout, or for spare parts programmes aligned to a specific installed relay type, purchasing genuine new 3TX7004-1LB00 units provides exact form-fit-function replacement — no panel modification, no terminal relabelling, no wiring changes. Where 3TX7004-1LB00 and 3RQ3018-1AB00 units would be mixed in the same installation, the wiring and terminal configurations should be verified for compatibility before mixing relay types on the same rail section.

Frequently Asked Questions

Q: What is the thermal current rating and how does it differ from the switching current?

A: The 3TX7004-1LB00 has a thermal current of 6A — this is the maximum continuous current the contact terminals can carry without exceeding the relay's temperature limits. The switching current (the current the contact actually interrupts when the relay opens) is lower and depends on the load type and voltage. For AC-15 (inductive AC loads) at 230V, the rated switching current is 3A. For DC switching at 24V, it is 1A, and at 110V DC, it is 0.2A. Thermal current is the ceiling for continuous carrying capacity; switching current ratings define how much the contact can reliably make and break at specific voltages and load types. Both limits must be respected in any application.

Q: Can the 3TX7004-1LB00 be used for safety relay applications?

A: No. The 3TX7004-1LB00 is a standard interface relay with non-guided contacts. It does not meet the force-guided (mirror-contact) contact requirements of IEC 60947-5-1 Annex L that are necessary for safety relay and safety circuit applications. For safety-rated applications requiring positively guided contacts and SIL or PLr compliance, Siemens offers dedicated safety relay products in the 3SK and SIGUARD series. The 3TX7004-1LB00 is appropriate for standard signal interfacing, output amplification, and logic-level translation duties outside of safety-rated circuits.

Q: What is the successor or replacement part for the 3TX7004-1LB00?

A: Siemens designates the 3RQ3018-1AB00 (from the current SIRIUS 3RQ3 series) as the replacement for the discontinued 3TX7004-1LB00. The 3RQ3 series is Siemens' current-generation coupling relay family. However, genuine new 3TX7004-1LB00 units remain available as original stock, providing exact replacement capability for existing installations without any panel modification.

Q: What DIN rail size does the 3TX7004-1LB00 mount on?

A: The relay mounts on TH35 standard DIN rail (35mm wide, also described as EN 60715 / TS 35). This is the most common DIN rail size in industrial panel construction and is compatible with the vast majority of control panel mounting rails. The DIN rail can be oriented horizontally or vertically; relay orientation follows the rail orientation.

Q: Is the 3TX7004-1LB00 compatible with jumper bridges for potential groups?

A: No — the 3TX7004-1LB00 does not include a jumper socket and is not compatible with jumper bridges for creating potential groups or linking coil supply across multiple units. Each relay is independently wired via its own screw terminals. For applications requiring grouped coil connections, pluggable relay modules with jumper socket accessories would be the appropriate choice within the 3TX7 family.

Q: What is the maximum ambient operating temperature?

A: The rated ambient operating temperature range is –25°C to +60°C. This covers both cold-climate storage and startup conditions and the elevated ambient temperatures found in loaded electrical panels during summer operation. Panel ambient temperature — the air temperature inside the cabinet at the relay mounting location — should be used for this assessment, not the external ambient temperature, as heat from other components in the panel can raise the local ambient meaningfully above the external air temperature.

Q: How many relays can fit on a standard 35cm DIN rail section?

A: At 6.2mm width per relay, approximately 56 units can be mounted on a 350mm rail section (less the end anchors, which typically occupy 10–15mm at each end, giving approximately 49–52 usable slots). This is one of the key reasons the 6.2mm format was developed — for high-density relay output assignments where 32 or more interface relay points are required in a single panel section.

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