Slim Interface Relays for High-Density DIN Rails
What are slim interface relays? A slim interface relay is a plug-in relay housed in a DIN-rail socket only 6-6.2 mm wide, roughly half the footprint of a standard 12.5-15 mm interface relay, built to the same IEC 61810 and IEC 60947-5-1 base standards as its wider siblings. That width cut lets a panel builder mount two to three times more isolation channels in the same length of rail next to a PLC's digital I/O terminals, which matters once a rack has 64 or 128 points to interface and the enclosure is already tight on space. This article covers what actually makes a relay "slim," the channel-density arithmetic, the contact and coil trade-offs against standard plug-in relays, terminal options, where slim relays belong in a panel, and how Schneider Zelio and ABB CR ranges cover the role.
What Makes a Relay "Slim"?
The relay mechanism inside a slim socket is not a shrunken version of anything. It is the same class of miniature electromechanical relay used in a standard plug-in relay and socket system: a coil, a set of change-over contacts, a plastic cage. What changes is the socket module and how tightly the manufacturer packages the terminal block, retaining clip and LED/suppression module around it. A standard interface relay module runs 12.5-15 mm wide on a TS35 DIN rail; a slim module runs 6-6.2 mm. Halve the width, and you roughly double how many channels fit in the same rail length, which is the entire point of the product.
Narrower is not free. Fitting the coil, contacts, terminals and status LED into 6 mm forces a smaller contact set and finer terminal spacing than a standard-width relay carries, so slim is a deliberate trade of contact capacity for rail real estate. Choose it because the channel count justifies it, not because it looks tidier on the drawing.
Channel Density and the DIN-Rail Power Budget
The math behind "slim saves space" is simple division, but it is worth doing before ordering hardware. Available rail length divided by module width gives channel count.
Formula: Channels per Metre of DIN Rail — Source: manufacturer DIN-rail layout data (module widths per IEC 60715 mounting)
N = L / Wmodule
| Symbol | Description | Unit |
|---|---|---|
| N | number of relay channels that fit on the rail | - |
| L | available DIN-rail length in the enclosure | mm |
| Wmodule | socket module width: 6-6.2 mm slim, 12.5-15 mm standard | mm |
Run the numbers on a 300 mm rail segment and a slim relay fits roughly 48-50 channels against 20-24 for a standard-width relay in the same space. That gap is why slim exists at all next to a 32- or 64-point transistor output card. What it does not change is total coil power. Pack twice the channels into the same length and you roughly double the watts of coil dissipation per rail-metre, even though each individual coil draws only a few hundred milliwatts. Enclosures sized around a standard-density layout can run warmer than expected once slim relays double the channel count in the same footprint — check the cabinet's ventilation and derating figures before assuming the space savings are free.
Contact and Coil Trade-offs vs Standard Plug-in Relays
Squeezing a relay into 6 mm costs contact capacity. Most slim interface relays top out at 1 CO (SPDT), where a standard-width interface relay commonly offers 1 or 2 CO, and a heavier power plug-in relay reaches 4 CO for switching contactor coils directly. Contact current class follows the same pattern: slim types sit at the low end of the AC1 range, standard relays stretch further, and DC breaking capacity falls on all of them as voltage rises because there is no natural current zero-crossing to quench the arc. See relay coil voltages for how 24 VDC, 110 VAC and 230 VAC coil options interact with this same contact-rating ceiling.
Coil suppression still matters at slim width. A DC-coil slim relay driven from a PLC transistor output needs the same freewheel diode a standard relay needs — skip it and the collapse spike takes the driving transistor with it. The suppression element just moves into a smaller clip-in module, or in some designs sits potted inside the relay body instead of a separate accessory.
Terminal Style on Slim Sockets
Screw terminals exist on some slim sockets, but push-in (spring-clamp) terminals dominate this end of the range, and for a practical reason: there is not much finger room at 6 mm, and a torque wrench does not fit where a push-in lever does. Push-in terminals are also faster to wire in bulk and hold up better under panel vibration than a screw that can back off over time. The trade-off is wire-gauge range, which tends to be narrower on a push-in slim terminal than on a full-size screw terminal, so check the conductor size against the terminal's stated range before specifying a run of 50 channels.
What we see in the field: panel builders standardize on push-in for slim relay banks and keep screw terminals for the handful of standard-width relays doing heavier switching elsewhere in the same cabinet. Mixing terminal styles by function, not by preference, keeps wiring crews from guessing which tool to grab.
Where Slim Relays Belong in the Panel
The natural home for a slim relay bank is next to a PLC's digital output or input terminal strip, where each point needs its own isolation and the channel count runs into the dozens. PLC interface relays that isolate and amplify I/O are exactly this use case, and slim width is what makes a 64-point card practical to interface without a second enclosure. The same logic applies in building-automation marshalling cabinets, where a BMS controller's I/O count often exceeds what a standard-width relay layout can fit on the available rail.
Slim is the wrong choice where a single channel switches a heavier load — a contactor coil at several amps, a solenoid bank — because the contact rating runs out before the width savings matter. That is where a standard or power plug-in relay from the broader interface relay range takes over. Matching relay type to load, not just to available rail space, is the actual selection call.
Schneider and ABB Slim Ranges
Schneider's Zelio RSL series is built specifically at slim width for high-density I/O next to a PLC, with push-in terminal options and the same LED/suppression module approach as the wider RXM and RSB interface relays in the same family. ABB's CR-P pluggable interface relays cover the equivalent interfacing role in a compact PCB/DIN footprint, though whether a given CR-P variant is offered at true slim (6 mm) width depends on the current catalogue — this depends on the specific model and generation, so confirm the module width against the datasheet rather than assuming parity with Zelio RSL on that one dimension. Both families share the underlying job: isolate the control signal, amplify it to contact level, and do it on a socket a technician can swap without rewiring.
| Criteria | Slim interface relay | Standard interface relay | Power plug-in relay |
|---|---|---|---|
| Module width | 6-6.2 mm | 12.5-15 mm | 17.5-22.5 mm |
| Typical contacts | 1 CO | 1-2 CO | 2-4 CO |
| Contact current class | Low end of AC1 range | Few amps to roughly 10 A, AC1 | Tens of amps, AC1 |
| Best fit | High channel count next to PLC/BMS I/O | General control interfacing | Contactor coils, motor auxiliaries |
Frequently Asked Questions
What does "slim" mean for an interface relay?
It refers to the DIN-rail socket module width, typically 6-6.2 mm against 12.5-15 mm for a standard interface relay. The relay mechanism inside is the same class of electromechanical device; only the socket and terminal packaging change.
How many slim relay channels fit per metre of DIN rail?
Divide available rail length by module width. A 6 mm slim relay fits roughly 160 channels per metre against 65-80 for a standard-width relay, though enclosure clearances and end-stop hardware reduce the real number somewhat.
Do slim interface relays cost more per channel than standard ones?
Per-channel pricing varies by brand and distributor, but the relevant comparison is total enclosure cost: fewer or smaller cabinets from higher channel density can offset a per-unit price difference. Check current pricing against the specific project's channel count.
Can slim and standard interface relays share the same DIN rail?
Yes. Both mount on the same TS35 rail profile, so a panel commonly runs a bank of slim relays for PLC I/O next to a few standard or power plug-in relays handling heavier switching, as long as the end-stop and wiring layout account for the width difference.
Do slim interface relays support push-in terminals?
Most slim ranges default to push-in (spring-clamp) terminals because screw terminals do not leave enough finger and tool room at 6 mm width. Some manufacturers still offer a screw-terminal slim variant; check the specific socket's terminal type before ordering.
When should I use a standard-width relay instead of a slim one?
Use a standard-width relay whenever the load needs more than 1 CO or a contact current class beyond what the slim relay's datasheet supports — a contactor coil or solenoid, for example. Slim relays are sized for low-current signal isolation, not for switching power loads.
Conclusion
Slim interface relays solve one problem well: fitting a large channel count of signal-level isolation into a rail length that would otherwise run out. They do it by trading contact capacity and terminal room for width, which is the right trade next to a dense PLC or BMS I/O card and the wrong one anywhere a relay needs to switch real current. Run the channel-count math against the rail length, check the enclosure's thermal budget once density doubles, and pick terminal style by wiring volume rather than habit. For the broader selection logic across the full interface and coupling relay family, see the interface and coupling relay engineering guide and browse the current interface and control relays catalogue at Stoklink.
