The fastest way to select the wrong interface relay is to start with only two values: “24 V” and “3 A”. A defensible selection starts with the complete controller signal, field load, environment and installation.
Controller-side checklist
- Is the relay interfacing a PLC/DCS output or a PLC/DCS input?
- Is the signal AC or DC, and what is its nominal voltage?
- What are the available channel current and power?
- What switch-on and switch-off thresholds are required?
- How long is the cable, and what leakage current or cable capacitance may be present?
- Is polarity relevant?
Field-side checklist
- Is the load AC or DC?
- What are the nominal and maximum operating voltages?
- What are the continuous current and inrush current?
- Is the load resistive, capacitive or inductive?
- For a coil, what are its L/R, stored energy, power factor or pull-in VA?
- How often will it switch, and what release time is required?
- What suppression is already fitted?
Relay-output checklist
- Load-voltage operating range
- Continuous current at the actual ambient temperature
- Short-duration surge/inrush limit and pulse duration
- Off-state leakage current
- On-state voltage drop
- Switch-on and switch-off times
- Minimum load, if any
- Inductive-load conditions and external suppression requirements
Isolation and environment checklist
- Input-to-output test voltage and test duration
- Insulation method
- Air/creepage distance
- Overvoltage category and pollution degree
- Operating-temperature range
- Derating for temperature, grouping and rail orientation
- Required EMC, UL or hazardous-area approvals
Mechanical and maintenance checklist
- Plug-in, PCB or multi-channel mounting
- Compatible socket and terminal type
- Terminal numbering and conductor size
- Required fuse and busbar accessories
- Panel space, ventilation and replacement access
How the checklist changes the product choice
For a 24 V DC PLC output driving a modest DC load, the SLO24CR may be a starting point. If the application requires higher DC voltage, current or inductive capability, a model such as the SLO24CRA5 may be more relevant. For an AC coil, start from an AC-output family such as the SLO24TR. These are examples only: the exact datasheet limits must match the real load.
For a field signal going into a PLC, use an input-relay family such as the SLI24CR. The low-current PLC input circuit has different requirements from a field power load, including leakage, thresholds and polarity.
Information to send with a selection request
To obtain a useful recommendation, provide:
- PLC/DCS model and I/O card;
- signal voltage and available channel current;
- load manufacturer and exact part number;
- continuous and inrush current or VA;
- load type and inductive data;
- switching frequency and ambient temperature;
- cable length, suppression and required approvals;
- preferred socket or multi-channel base.
Use the Delcon product range to identify candidates and the technology page to understand the isolation architecture. Final selection must always be based on the current product datasheet and the complete application data.