Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
IEC 61811-50: Electromechanical Telecom Relays – Blank Detail Specification
Key Insight
IEC 61811-50 provides a standardized blank detail specification for electromechanical telecom relays of assessed quality, defining the test schedule, inspection levels, and performance limits that govern relay qualification for telecommunications infrastructure.
IEC 61811-50 provides a standardized blank detail specification for electromechanical telecom relays of assessed quality, defining the test schedule, inspection levels, and performance limits that govern relay qualification for telecommunications infrastructure.
1. Standard Role and Capability Approval Framework
IEC 61811-50, published in 2002, is a blank detail specification (BDS) within the IEC 61811 series that covers electromechanical all-or-nothing telecom relays. The series operates within the IEC Quality Assessment System for Electronic Components (IECQ), which provides a framework for third-party certification of component quality and reliability. Part 50 specifically addresses relays with two changeover contacts (2 CO, DPDT) — the most common configuration for telecom signaling applications.
As a blank detail specification, IEC 61811-50 does not define a specific commercial relay product. Instead, it provides a template that relay manufacturers complete with their specific product parameters. The BDS structure includes: a front sheet for general data (contact arrangement, coil ratings, dimensions), an inspection schedule defining test groups and sample sizes, and performance limits for each test parameter. When completed by a manufacturer, the resulting detail specification becomes the basis for IECQ certification of the relay series.
Engineering Insight: The blank detail specification format enables telecom equipment manufacturers to compare relays from different suppliers on a common basis. Because each manufacturer fills in the same template, parameters such as “operate time at nominal voltage” or “minimum mechanical life” are presented identically across datasheets, eliminating the variability in how different manufacturers might otherwise present performance data.
2. Test Schedule and Inspection Levels
2.1 Qualification and Lot-by-Lot Testing
IEC 61811-50 defines three test categories: A-group (routine, 100% production tests), B-group (periodic sampling tests, daily or weekly), and C-group (qualification tests, performed once on initial design and after major changes). The inspection levels follow ISO 2859-1 (successive lot-by-lot inspection) with AQL (Acceptable Quality Level) values ranging from 0.1% for critical parameters to 1.0% for visual characteristics.
| Test Group | Test Items | Sample Size | Frequency | AQL / Criterion |
|---|---|---|---|---|
| A1 | Dimensions, marking, visual inspection | Full lot (100%) | Every lot | 1.0% |
| A2 | Coil resistance, operate/release voltage | Full lot (100%) | Every lot | 0.65% |
| B1 | Contact resistance, insulation resistance | 20 samples | Weekly | 0.1% |
| B2 | Dielectric strength, timing | 20 samples | Monthly | 0.1% |
| C1 | Electrical endurance at rated load | 10 samples | Qualification only | No failures allowed |
| C2 | Mechanical life (107 operations) | 6 samples | Qualification only | No failures allowed |
| C3 | Environmental (damp heat, cold, vibration) | 6 samples | Qualification only | No failures allowed |
2.2 Critical Performance Parameters for Telecom Relays
Telecom relays differ from general-purpose industrial relays in several key respects:
- Low-level switching capability: Must reliably switch dry circuits (10 mA at 10 mV) without contact contamination issues — this requires gold-plated bifurcated contacts specified in IEC 61811-50.
- High mechanical life: Minimum 107 operations (vs. 105–106 for typical industrial relays) to match the service life of telecommunications equipment.
- Stable contact resistance: Maximum 50 mΩ initial and 100 mΩ after life — tighter than the 100/200 mΩ typical for industrial relays.
- Low coil power consumption: Typically 200–400 mW for telecom relays to enable direct drive from line-card logic without external power transistors.
Design Warning: The low-level switching requirement (10 mA at 10 mV) is one of the most challenging for telecom relay qualification. At these levels, the electrical energy is insufficient to break through surface oxide or sulfide films that form on non-precious metal contacts. IEC 61811-50 mandates that relays use bifurcated cross-bar contacts with gold plating (minimum 0.8 μm on the mating surface) to achieve reliable dry-circuit switching. Using a relay not qualified for low-level switching in a telecom application will result in intermittent open circuits due to contact film resistance.
3. Constructional Requirements and Relay Dimensions
IEC 61811-50 references standardized dimensions for telecom relays, primarily the DIL (Dual In-Line) package format originating from the integrated circuit industry. The standard covers 8-pin, 14-pin, and 16-pin DIL configurations with 2.54 mm pin spacing, along with the more compact SOP (Small Outline Package) variants with 1.27 mm spacing for high-density applications. Key constructional requirements include:
- Sealing: All telecom relays must be hermetically sealed or at least flux-proof to withstand wave soldering and aqueous cleaning processes used in PCB assembly.
- Contact material: Gold overlay on palladium-ruthenium or silver-palladium alloy base for bifurcated contacts, with minimum gold thickness specified in the detail specification.
- Coil insulation: Class B (130 °C) minimum, with Class E (155 °C) recommended for relays operating in elevated ambient temperatures within telecommunications cabinets.
- Pinout conformity: Standardized pin assignments per IEC 61811-1 to ensure socket compatibility across manufacturers.
4. Frequently Asked Questions
Q1: What is the difference between IEC 61811-50 and IEC 61811-1?
IEC 61811-1 is the generic specification that establishes the general requirements, test methods, and quality assessment procedures for the entire IEC 61811 series. IEC 61811-50 is a blank detail specification for a specific relay type (2 CO changeover contacts) within the telecom relay family. The generic specification (Part 1) defines “what to test,” while the blank detail specification (Part 50) defines “what values are acceptable” for that relay category.
Q2: Are IEC 61811 relays still relevant in the age of digital and optical switching?
Yes. While optical fiber carries the bulk of long-distance telecommunications traffic, the “last meter” to subscriber equipment and the internal switching within exchange equipment still require physical metallic contacts for functions such as line testing, ringing injection, and protection switching. Telecom relays according to IEC 61811 remain essential in DSL access multiplexers (DSLAMs), PBX systems, and cellular base station controllers for these metallic interface functions.
Q3: What is the significance of the “bifurcated contact” requirement?
Bifurcated contacts (each moving contact is split into two parallel fingers) provide redundant contact points, dramatically improving reliability at low signal levels. If one contact finger is contaminated or fails to make contact, the parallel finger maintains circuit continuity. For telecom relays specified in IEC 61811-50, bifurcated contacts reduce the probability of intermittent open circuits by a factor of 10–100 compared to single-point contacts under dry-circuit conditions.
Q4: Can IEC 61811-50 relays be used for power switching?
The standard’s relays are designed for telecom signaling (typically 50 V / 0.5 A maximum switching capacity). They are not suitable for power switching applications such as motor control, lighting, or heater loads. Attempting to switch loads beyond the rated capacity will cause contact welding or rapid erosion. For power switching, refer to IEC 61810 (electromechanical elementary relays) or IEC 60947 (low-voltage switchgear) series standards.
