Introduction Miniature intermediate relays are crucial to the field operational reliability of power system automation devices. How to properly select and rationally use miniature intermediate relays, and how to strengthen the design, manufacturing, and screening of miniature intermediate relays to effectively improve their inherent reliability, are urgent issues facing power system automation device manufacturers, operating units, and miniature intermediate relay manufacturers. Therefore, based on the opinions of power system automation device manufacturers, and striving to simulate the actual operating conditions of automation devices as closely as possible, a series of comparative preliminary tests were conducted on some miniature intermediate relays. 1. Series Product Comparison and Preliminary Test 1.1 Grouping and Numbering of Test Relays Group 1: Domestic metal-encased sealed relays (model JHX-1M/A-2Z 024, which can represent JRX-31M, JZX-29M, JMX-13M) Group 2: Imported plastic-encased relays Group 3: Imported plastic-encased relays Group 4: Imported plastic-encased relays Group 5: Imported plastic-encased relays 1.2 Test Conditions and Basic Results All test samples underwent 13 comparative tests, including visual inspection, coil resistance, functional testing, contact resistance, time testing (① excitation at rated excitation value; ② excitation at 80% of rated excitation value), dielectric withstand voltage, insulation resistance, coil temperature rise, high temperature, low temperature, high and low temperature cycling, steady-state damp heat, and load capacity (3 A, 5 A, 8 A at AC 220 V). The basic results are as follows. 1.2.1 High-Temperature Comparison Test: Temperature 70±2℃, coil excited by DC 24 V, maintained for 2 hours. During the pull-in and release tests inside the chamber: Product No. 6 in Group 5 had an operating value of 20.5 V, exceeding the maximum value of 19.2 V, and failed. All other samples passed. 1.2.2 Steady-State Damp Heat Comparison Test: Temperature 40±2℃, relative humidity (95±3)%, maintained for 96 hours. During the external dielectric withstand voltage and insulation resistance tests: Sample No. 2 in Group 5 had an insulation resistance <25 MΩ, indicating dielectric withstand voltage breakdown, and failed. All other samples passed. 1.2.3 Load Capacity Comparison Test: Temperature 40±2 ℃, relative humidity (95±3)%, contact switching load voltage AC 220 V, current from low to high, 3 A, 5 A, 8 A, 1000 switching tests each, switching frequency 30 times/min. During the test: The fourth group of products only passed the 3 A, 1000-cycle test. Under the 5 A load, products 4 and 5 experienced open circuit failure, and product 4 showed deformation. Product 3 failed the withstand voltage test. The 8 A load test was not conducted. The fifth group passed the 3 A, 1000-cycle test. Under the 5 A, 1000-cycle test, contacts 1 and 2 failed due to sticking and deformation. Products 4, 5, and 6 failed the withstand voltage test. All other samples passed. 1.2.4 Conclusion All relays in the first and second groups successfully passed the 13 comparative baseline tests. In the time test, the operation and reset times when excited at 80% of the rated value were within the normal range specified in the original product standard and passed the test. In the load capacity test, all relays passed the 1000 cycles of switching and disconnecting climb-up tests at 3A, 5A, and 8A under harsh environmental conditions (40±2℃, (95±3)% relative humidity) at AC 220 V. The first group (domestic) and the second group (imported) relays performed well and can be recommended as preferred products. 2 Relay Capability Test for DC 220 V Resistive Load Since most power system automation devices use small intermediate relays to directly switch DC 220 V loads, testing and verifying the actual switching capability of various small intermediate relays for DC 220 V loads is particularly important for improving the field reliability of small intermediate relays. Therefore, a series of comparative baseline tests were conducted on various relay products according to the following specifications. 2.1 Test Technical Requirements and Environmental Conditions: Normal laboratory environmental conditions; Load Type: Resistive; Contact Open Circuit Voltage: DC 220 V; Contact Load Current: Start with 0.4 A, 10⁴ cycles. If successful, proceed sequentially with 0.5 A, 10⁴ cycles; 0.6 A, 10⁴ cycles; 0.7 A, 10⁴ cycles; ..., and continue the climb-up test until failure. 2.2 Basic Results: The JAG-5 reed relay has the strongest capacity to withstand a DC 220 V load. JHX-1M (including JRX-31M, JZX-29M, JMX-13M), JHX-2F, JHX-3F, and the improved JZX-39F can all withstand loads above 0.5 A at DC 220 V. For the DC 220 V, 50 W load specified in the product standard, it has an overload capacity exceeding twice the specified load. 3. Rational Selection of Small Intermediate Relays Faced with the complex array of modern relay products, how to rationally select and correctly use them is a crucial issue directly affecting the overall performance and practical reliability of the system. It is also a practical problem that the system designers and developers must closely monitor and prioritize. Given the special nature of power system automation devices during operation, and the potentially severe consequences of accidents, rational selection and correct use require a thorough analysis of the actual operating conditions and technical parameter requirements of the system. Following the principles of value engineering, the technical performance requirements that the selected relay products must meet should be appropriately proposed. Specifically, the following factors can be analyzed item by item to confirm the required level and value range. 3.1 Climatic stress factors mainly refer to temperature, humidity, atmospheric pressure (altitude), coastal atmosphere (salt spray corrosion), sand and dust pollution, chemical atmosphere, and electromagnetic interference. Considering the universal applicability of power system automation devices to natural environments across the country, and taking into account the special requirement of reliable operation over many years, key components of the devices must use fully sealed (metal-encased or plastic-encased, with metal-encased products being superior to plastic-encased products) small intermediate relays with high insulation and strong dielectric properties. This is because only fully sealed relays possess excellent long-term resistance to harsh environments, good electrical contact stability, reliability, and stable load switching capability (unaffected by external climate). 3.2 Mechanical stress factors mainly refer to stress factors such as vibration, impact, and collision. For power system automation devices, the primary consideration is resistance to seismic stress. To improve resistance to mechanical stress, small intermediate relays with balanced armature mechanisms, such as JHX-1M, JHX-3M, and JHX-2F, are recommended. 3.3 Excitation coil input parameter factors mainly refer to over-excitation, under-excitation, isolation between low-voltage excitation and high-voltage (220 V) output, the influence of temperature changes, long-distance wired excitation, and electromagnetic interference excitation. These are all factors that must be carefully considered to ensure the reliable operation of power system automation devices. Excitation according to the specified excitation amount of the small intermediate relay is a necessary condition to ensure its reliable and stable operation. 3.4 Contact output (switching circuit) parameter elements mainly refer to the nature of the contact load, such as lamp load, capacitive load, motor load, inductor, solenoid, contactor (relay) coil, choke load, resistive load, etc.; contact load value (open circuit voltage value, closed circuit current value), such as low-level load, dry circuit load, small current load, large current load, etc. Any automated equipment must accurately determine the actual required load nature and load value, and it is particularly important to select appropriate relay products. The failure or reliability of the relay mainly refers to whether the contacts can complete the specified switching circuit function. If the actual switching load is inconsistent with the switching load specified by the selected relay, reliability is out of the question. 4 Special issues to note when using small intermediate relays 4.1 Key issues when using export intermediate relays Five relays of a certain model failed during the operation of the static relay protection device, and all of them passed the dielectric withstand voltage test.