The operation and maintenance personnel of a certain substation discovered that the temperature of a group of 35kV isolation switch contacts was abnormal, reaching up to 120 ℃. They repeatedly tightened all the bolts of the wiring board, but the temperature did not drop at all. It was only discovered during the final disassembly that the silver plated spring inside the contact finger had been annealed at high temperature, causing the pressure to drop from the standard 200N to less than 50N, resulting in poor contact between the contacts, forming a vicious cycle, and ultimately burning out.
Why is spring pressure more important than bolt tightening?
The conductive circuit of the isolation switch is a "spring compression system", which works similarly to the relationship between a power plug and a socket:
-Bolts (Terminal block screws): only provide static and rigid connections, and cannot compensate for thermal expansion, contraction, and electric force impact.
-Spring (contact finger spring): Provides dynamic and constant contact pressure, which is the key to maintaining stable contact resistance.
-Compensate for metal creep caused by heating
-Resist the vibration caused by short-circuit electric force
-Ensure that each finger contacts evenly
The consequences of insufficient pressure: increased contact resistance → intensified heating → increased spring temperature annealing (softening) → further decrease in pressure → increased contact resistance, forming thermal runaway, ultimately leading to contact melting.
How to measure contact pressure of contacts? (On site practical operation method)
The following methods do not require expensive equipment and can be completed in the maintenance workshop.
Method 1: Spring scale method (most commonly used)
Required tools: Digital push-pull force gauge (range 0-500N), self-made towing hook, fixture
Measurement steps:
1. Disassemble the contact: Remove the contact module from the main pole and clean the contact surface.
2. Installation tool: Fix the contact seat with a fixture. Insert the towing hook into the gap between the contact finger and the contact.
3. Connection measurement: Hook the tension gauge onto the traction hook and slowly apply tension in the opposite direction to the pressing direction of the touch finger.
4. Read data: Record the maximum reading of the tension meter at the moment when the touch finger is just separated from the contact. This value is the contact pressure of the finger.
Measurement standard: Take the average of three measurements. The measured value should not be lower than 80% of the factory value (generally, the main contact pressure standard for 110kV isolation switches is around 200-250N, please refer to the product manual for details).
Method 2: Pressure sensitive test paper method (qualitative judgment)
Required tools: Prescale pressure distribution test paper
Operation steps:
1. Insert a small pressure test strip between the contact finger and the contact surface of the contact.
2. Manually or with tools, restore the contact to its working state and press the test paper tightly.
3. Hold the pressure for a few seconds and then remove the test paper.
4. By comparing the color changes on the test paper with the color chart, the uniformity of pressure distribution can be qualitatively analyzed.
Applicable scenario: Quickly determine whether the pressure between multiple touch fingers is balanced, but cannot measure accurate values.
Solution for Insufficient Stress
Take different measures based on pressure and spring status:
Pressure measurement results | Spring condition | Treatment plan |
≥ 80% of the standard value | Good elasticity, no rust | Clean the contact and apply conductive paste before continuing to use |
50% -80% of the standard value | Slight deformation, decreased elasticity | The spring must be replaced and the wear of the contact finger checked |
<50% of the standard value | Severe annealing, rusting, or fracture | Replace the entire contact assembly and check if the main pole is burned |
Preventive maintenance recommendations
1. Establish a pressure test file: Include contact pressure testing in the standard project for isolating switch overhaul, record each measurement data, and track the trend of spring performance degradation.
2. Regular infrared temperature measurement: At least one infrared general measurement should be conducted every month, and contacts with a temperature difference of ≥ 20K should be given special attention and dealt with in a timely manner.
3. Choose high-quality spare parts: When replacing the spring, it is necessary to choose the original factory or a reputable brand to ensure its high temperature resistance (usually requiring it to withstand high temperatures of at least 150 ℃ for a long time without annealing).
4. Proper use of conductive paste: Apply an appropriate amount of electrical contact grease (conductive paste) evenly on the contact surface of the contact, which can effectively inhibit oxidation, reduce temperature rise, and extend the life of the spring.
Summary: Operation and maintenance mnemonic
To deal with the heating of isolation switch contacts, remember the following mnemonic:
>Don't panic when your hair feels hot, first measure the temperature and determine if it's abnormal.
>Although bolt tightening needs to be done, spring pressure is the king.
>Tension meter, hook, read the value at the moment of separation.
>Less than 80% must be replaced to prevent heat loss and protect the power grid.
Finally, remember: the spring is the "heart" of the isolation switch, and maintaining its pressure is to keep the device healthy. A simple stress test is much cheaper than emergency repairs afterwards.
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