To judge the quality of discharge tubes (such as gas discharge tubes, varistors, etc.), it is necessary to combine their working principles and common failure modes. Here is a set of original, logically clear, and fluent language judgment methods:
Core principle: The core function of a discharge tube is to maintain high impedance (insulation state) under normal voltage, and instantly become low impedance (conducting state) when there is abnormal overvoltage (higher than its nominal voltage), to discharge amplified current to protect subsequent circuits. Failure usually manifests as:
Short circuit: Maintaining low impedance even at normal voltage, causing circuit misoperation or burnout.
Open circuit: unable to conduct under overvoltage, losing its protective function.
Parameter degradation: such as triggering voltage increase or decrease, slow response time, decreased current carrying capacity, etc., although not completely ineffective, the performance is insufficient.
Judgment method (from easy to difficult, from routine to professional):
1、 Routine appearance and basic electrical inspection (suitable for preliminary judgment)
Visual inspection:
Observation of appearance: Check for obvious physical damage to the discharge tube casing, such as cracks, breakage, burnt marks, discoloration (especially blackening around the metalized area or electrode), broken pins, or severe corrosion. If there is obvious damage, it can be basically judged as damaged.
Check the model and parameters: Confirm whether the discharge tube model matches the design or replacement requirements, and verify key parameters such as voltage and current.
Measurement of multimeter resistance range (static insulation resistance):
Method:
Place the multimeter in the highest resistance range (such as 20M Ω or higher).
Ensure that the tested discharge tube is completely disconnected from the circuit (at least one end is disconnected), and there is no residual charge at both ends of the discharge tube (which can be discharged by short circuiting the pins).
Connect the multimeter probes to the two electrodes of the discharge tube.
Judgment:
Normal: It should display a very high resistance value (close to or exceeding the upper range limit of the multimeter, usually displaying "OL" or "1"). This indicates that under normal voltage, its insulation performance is good.
Short circuit: The displayed resistance value is very low (close to 0 Ω). Indicating that the insulation has been damaged and cannot be restored.
Open circuit: This method cannot directly determine the failure of an open circuit (as it is normally high resistance). Further testing is required.
Leakage (deterioration): Display a certain resistance value (such as several hundred kiloohms to several megaohms), far below the normal value (which should be above several tens of megaohms). Indicating that the insulation performance has severely deteriorated and there is a risk of leakage current, it should be considered as a defective product.
Megohmmeter measurement (more accurate insulation resistance):
Method: Use a megohmmeter (shaking table or electronic), apply a test voltage of 500V or 1000V DC (be careful! Confirm that the tested equipment can withstand this voltage), and measure the insulation resistance between the two poles of the discharge tube.
Judgment: The measured value should be much higher than the range of the multimeter. The specific qualification standards need to refer to the device specification book (usually requiring>100M Ω or even>1G Ω). If it is lower than the specification requirements, it will be judged as poor insulation or deterioration.
2、 Function and parameter verification (requires professional equipment/environment, cautious operation)
Important safety reminder: The following tests involve high voltage or high current and must be operated by personnel with corresponding qualifications and safety knowledge in a safe environment, using professional equipment and strictly following operating procedures. Non professionals are not allowed to try!
DC trigger (breakdown) voltage test:
Principle: Measure the minimum DC voltage required for a discharge tube to transition from an insulating state to a conducting state.
Method: Use an adjustable high-voltage direct current power supply (with current limiting protection) to slowly increase the voltage applied to both ends of the discharge tube while monitoring the current. When the current suddenly increases sharply (reaching a set threshold, such as 1mA), record the voltage value at this time.
Judgment:
The measured trigger voltage should be within the range specified in the specifications (such as ± 10% or ± 20% of the nominal voltage).
Too low: may cause misoperation under normal voltage.
Excessive: It may not be able to act in a timely manner under expected overvoltage, losing its protective function.
Unable to trigger: Even if the voltage rises very high (far beyond the nominal value), there is no sign of conduction, and it is judged as an open circuit failure.
Leakage current test (at specified voltage):
Principle: Measure the small current flowing through the discharge tube at a DC voltage slightly lower than its nominal trigger voltage (such as 80% of the nominal voltage).
Method: Apply a specified voltage and measure the current using a high-precision ammeter (microampere meter or picoammeter).
Judgment: The measured leakage current should be much smaller than the maximum value specified in the specification book (usually at microampere or even nanoampere level). A large leakage current indicates a deterioration in insulation performance, and the device may generate heat or be unstable.
Pulse impact test (current capacity verification - strictest, simulating actual working conditions):
Principle: Apply simulated lightning current or operating overvoltage waveform (such as 8/20 μ s, 10/350 μ s) that meets specific standards (such as IEC 61643-1) to the discharge tube to verify whether it can reliably operate and discharge the rated impulse current for the specified number of times without damage.
Method: Use a professional surge generator (surge current generator) and strictly set the waveform, amplitude, and frequency according to the standard. Measure the changes in DC trigger voltage and insulation resistance before and after testing.
Judgment:
Is the action reliable: Each impact should trigger conduction normally.
Parameter change: After testing, the triggering voltage change should be within the allowable range (as specified in the specification, such as ± 10%), and the insulation resistance should not significantly decrease (such as not less than 50% of the initial value).
Appearance: After testing, the appearance should be undamaged.
If the action fails, the parameters exceed the tolerance, or the appearance is damaged, it is determined that the flow capacity is insufficient or ineffective. This is a key test to verify the ultimate performance of the discharge tube.
3、 Comprehensive judgment and precautions
Priority order: Usually, visual inspection and multimeter/megohmmeter resistance testing are performed first for quick screening. If high reliability is suspected or required, conduct DC trigger voltage and leakage current tests. Pulse impact testing is mainly used in research and development, type testing, or high reliability requirements.
Safety first: High voltage testing carries extremely high risks! Be sure to use insulated tools, maintain a safe distance, follow the equipment operation manual, and preferably perform on a well grounded test bench.
Reference specifications: All judgment criteria (trigger voltage range, leakage current limit, impulse testing requirements, allowable parameter changes) should be based on the official specifications provided by the device manufacturer.
Deterioration failure: Even after passing static resistance and trigger voltage tests, discharge tubes that have undergone multiple impacts (even if they have not reached the nominal limit) may still experience performance degradation (trigger voltage drift, decreased current carrying capacity). After critical applications or multiple lightning strikes, it is recommended to consider them as potential defective products for replacement.
The difficulty of open circuit failure: Static testing cannot detect open circuit failure. The most reliable verification is the DC trigger voltage test (to see if it can be triggered) or observing the action during the pulse impulse test.
The particularity of varistors: The determination method of varistors is similar (appearance, static resistance, leakage current, varistor voltage V1mA test, pulse impulse test). Its leakage current test is more commonly used, and the voltage V1mA (the voltage at both ends when 1mA DC current flows through) is its core parameter, which is equivalent to the DC trigger voltage of a gas discharge tube.
Summary and judgment process:
Appearance: Is there any obvious damage? → Bad.
Measure resistance (high resistance range):
Short circuit (resistance ≈ 0 Ω)? → Bad (short circuit).
Serious leakage (resistance significantly lower than normal/specification)? → Bad/deteriorated.
High resistance (normal)? → Enter the next step.
Measure the DC trigger voltage (if possible/necessary):
Beyond the specification range? → Bad/deteriorated.
Can't trigger (open circuit)? → Broken (open circuit).
Within the specification range? → Preliminary judgment is good.
Leakage current measurement (when high demand/suspicion):
Exceeding the maximum specification value? → Bad/deteriorated.
(Most reliable/type test) Pulse impact test:
Action failure, parameter deviation, appearance damage? → Poor/insufficient flow capacity.
Core conclusion:
The most basic "good" standard: intact appearance+extremely high static insulation resistance (close to infinity).
A more reliable "good" standard: basic standard+DC trigger voltage within nominal range+leakage current meets specification requirements.
The "good" standard for comprehensive verification: above+can reliably withstand the specified number and amplitude of pulse surge currents, and the parameter changes are within the allowable range.
Any test that clearly does not meet the specifications or physical damage is observed can be judged as bad.
Always remember the safety operating procedures, especially when conducting high-voltage testing. For non professionals, visual inspection and multimeter high resistance testing are relatively safe and effective preliminary judgment methods. If there are any questions or concerns about critical protection, it is recommended to replace the product with a new one or have it tested by a professional.