Routine test for electrical performance of plug power cord

Why did you talk about this topic? In fact, it's simple and depressing. I thought this was originally a very simple problem, but I visited so many factories and found that quite a few engineering and quality control technicians were also unable to clearly understand the purpose, principle, and parameters of electrical testing; Causes and consequences of adverse events; How to use the instrument correctly, etc. Therefore, taking the opportunity of this exchange meeting, I would like to share my own experiences over the years with you. I also hope that colleagues and advanced critics can make corrections.
The most commonly used test items for electrical routine inspection of plug power cords are:
Generally, it includes: polarity (line position, phase sequence), conduction (on/off, continuity), inter pole insulation resistance (internal insulation), external insulation resistance (external insulation,) inter pole withstand voltage (internal voltage, internal high voltage, inter line high voltage, hit), external withstand voltage (external voltage, external high voltage, skin), grounding resistance (some customers test all conductors with high current), etc.
1、 Explanation of terms:
Conduction: Conduction indicates whether the power cord can conduct current. The explanation in UL is that electric current continuously flows through the entire length of conductors and connectors.
For power lines, conduction means that plugs, wires, connectors, or tail connectors can continuously pass current.
Polarity: Polarity indicates the correct connection of the wire to the connector.
For power lines, polarity means the correct connection of phase line L, neutral line N, and ground line E equal to the corresponding color, wire stripe, etc. Of course, for some products in some countries and regions, phase and zero lines are not mandatory, except for the ground wire connection requirements of green or yellow/green.
Insulation resistance: Apply a DC voltage to the dielectric, and after a certain period of polarization, the resistance corresponding to the leakage current flowing through the dielectric is called insulation resistance. That is, the quotient of the DC voltage applied between the two electrodes in contact with the insulator or sample divided by the current flowing through the two electrodes. It depends on the volume resistance and surface resistance of the sample.
For power lines, the insulation resistance is divided into internal and external parts: between all connected poles and the body (externally insulated), and sequentially between each pole and all other poles connected to the body (inter pole insulation). Note here that the term "body" includes easily accessible metal parts, external mounting screws, grounding terminals, grounding pins, and metal foils that come into contact with the outer surface of easily accessible parts made of insulating material (except the mating surface). It can be seen that the grounding pin is also considered a part of the body. Before, many people said that the ground wire should also be tested for external insulation, which is not up to standard.
Electrical strength: UL refers to the ability of dielectric to withstand voltage. Some people also describe it as insulation strength - a numerical value indicating the ability of insulating materials to withstand breakdown voltage, called insulation strength (also known as breakdown strength). The types of breakdown are: electrical breakdown, thermal breakdown, and electrochemical breakdown. Insulation breakdown is considered to have occurred when the current caused by applying the test voltage rapidly increases in a runaway manner, that is, the insulating material has no limited current. Under the action of high voltage, the destructive discharge of gas or liquid media along the insulating surface is called flashover.
For power lines, we are commonly referred to as high voltage or withstand voltage testing. This is exactly the same as the test location for insulation resistance, and is divided into internal and external parts. Like insulation resistance, ground wires do not need to be tested for exposed copper.
Grounding resistance: This term is not very clearly defined. Generally speaking, grounding resistance refers to the resistance encountered when current flows from a grounding device to the ground and then flows through the ground to another grounding body or spreads to a distance. It includes the resistance of the grounding wire and the grounding body itself, the contact resistance between the grounding body and the ground resistance, and the resistance of the ground between two grounding bodies or the ground resistance from the grounding body to an infinite distance.
For power lines, the ground resistance we refer to only refers to the total resistance of the plug terminals and wires.
2、 Purpose of the test:
Conduction: This is to verify whether the power cord can transmit current. Unable to transmit current, it loses its function as a power cord. However, generally speaking, open circuit only causes electrical products to fail to work, and generally does not cause major safety accidents.
Polarity: This is to confirm the safety of the power cord against electric shock and the functionality of certain products (such as the direction of rotation of the motor, inability to use electrical appliances, etc.). In particular, if the polarity of the ground wire is incorrect, it will directly electrifie the surface of the electrical product, causing personal injury. In theory, the incorrect connection of phase and neutral lines may also lead to electric shock (the power supply entering the appliance cannot be cut off when the switch is disconnected), and may also lead to some appliances not working properly. However, for some countries, when the product design is sufficiently safe, it is not ruled out that the mutual positions of the phase and neutral lines are not limited, which is often referred to as non polarity.
Insulation resistance: Insulation resistance is a physical quantity that measures the insulation performance of a medium, and can also indirectly determine whether the internal structure design is reasonable or not. Changes can be caused by moisture, heat, aging, surface contamination, mechanical pressure, deformation, etc. If the insulation resistance is too small, during normal use of the electrical appliance, there may be an increase in electrical leakage, thermal degradation of insulation materials, reduced anti pollution ability, and normal voltage resistance, leading to early product failure and safety hazards.
Electrical strength: Electrical strength measures the ability of electrical products to withstand overvoltage (such as surge voltage), insulation design (such as thickness, electrical clearance, and creepage distance), material selection (incorrect or poor quality materials), internal wiring, and the quality of processing, assembly, and transportation links. It can detect potential defects in products and reduce the early failure rate of products.
Grounding resistance: The grounding resistance is a measure of the low resistance performance of electrical product grounding. In addition to eliminating induced electricity and reducing electromagnetic interference, its most important purpose is to protect the human body from the threat of electrical leakage. If there is a correct grounding and a low resistance, when the housing of an electrical product is energized, the voltage will be directly shorted to the ground and will not form a high voltage. The human body will not be electrocuted when touched. This is a very important parameter. But why is testing rarely required in the power cord industry? My personal understanding is: 1. The difficulty of testing is relatively high (for example, BS1363 requires 60 seconds); 2. Installing the power cord on the entire machine will also test the ground resistance of the entire machine.
3、 Test method, parameters, and judgment:
Conduction: In UL standards, conduction is defined as the continuous flow of current through the entire length of a conductor and connector. That is, if there is no current flowing, it is not considered to be conductive. Currently, in the power cord industry, there are generally two methods to test continuity: 1. Loop method, which allows current to flow through a conductor and generate acoustic, optical, and electrical signals. This method fully meets the requirements of the standard, and there is no doubt about the measurement results; 2. Capacitance method, which uses the principle of measuring the capacitance between two wires and comparing it with the instrument's set capacitance to determine whether the power cord is interrupted within a certain length, or whether the total length is lower than the set value. This method is not recognized by the standard because there is no current flowing through the conductor; If the middle of the power line is interrupted, it often cannot be detected due to improper adjustment of the instrument; If there is an interruption at the end, it is almost impossible to detect. Although the loop method meets the standards, the speed of testing, especially for single ended testing, appears to be particularly slow; The capacitance method is particularly fast (especially for non polarity requirements).
Based on our experience, the capacitance method has no problem with the reliability of single ended testing (A-end, single ended) without polarity requirements, if the instrument is properly adjusted; Of course, if there are terminals attached to the other end that are prone to open circuit phenomena, it is recommended to use the loop method instead. If two end testing (AB end, double end) or products with polarity requirements are required, loop testing is recommended.
Poor product conduction is often caused by the following factors:
Broken wires due to riveting and pressing, broken or sliding wires due to molding and rubber stamping, and broken wires due to the wire itself (process collision or self connector). However, it should also be noted that misjudgment often occurs due to poor contact of test fixtures. When using the capacitance method, the set length of the instrument is too long, which can misjudge a good product as a defective product; Conversely, if the set length is too short, it is easy to misjudge defective products as good ones.
How long should the conduction test take? Probably few people ask this question. Although the standard does not directly mention this parameter, IEC60884-1:2002 provides an indirect explanation of this issue. I will explain it in the "polarity" section below.
Polarity: The testing of polarity seems very simple. Generally, it can be tested and verified using electronic circuits, even simple bulbs, buzzers, etc. However, unreliable testing instruments, methods, and work habits can have dire consequences.
First of all, how long should the polarity test take? We asked the question above, how long does the continuity test take? According to IEC60884-1 and VDE0620:
A.2 Polarity system, phase line (L) and neutral line (N) - Correct connection For polarity systems, a safe extra-low voltage SELV of not less than 2 seconds shall be applied for testing.
A.3 Test the continuity of grounding by applying SELV for not more than 2 seconds.
Note: If there is an automatic timer, 2 seconds can be reduced to no less than 1 second.
What is the relationship between polarity and conduction test time? In fact, the polarity test is also to check the performance of conduction, and also requires current to flow through the conductor, which is exactly the same as conduction. The requirement of no less than 1 second is to confirm the stability of the test results. Too short a test time may not confirm "continuity" due to poor contact, transient interruptions, etc. These are the items for the VDE factory to review and confirm the instruments. But I have hardly seen any manufacturer test it for so long.
In addition, the test voltage requirement is SELV (not greater than AC42.4V). In addition to ensuring personal safety, low voltage will also prevent arcing between previously disconnected wires and display continuity. You may also have seen the effect of high-voltage discharge.
Poor polarity of the product is often caused by the following factors: incorrect wiring during terminal riveting, incorrect mounting of the internal rack, incorrect insertion of the mold, and short circuit between wires (of course, poor insulation or voltage resistance will be detected in subsequent tests).
Insulation resistance: This is a controversial issue, which is whether to conduct insulation resistance testing on the production line. Based on national standards and factory inspection requirements, except for the slightly special requirements of ASTA (which will be discussed below), there is no requirement to test this project. Therefore, my understanding is that insulation resistance belongs to the category of type test, not a routine test item. According to IEC60884-1, the insulation resistance is conducted immediately after the moisture resistance test, which cannot be a routine test. Its provisions are as follows:
The insulation resistance shall be measured with a DC voltage of about 500 V, and the measurement shall be conducted 1 min after the voltage is applied.
The insulation resistance shall not be less than 5 M Ω
For plugs, the insulation resistance should be measured sequentially on the following components:
a) Between all the connected poles and the body,
b) Between each pole and all other poles connected to the body in turn;
Note: While wrapping the outer surface of the insulating material component with a metal box or placing the metal box in contact with the inner surface of the insulating material component, press the metal box into a hole or groove with an unobvious force using a knuckled test finger.
Note that due to the DC current applied to measure the insulation resistance, there is a charging process. To obtain a stable reading, it is necessary to have sufficient charging time, that is, to take a reading after 1 minute. This is one reason why it cannot be included in routine tests.
Another important reason is that in routine tests, for reasons such as improving efficiency, the two test wires of an insulation resistance tester are not usually directly connected to the terminals and wires of the plug. Instead, the product is connected to the tester through some test fixtures, connecting wires, relays, switches, and circuit boards within the instrument. All these intermediate links have their own insulation resistance. Moreover, these resistances can vary greatly depending on factors such as temperature, humidity, surface contamination, and aging of the insulation itself. Therefore, the actual measured insulation resistance is not just the resistance of the plug, but the total insulation resistance. Due to the parallel connection, according to the formula
1/R total=1/R plug+1/R1+1/R2+......+1/Rn
If any one factor changes, it will lead to a change in the total resistance. To simplify the formula, if there is only one instrument's resistance, then
1/R total=1/R plug+1/R instrument,
Rtotal=R plug XR instrument/(R plug+R instrument). If the insulation resistance of the plug is approximately 150M Ω and the instrument is 200M Ω, then the total resistance Rtotal=150X200/(150+200)=85.7 M Ω
Please note that the total resistance measured at this time is less than the actual resistance of the plug. It can be seen that this resistance cannot represent the real situation. Of course, this parameter has certain reference significance. Because the test value is smaller than the actual value, that is, it is actually more stringent than the standard. However, there is another problem. Generally speaking, the time required for testing insulation resistance using so-called comprehensive testing machines is extremely short (less than 1 second), which is far from the standard requirement of 1 minute. Therefore, I believe that conducting insulation resistance testing on production lines is unrealistic and meaningless. Moreover, everyone may have encountered problems with almost this parameter every time an instrument is calibrated for metrology.
In addition, for plug power cord products, insulation resistance is basically a parallel relationship between volume resistance and surface resistance. If the product is not qualified, it is necessary to analyze which aspect is unqualified.
Poor insulation of products is often caused by the following factors:
The insulation material itself is poor (including plugs, wires, internal racks, etc.), the distance between the two pole conductors is too close, moisture is affected, and the surface is dirty.
Electrical strength: The position where electrical strength is applied is exactly the same as the insulation resistance. According to IEC60884-1:
Apply a voltage that is essentially sinusoidal and has a frequency of 50 Hz for 1 minute
The test voltage shall be as follows:
- 1250 V for electrical accessories with a rated voltage of 130 V and below;
- For electrical accessories with a rated voltage above 130 V, 2000 V
Initially, the applied voltage should not be greater than half the specified value, and then rapidly increase to the specified value.
During the test, there shall be no flashover or breakdown.
This is a requirement for type testing. For routine tests on production lines, the test time is generally shortened. Some countries will increase the test voltage (such as UL), but the IEC system still maintains the same voltage. According to IEC60884-1:
Apply the following voltage to the power supply terminal (i.e. plug) for at least 2 seconds for the test:
- 1250V ± 10% for electrical accessories with a rated voltage of less than or equal to 130V;
- 2000V ± 10% for electrical accessories with a rated voltage greater than 130V;
Note 1: If there is an automatic timer on the test device, 2 seconds can be reduced to not less than 1 second.
Perform a 1.2/50us pulse voltage with a peak value of 4kV for all voltages, with an interval of not less than 1s:
Between the phase line and the ground line,
Between the neutral line and the ground line.
Note 2: During this test, the phase and neutral lines can be connected together.
There should be no arcing.
At present, pulse voltage testing is not well understood. It is mainly used to confirm the creepage distance and electrical clearance, which is also a requirement for future testing