# 絕緣電阻測試 insulation resistance testing

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Lecture 03 絕緣電阻測試 insulation resistance testing

R=V/I , where is the current from?
Total leakage current : Conductive leakage current (IL ) Capacitive charging leakage current (IC ) Polarization absorption leakage current (IA ) Surface Leakage current (Is) 測量開始後一分鐘: I= IL+ IA+ Is 測量開始後十分鐘: I= IL+ Is

Spot Reading Test Method For this test, the megohmmeter is connected across the insulation of the windings of the machine being tested. A test voltage is applied for a fixed period of time, usually 60 seconds and a reading is taken. The spot reading test should only be carried out when the winding temperature is above the dew point (1). The operator should make a note of the winding temperature, so that it will be possible to correct the reading to a base temperature of 20 ℃. (1) Dew point temperature is the temperature at which the moisture vapor in the air condenses as a liquid. Test Duration To obtain comparable results, tests must be of the same duration. Usually the reading is taken after 60 seconds. Interpretation of Results Proper interpretation of spot reading tests requires access to records of results from previous spot reading tests. For conclusive results, only use results from tests performed at the same test voltage for the same amount of time, and under similar temperature and humidity conditions. These readings are used to plot a curve of the history of insulation resistance. A curve showing a downward trend usually indicates a loss of insulation resistance due to unfavorable conditions such as: humidity, dust accumulation, etc. A very sharp drop indicates an insulation failure. See Figure 1.

Example of the variation of insulation resistance over a period of years:  A, 絕緣電阻漸漸下降，代表絕員可能老化或受到塵埃汙染。  B, 絕緣電阻尖銳的驟降點代表絕緣破壞。  C, 馬達線圈重繞處理後絕緣電阻回升。

Dielectric Absorption Ratio (DAR) or Time-Resistant Tests
This method is fairly independent of temperature and often can give you conclusive information without records of past tests. It is based on the absorption effect of good insulation compared to that of moist or contaminated insulation. Simply take successive readings at specific times and note the differences in readings (see curves, Figure 2). Tests by this method are sometimes referred to as absorption tests. Good insulation shows a continual increase in resistance (see curve D) over a period of time (in the order of 5 to 10 minutes). This is caused by the absorption; good insulation shows this charge effect over a time period much longer that the time required to charge the capacitance of the insulation. If the insulation contains moisture or contaminants, the absorption effect is masked by a high leakage current which stays at a fairly constant value Ð keeping the resistance reading low (R = E/I) (see curve E). The time-resistance testing is of value because it is independent of equipment size. The increase in resistance for clean and dry insulation occurs in the same manner whether a motor is large or small. You can compare several motors and establish standards for new ones, regardless of their horsepower ratings.

Figure 2 shows how a 60-second test would appear for good and bad insulation. When the insulation is in good shape, the 60-second reading is higher that the 30-second reading. A further advantage of this two reading test is that it gives you a clearer picture, even when a “spot reading” says the insulation looks ok. Time-resistance tests on large rotating electrical machinery - especially with high operating voltage - require high insulation resistance ranges and a very constant test voltage. A heavy-duty megohmmeter serves this need. Similarly, such an instrument is better adapted for cables, bushings, transformers, and switchgear in the heavier-duty sizes. DAR Test Methods DAR =R(60 sec)/R(30 sec) less than 1 = failed 1.0 to 1.25 = OK 1.4 to 1.6 = excellent Note: This is not a commonly used test

polarization index (PI)
Absorption curve of test conducted on 350 HP Motor:  Curve D indicates a good insulation with an excellent polarization index of 5. Curve E indicates a potential problem. The polarization index is only 140/95, or 1.47.

R=V/I , How many volts? Voltage Level IR Tester 650V 500V DC 1.1KV
1KV DC 3.3KV 2.5KV DC 66Kv and Above 5KV DC MEGGER FLUCK 絕緣阻抗的測試與耐壓測試其接線方式大致相同，主要是量測兩個端點之間及其週邊連接在一起的各項關聯網路所形成的等效電阻值， 絕緣電阻是指用絕緣材料隔開兩部分導體之間的電阻稱絕緣電阻，為確保電氣設備運行的安全，對其不同極性（不同相）的導電體之間，或導電體與外殼之間的絕緣電阻均須滿足一定的要求。 屏蔽端子

Q:如何量測以下低壓配電盤之絕緣電阻?

Ans: 低壓配電盤 線間絕緣電阻測量 低壓電路之導線間及導線對大地之間的最低絕緣電阻應符合下表之規定: 線路電壓 絕緣電阻 (MΩ)
300V 以下 對地電壓150V以下 0.1 對地電壓150V以上 0.2 300V 以上 0.4 Ans: 低壓配電盤 線間絕緣電阻測量 插座

Ans: 低壓配電盤 導線與大地絕緣電阻測量