A customer recently

asked me why we specify leakage current on a Class II power supply, when a

Class II power supply has no ground terminal.

A good question, but first some background.

一位客戶最近詢問，為什么II類電源并沒有接地端子，卻要指定II類電源的漏電流。

As part of the

testing for IEC60950, power supply manufacturers measure leakage current

to the IEC60990 standard.

作為 IEC60950 測試的一部分，電源的制造商要根據 IEC60990 標準來測試漏電流。

To be more accurate,

the terms “Touch Current” and “Protective Conductor Current” replace the term

“Leakage Current”.

更加準確的說，“接觸電流”和“保護導體電流”這兩個術語將取代“漏電流”這一術語。

Protective

Conductor Current (PCC)

保護導體電流 (PCC)

Is the current that

flows through the protective conductor; commonly referred to as the ground

connection.

流過一般稱為接地連接的保護導體的電流。

AC Supply 交流電

Protective Conductor Current 保護導體電流

Power Supply 電源

Enclosure 外殼

As a note, the

withstand voltage and insulation resistance tests measure the current flowing

through the insulation of the unit under test.

注意，耐受電壓和絕緣電阻測試時會測量流經被測電源絕緣部分的電流。

Touch Current (TC)

接觸電流 (TC)

Is the current that

flows when a human body touches the equipment, simulated by a body impedance

network.

人體接觸設備時流過的電流，如下示意是由人體阻抗網絡進行模擬。

AC Supply 交流電

Isolation Transformer 絕緣變壓器

Switches 開關

Power Supply 電源

Enclosure 外殼

Accessible Conductive Part 可觸及導電部分

Touch Current 接觸電流

Body Impedance Network 人體阻抗網絡

The switches are used

to simulate a line, neutral or ground fault, referred to as a single fault

condition (S.F.C.). Usually there is a

polarity reversal switch to reverse the line and neutral connections to the

power supply.

開關用于模擬火線，零線或接地線故障，稱為單一故障條件 (S.F.C.)。通常，使用極性反轉開關來改變電源上的火線和零線連接。

So back to theoriginal customer question, if a Class II power supply is used, there will be

current that flows through a human body upon touching conductive parts in a

system (like a USB port or a conductive product case). That measured current is usually listed on

the power supply datasheet.

回到客戶的問題，如果使用II類電源，在接觸系統(tǒng)的導電部分時（例如 USB 端口或產品的導電外殼），電流將流過人體。所測得的電流一般列在電源數據表中。

Here is an excerpt

from a CB report showing the test, input voltage, frequency and the measured

touch current. Note half the tests

conducted were with the simulated human body touching the “output connector” or

pins of the power supply.

這是從CB 測試報告里摘錄，輸入電壓、頻率，以及測得的接觸電流。請注意，所執(zhí)行的一半測試使用了模擬的人體來接觸“輸出連接器”或電源的引腳。

Enclosure leakage current (normal conditions,

normal polarity)

機殼漏電流（正常狀態(tài)，正常極性）

Enclosure leakage current (normal conditions,

reverse polarity)

機殼漏電流（正常狀態(tài)，反向極性）

Enclosure leakage current measured on output

connector (normal conditions, normal polarity)

輸出連接器上測得的機殼漏電流（正常狀態(tài)，正常極性）

Enclosure leakage current measured on

output connector (normal conditions, reverse polarity)

輸出連接器上測得的機殼漏電流（正常狀態(tài)，反向極性）

Enclosure leakage current (single fault

conditions, neutral open, normal polarity)

機殼漏電流（單一故障條件，零線開路，正常極性）

Enclosure leakage current (SFC, neutral

open, reverse polarity)

機殼漏電流（單一故障條件，零線開路，反向極性）

Enclosure leakage current measured on

output connector (SFC, neutral open, normal polarity)

輸出連接器上測得的機殼漏電流（單一故障條件，零線開路，正常極性）

Enclosure leakage current measured on

output connector (SFC, neutral open, reverse polarity)

輸出連接器上測得的機殼漏電流（單一故障條件，零線開路，反向極性）

The ammeter used is a

specialized meter; do not use a regular hand-held multi-meter!

所用的電流表為專用電流表，請勿使用常規(guī)的手持式萬用表！

For more details,

including the limits of the measured currents, please consult a professional

safety engineer.

有關更多詳細信息，包括測得電流極限等內容，請咨詢專業(yè)安規(guī)工程師。

(link to medical

brochure)