Power Supply Leakage Current Testing to IEC60990

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.  

A customer recently asked why leakage current is specified for a Class II power supply when the Class II power supply does not have a ground terminal.

As part of the

testing for IEC60950, power supply manufacturers measure leakage current

to the IEC60990 standard.  

As part of the IEC60950 testing, power supply manufacturers test leakage current according to the IEC60990 standard.

To be more accurate,

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

“Leakage Current”.

To be more precise, the terms "touch current" and "protective conductor current" will replace the term "leakage current".

Protective

Conductor Current (PCC)

Protective Conductor Current (PCC)

Is the current that

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

connection.  

The current flowing through the protective conductor generally known as the earth connection.

AC Supply

Protective Conductor Current Protective Conductor Current

Power Supply

Enclosure

As a note, the

withstand voltage and insulation resistance tests measure the current flowing

through the insulation of the unit under test.    

Note that the withstand voltage and insulation resistance tests measure the current flowing through the insulation of the power supply under test.

Touch Current (TC)  

Contact current (TC)

Is the current that

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

network.  

The current that flows when the human body touches the device is simulated by the human body impedance network as shown below.

AC Supply

Isolation Transformer

Switches

Power Supply

Enclosure

Accessible Conductive Part

Touch Current

Body Impedance Network

The switches are used

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

condition (SFC). Usually there is a

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

power supply.  

Switches are used to simulate a live, neutral, or ground fault, known as a single fault condition (SFC). Typically, a polarity reversal switch is used to change the live and neutral connections on the power supply.

So back to the original 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.  

Returning to the customer's question, if a Class II power supply is used, current will flow through the human body when touching a conductive part of the system (such as a USB port or the conductive housing of the product). The measured current is generally listed in the power supply data sheet.

Here is an example

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

touch current. Note half the tests

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

pins of the power supply.  

This is an extract from the CB test report, the input voltage, frequency, and measured touch current. Note that half of the tests performed used a simulated human body to touch the "output connector" or pins of the power supply.

Enclosure leakage current (normal conditions,

normal polarity)

Case leakage current (normal state, normal polarity)

Enclosure leakage current (normal conditions,

reverse polarity)

Case leakage current (normal state, reverse polarity)

Enclosure leakage current measured on output

connector (normal conditions, normal polarity)

Chassis leakage current measured at output connector (normal state, normal polarity)

Enclosure leakage current measured on

output connector (normal conditions, reverse polarity)

Chassis leakage current measured at output connector (normal condition, reverse polarity)

Enclosure leakage current (single fault

conditions, neutral open, normal polarity)

Chassis leakage current (single fault condition, open neutral, normal polarity)

Enclosure leakage current (SFC, neutral

open, reverse polarity)

Chassis leakage current (single fault condition, open neutral, reverse polarity)

Enclosure leakage current measured on

output connector (SFC, neutral open, normal polarity)

Chassis leakage current measured at output connector (single fault condition, open neutral, normal polarity)

Enclosure leakage current measured on

output connector (SFC, neutral open, reverse polarity)

Chassis leakage current measured at output connector (single fault condition, open neutral, reverse polarity)

The ammeter used is a

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

The ammeter used is a special ammeter, please do not use a regular handheld multimeter!

For more details,

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

safety engineer.

For more detailed information, including measured current limits, please consult a professional safety engineer.

(link to medical

brochure)