LeakageCurrent的基本知识(英文)

LeakageCurrent的基本知识（英文）

LEAKAGE CURRENT AN-113

Leakage Current

WHAT IS LEAKAGE CURRENT?

Leakage current is the current that flows through the

protective ground conductor to ground. In the absence of a

grounding connection, it is the current that could flow from any

conductive part or the surface of non-conductive parts to ground

if a conductive path was available (such as a human body). There

are always extraneous currents flowing in the safety ground

conductor.

WHY IS IT IMPORTANT?

Electrical equipment commonly includes a grounding system

to provide protection against a shock hazard if there is an

insulation failure. The grounding system usually consists of a

grounding conductor that bonds the equipment to the service

ground (earth). If there is a catastrophic failure of the insulation

between the hot (power) line and touchable conductive parts, the

voltage is shunted to ground. The resulting current flow will

cause a fuse to blow or open a circuit breaker; preventing a shock

hazard. Obviously, a possible shock hazard exists if the grounding

connection is interrupted, either intentionally or accidentally. The

shock hazard may be greater than supposed because of the

leakage currents. Even if there is no insulation failure,

interruption of the leakage currents flowing through the ground

conductor could pose a shock hazard to someone touching the

ungrounded equipment and ground (or other grounded

equipment) at the same time. This possibility is of much more

concern in medical applications, where a patient may be the

recipient of the shock. A fatal shock could result if the patient is

in a weakened condition or unconscious, or if the leakage current

is applied to internal organs through patient contacts. The

double insulation provided in non-grounded equipment

provides protection by using two separate layers of insulation.

The protection in this case is ensured because both layers of

insulation are unlikely to fail. However, the conditions that

produce leakage currents are still present, and must be

considered.

WHAT CAUSES LEAKAGE CURRENT?

There are two types of leakage current: ac leakage and dc

leakage. Dc leakage current usually applies only to end-product

equipment, not to power supplies. Ac leakage current is caused

by a parallel combination of capacitance and dc resistance

between a voltage source (ac line) and the grounded conductive

parts of the equipment. The leakage caused by the dc resistance

usually is insignificant compared to the ac impedance of various

parallel capacitances. The capacitance may be intentional (such

as in EMI filter capacitors) or unintentional. Some examples of

unintentional capacitances are spacings on printed wiring boards,

insulations between semiconductors and grounded heatsinks,

and the primary-to-secondary capacitance of isolating

transformers within the power supply.

HOW IS IT MEASURED?

A meter specially designed for measuring leakage currents is

used. The current flowing in the ground conductor is measured

by connecting the meter in series with the grounding connection.

For information processing equipment, the ground connection is

opened and the current flowing to the neutral side of the power

line is measured. For medical equipment, the current flowing to

ground is measured. The meter may also be connected between

the outputs of the power supply and ground. The test conditions

include swapping the ac line and neutral connections, and

turning power switches off and on while monitoring the current.

The test is performed after the equipment has warmed to normal

operating temperature and, in some cases, following certain tests

that cause abnormally high temperatures within the equipment.

The intent is to identify and measure the worst-case leakage

current.

For very low leakage currents, the meter is replaced with a

network consisting of either a resistor or a resistor and capacitor

combination. The voltage drop across the network is then

measured using a sensitive ac voltmeter. Ungrounded or double-insulated equipment is checked by connecting the meter

between any touchable conductive part and ground. In the case

of non-conductive housings, a copper foil of a specific size is

placed on the housing, and the current flowing from it to ground

is measured.

WHAT IS A SAFE LEVEL?

For non-medical equipment, the safe levels have been

determined by an international organization and documented in

the IEC 950 safety standard. Most countries around the world

have adopted this standard. The limits are defined in Table 1.

Table 1. IEC 950 Safety Standards

Equipment Type Max. Leakage Current Double Insulated All

0.25 mA

Grounded Hand Held 0.75 mA

Movable (other than hand-held) 3.5 mA

Stationary (Permanently connected) 3.5 mA (Note 1)

1. Leakage currents greater than 3.5 mA are allowed under

certain conditions.

Medical equipment leakage current limits are much lower.

The requirements are summarized in Table 2. Because of the

lower values of allowable leakage current in medical power

supplies, it is important to substantially reduce the capacitances

that cause leakage currents. This poses a special design problem

for the power supply and especially EMI filters. For example, line-to-ground capacitors are an important part of the EMI filter's

ability to perform properly. Reducing

their value can severely reduce the filter's effectiveness.

Condor's medical designs and patented EMI filtering techniques

have overcome these problems.

Table 2. IEC601-1, UL2601-1 Safety Standards

Medical Device Category Type B Type BF Type CF Conditions

N.C. S.F.C. N.C. S.F.C. N.C. S.F.C. Earth Leakage Current, Portable

0.51 1.0 0.51 1.0 0.51 1.0 Earth Leakage Current, Fixed 2.5 5.0 2.5

5.0 2.5 5.0 Leakage

Current 0.1 0.5 0.1 0.5 0.1 0.5 Enclosure

Current 0.1 0.5 0.1 0.5 0.01 0.05 Patient

Leakage

Notes:

All leakage currents are in mA

N.C. = Normal Condition

S.F.C. = Single Fault Condition

(1) 0.3 mA for UL 2601-1.