What Is a Leakage Current?

Leakage Current Definition and Meaning

Leakage current: electric current in an unintended conductive path under normal conditions [defined in the IEC 60050-195-2021].

The definition in question precisely establishes the conditions under which the leakage current flows. However, the definition of the term does not contain the same unambiguous information about the path of leakage current.

In the previous standard IEC 60050-826:1982, the term “leakage current (in installation)” was defined more correctly: the current in a circuit which, in the absence of damage, flows to earth or to extraneous conductive parts. A note to the definition of the term stated that this current may have a capacitive component, which includes that resulting from the deliberate use of capacitors.

IEC 60050-442 defines the term “earth leakage current” as follows: current flowing from live parts of the installation to earth in the absence of an insulation fault. This definition specifies both the conditions under which the leakage current flows and the main path through which it flows. It is therefore formulated more correctly than in IEC 60050-195, IEC 60050-826 and IEC 60050-151.

British Standard BS 7671 defined the term “leakage current” in the same way as IEC 60050-195:

Electric current in an unwanted conductive path under normal operating conditions.

BS 7671

The term “earth leakage current” has been incorrectly replaced in BS 7671 with “protective conductor current”: electric current appearing in a protective conductor, such as leakage current or electric current resulting from an insulation fault. Earth leakage current flows into the earth under normal conditions, while protective conductor current, according to its definition, can be both under normal conditions and under single fault conditions.

Causes of Leakage Current

It follows from the above definition that leakage current occurs under normal operating conditions when the insulation of live parts of the low-voltage electrical installation is not faulty. These conditions are called normal conditions.

Leakage current flows from live parts to the earth or to extraneous conductive parts. It should be borne in mind that the leakage current of class I equipment usually flows along the following conductive pathway: from live parts to its exposed conductive parts and then to the protective conductors connected to them.

The active insulation resistance of live parts of electrical equipment cannot be infinitely high and their capacitance to earth or to conductive parts connected to earth cannot be equal to zero.

Therefore, a small electric current, referred to in standards as leakage current, constantly flows from any live part to the earth as well as to the conductive parts electrically connected by protective conductors to the earthing arrangement of the electrical installation of building and to the earthed conductive part of the power supply. That is, under normal conditions there is always electrical current leakage from live parts of functioning electrical equipment to earth, exposed and extraneous conductive parts and protective conductors.

The only way to eliminate leakage currents is to disconnect the electrical installation of building.

All good quality electrical equipment has some leakage currents that start flowing in the conductors of electrical circuits when it is switched on. If leakage current protection is implemented, the electrical equipment cannot be used because any activation of the equipment will initiate protective devices which will shut down the circuits. Under fault conditions, when earth faults occur, earth-fault currents flow. Protective devices detect earth-fault currents and disconnect the circuits they protect or signal the occurrence of earth faults.

If a person touches an energized live part, an earth-fault current will flow through the person’s body, not a leakage current. Earth fault current also occurs when there is a fault in the “insulation to enclosure or earth”.

The residual current is the vectorial sum of the currents in the main circuit conductors of the residual current device, i.e. it is a calculated value. Under normal conditions it is approximately equal to the leakage current, and under fault conditions to the sum of the leakage current and the earth-fault current. And in the TN-C, TN-S, TN-C-S and even TT types of earthing system, the leakage current is negligible compared to the earth-fault current.

In three-phase 3-wire electrical circuits and networks, three leakage currents flow through three-phase conductors. Three-phase conductors can carry three leakage currents whose values are either approximately equal to each other or significantly different from each other. Moreover, the protective conductor of these circuits and networks carries a leakage current which is the vectorial sum of the three-phase conductor leakage currents.

Where Does the Leakage Current Flow?

The path through which leakage currents flow depends on the type of system earthing. In electrical installations of buildings corresponding to the TT and IT types of system earthing, class I equipment leakage currents flow from live parts to their exposed conductive parts via undamaged basic insulation. From exposed conductive parts, earth-leakage currents flow to earth via protective conductors, main earthing terminals, earthing conductors and earth electrodes.

If the electrical installations of buildings conform to the TN-S, TN-C and TN-C-S types of system earthing, most of the leakage currents do not flow into the earth, but through the protective conductor in the TN-S system and the PEN conductors in the TN-C and TN-C-S systems of LV distribution networks to the earthed live parts of the power supply. In other words, the leakage currents of class I electrical equipment flow along the same conductive paths as the protective conductor currents.

Leakage currents from electrical equipment of classes 0, II and III flow along less defined conductive pathways, e.g. through the sheath of the electrical equipment into the earth or through extraneous conductive parts. Part of the conductive pathway may be the body of a person holding portable electrical equipment or in electrical contact with accessible parts of mobile or fixed electrical equipment. Leakage currents can flow through floors, walls and other building components if, for some reason (e.g. because of increased humidity), their resistance has decreased dramatically, and through other unwanted conductive pathways.

Leakage currents always occur in electrical circuits during normal operation of the electrical installation of building (under normal conditions). Their values in the final electrical circuits depend little on the type of system earthing and rarely exceed a few tens of milliamperes (usually less than 10 mA). If the electrical installation of building uses electrical equipment with increased leakage currents, additional electrical protection measures must be taken in accordance with the requirements of the relevant international standards.

Leakage Current Maximum Values

If electrical equipment has a leakage current not exceeding the normative value, it is to be considered as fault-free electrical equipment. Otherwise, it should be considered as faulty electrical equipment which must be repaired or scrapped. Consider the maximum permissible leakage currents specified in the regulations for certain types of electrical equipment.

Section 13 “Leakage current and electrical strength at operating temperature” of IEC 60335-1-2020 specifies the following maximum allowable leakage currents for major types of household electrical equipment:

  • for class II appliances and for parts of class II construction – 0,35 mA peak;
  • for class 0 and class III appliances – 0,7 mA peak;
  • for class 0I appliances – 0,5 mA;
  • for portable class I appliances – 0,75 mA;
  • for stationary class I motor-operated appliances – 3,5 mA;
  • for stationary class I heating appliances – 0,75 mA or 0,75 mA per kW rated power input of the appliance with a maximum of 5 mA, whichever is higher.

For combined appliances, the total leakage current may be within the limits specified for heating appliances or motor-operated appliances, whichever is the greater, but the two limits are not added.

In some standards, IEC 60335 “Household and similar electrical appliances – Safety” for certain types of household electrical equipment set other values of maximum allowable leakage currents. Example, in IEC60335-2-6-2014+Amd1-2018 for stationary class I appliances, the leakage current shall not exceed the following values:

for appliances with heating elements that are detachable or can be switched off separately1 mA, or 1 mA per kW power input for each element with a limit of 10 mA, whichever is higher. If the appliance has more than three heating units, only 75 % of the measured leakage current is taken into account;
for other appliances1 mA, or 1 mA per kW rated power input with a limit of 10 mA, whichever is higher.

Section 13 “Leakage current” of IEC 60745-1-2006 specifies the following maximum allowable leakage currents for the main types of electric tools:

  • for class I tools 0,75 mA;
  • for class II tools 0,25 mA;
  • for class III tools 0,5 mA.

The compliance of the actual leakage current of the electric tool with the maximum permissible leakage current in IEC 60745-1-2006 is checked by following of a special test, which is carried out at a supply voltage equal to 1.06 times the rated voltage. Protective impedance is disconnected from live parts before carrying out the tests. The leakage current test is made with a.c. unless the tool is for d.c. only, in which case the test is not made.

The following are typical examples of leakage current levels likely to be produced by common appliances [IEC TR 62350-2006]:

  • 1 mA to 2 mA for computers;
  • 0,5 mA to 1 mA printers;
  • 0,5 mA to 0,75 mA for small portable appliances;
  • 0,5 mA to 1 mA for telecopiers;
  • 0,5 mA to 1,5 mA for photocopiers;
  • about 1 mA for filters.

Measurement of Leakage Current

IEC 60335-1-2020 requires that the leakage currents of electrical equipment be measured during normal operation of the equipment under the most adverse conditions of use, over a period of time that may consist of more than one operating cycle.

During testing of household electrical equipment heating appliances are operated at 1,15 times the rated power input. Motor-operated appliances and combined appliances are supplied at 1,06 times rated voltage. Three-phase appliances, which according to the instructions for installation are also suitable for single-phase supply, are tested as single-phase appliances with the three circuits connected in parallel. Protective impedance and radio interference filters are disconnected before carrying out the tests.

The leakage current is measured by means of the measuring network shown in Fig. 4 of IEC standard 60990-2016, between any pole of the power supply and the accessible metal parts connected to a metal foil having an area of at least 20 × 10 cm, which is in contact with accessible surfaces of insulating materials.

Therefore, the leakage current measured in accordance with IEC 60335-1-2020 is equal to the touch current measured in accordance with IEC 60990-2016.

Measuring network touch current weighted for perception or startle-reaction
Measuring network, touch current weighted for perception or startle-reaction [Figure 4 in the IEC 60990-2016]

For single-phase appliances, the measuring circuit is shown in the following figures:

  • if they are class II appliances or parts of class II construction, Figure 1 IEC 60335-1-2020;
  • if they are neither class II appliances nor parts of class II construction, Figure 2 IEC 60335-1-2020.

The leakage current is measured with the selector switch in each of the positions a and b.

Circuit diagram for leakage current measurement at operating temperature
Circuit diagram for leakage current measurement at operating temperature for single-phase connection of class II appliances and for parts of class ll construction [Figure 1 IEC 60335-1-2020]

Key:

  • C – circuit of Figure 4 of IEC 60990:2016;
  • 1 – accessible part;
  • 2 – inaccessible metal part;
  • 3 – basic insulation;
  • 4 – supplementary insulation;
  • 5 – double insulation;
  • 6 – reinforced insulation.

If the appliance incorporates capacitors and is provided with a single-pole switch, the measurements are repeated with the switch in the off position. If the appliance incorporates a thermal control which operates during the test of Clause 11 IEC 60335-1-2020, the leakage current is measured immediately before the control opens the circuit.

Figure 2 – Circuit diagram for leakage current measurement at operating temperature for single-phase connection of other than class II appliances or parts of class ll construction
Circuit diagram for leakage current measurement at operating temperature for single-phase connection of other than class II appliances or parts of class ll construction [Figure 2 IEC 60335-1-2020]

C – circuit of Figure 4 of IEC 60990:2016.

NOTE. For class 0I appliances and class I appliances, C can be replaced by a low impedance ammeter responding to the rated frequency of the appliance.

For three-phase with neutral (3N~) connected appliances, the measuring circuit is shown in the following figures:

  • if they are class II appliances or parts of class II construction, Figure 3 IEC 60335-1-2020;
  • if they are neither class II appliances nor parts of class II construction, Figure 4 IEC 60335-1-2020.

The leakage current is measured with the switches a, b and c in the closed position. The measurements are then repeated with each of the switches a, b and c open in turn, the other two switches remaining closed. For three-phase without neutral (3~) connected appliances, the measuring circuit in Figure 3 or Figure 4 shall be used as applicable, but the neutral is not connected to the appliance.

Circuit diagram for leakage current measurement at operating temperature for three-phase with neutral class II
Circuit diagram for leakage current measurement at operating temperature for three-phase with neutral class II appliances and for parts of class ll construction [Figure 3 IEC 60335-1-2020]

NOTE. If the test laboratory is supplied from a TN or TT distribution system, then Z will be zero. Consequently, always connecting “C” to the neutral conductor will ensure reproducibility of the test result regardless of the type of distribution system (TN, TT or IT) used by the test laboratory and will cover the most onerous condition likely to be encountered during normal use of the appliance.

Key:

  • C – circuit of Figure 4 of IEC 60990:2016;
  • 1 – accessible part;
  • 2 – inaccessible metal part;
  • 3 – basic insulation;
  • 4 – supplementary insulation;
  • 5 – double insulation.

Connections and supplies:

  • L1, L2, L3, N – supply voltage with neutral;
  • PE – protective earth conductor;
  • Z – IT system neutral to earth high impedance.
Figure 4 – Circuit diagram for leakage current measurement at operating temperature for three-phase with neutral appliances other than those of class II or parts of class ll construction
Circuit diagram for leakage current measurement at operating temperature for three-phase with neutral appliances other than those of class II or parts of class ll construction [Figure 4 IEC 60335-1-2020]

NOTE 1. For class 0I appliances and class I appliances, C can be replaced by a low impedance ammeter responding to the rated frequency of the appliance.
NOTE 2. If the test laboratory is supplied from a TN or TT distribution system,< then Z will be zero. Consequently, always connecting “C” to the neutral conductor will ensure reproducibility of the test result regardless of the type of distribution system (TN, TT or IT) used by the test laboratory and will cover the most onerous condition likely to be encountered during normal use of the appliance.

Key:

  • C – circuit of Figure 4 of IEC 60990:2016;
  • 1 – accessible part;
  • 2 – basic insulation.

Connections and supplies:

  • L1, L2, L3, N – Supply voltage with neutral;
  • PE – protective earth conductor;
  • Z – IT system neutral to earth high impedance.

FAQ

When the threshold voltage is more, leakage current will be?

Less.

The leakage current is measured in ________

mA.

In a transistor leakage current mainly depends on ________

Temperature.

The leakage current in an NPN transistor is due to the flow of ________

Holes from collector to base.

The leakage current in a diode is due to ________

Minority carriers.

The leakage current flowing in an installation is due to ________

Insulation failure.

References