Touch current: electric current passing through a human body or through livestock when it touches one or more accessible parts of an installation or of equipment (defined in the IEC 60050-195-2021).
This analysis of the term “touch current” from various international standards describes the electric current that can flow through the human or animal body when they touch one or more accessible parts of an electrical installation or piece of electrical equipment. Under normal conditions, accessible parts are conductive parts of electrical equipment that are not live.
Such parts are, for example, the exposed conductive parts of electrical equipment of classes 0, I and III and the conductive sheaths of Class II electrical equipment. Some electrically live parts of class III electrical equipment under extra-low voltage may also be accessible parts. Under single or multiple fault conditions, accessible parts, such as exposed conductive parts, may become dangerously live, and hazardous live parts may also become accessible if, under these conditions, their insulation has been damaged and the sheaths of the electrical equipment have been destroyed, for example.
The term “touch current” is used in the IEC 60479-1 technical specification to define the thresholds of perception (see note below), reaction, let-go and ventricular fibrillation, which establish the four characteristic responses of the human body to an electric current flowing through its body. Under normal conditions, when there is no damage to the insulation of live parts, a person cannot touch live parts.
Threshold of perception: minimum value of touch current which causes any sensation for the person through which i t is flowing.IEC 60479-1
If the standards for a particular piece of electrical equipment provide for such touching, the touch current is usually less than the threshold of reaction.
Consequently, the term “touch current” in the definitions of ” threshold of let-go” and especially “threshold of ventricular fibrillation” refers to the occurrence of an emergency condition when, for example, single or multiple faults have energized conductive parts of electrical equipment accessible to human touch, or have made live parts of electrical equipment accessible to human touch.
In other words, according to IEC 60479-1 technical specification information, touch current can flow under both normal and fault conditions. However, this contradicts the above requirements of IEC 60990, which states that the touch current is to be measured when there is no fault in the electrical equipment. Since the term “touch current” has replaced “leakage current” in this standard, the touch current, like leakage current, can only flow under normal conditions.
From the point of view of the fundamental rule of protection against electric shock laid down by IEC 61140, the accessible parts of the installation and of the electrical equipment mentioned in the definitions of “touch current” in IEC 60050-195 and IEC 60050-826 must not be live either under normal conditions or under single fault conditions. Therefore, a touch current can flow under these conditions. However, when a single fault occurs in the insulation of a live part of class I electrical equipment and the insulation is shorted to an exposed conductive part, an earth-fault current flows. This same current flows through the body of a person who touches an exposed conductive part that is live.
Therefore, to avoid confusion between “touch current” and “earth-fault current”, the touch current for both electrical equipment and electrical installations of buildings should only be determined for normal conditions when there is no fault. That is, touch current is the electric current that flows through the human or animal body when it touches any conductive part of an electrical installation of a building or electrical equipment under normal conditions. And this current cannot injure a person.
Touch Current Measurements
Touch currents must be measured according to the requirements of IEC 60990. If the technical committees allow additional touch currents under fault conditions, they must explicitly identify the conditions and the additional current allowed in their standards.
IEC 60990, also a fundamental safety standard, has established methods for measuring touch current and protective conductor current, which are gradually replacing leakage current measurement methods in international standards.
The international standard provides for the measurement of d. c. or a. c. current of sinusoidal or non-sinusoidal waveform, which could flow through the human body and through the protective conductor, except for the following electrical currents:
- touch currents having less than 1 s duration,
- patient currents as defined in IEC 60601 -1,
- a. c. at frequencies below 15 Hz, and
- currents above those chosen for electric burn limits.
The requirements of this standard apply to electrical equipment of classes 0, I, II, III. The measuring methods recommended for touch current are based upon the possible effects of current flowing through a human body. Measurements of current through networks representing the impedance of the human body are referred to as measurements of touch current. These networks are not necessarily valid for the bodies of animals.
In the past, equipment standards have used two traditional techniques for measurement of leakage current. Either the actual current in the protective conductor was measured, or a simple resistor-capacitor network (representing a simple body model ) was used, the leakage current being defined as the current through the resistor.
The IEC 60990 standard now provides measurement methods for the four body responses to electric current (perception, reaction, let-go, and electrical burn), using a more characteristic model of the human body.
This body model was chosen for most common cases of electric shock in the general sense. With respect to the path of current flow and conditions of contact, a body model approximating full hand-to-hand or hand-to-foot contact in normal conditions is used. For small areas of contact (for example, small, finger contact), a different model may be appropriate but is not covered here.
IEC 60990 specifies that of the four responses, startle-reaction and letgo-immobilization are related to the peak value of touch current and vary with frequency. Traditionally, concerns for electric shock have dealt with sinusoidal waveforms, for which r.m. s. measurements are most convenient. Peak measurements are more appropriate for non-sinusoidal waveforms where significant values of touch current are expected, but are equally suitable for sinusoidal waveforms. The networks specified for the measurement of startle-reaction and letgo-immobilization are frequency responsive and are so weighted that single limit power-frequency values can be specified and referenced.
Electric burns, however, are related to the r.m. s. value of touch current, and are relatively independent of frequency. For equipment where electric burns may be of concern, two separate measurements are made, one in peak value for electric shock and a second in r.m. s. value for electric burns each using the appropriate test circuit.
IEC technical committees on electrical equipment, as specified in IEC 60990, must decide which physiological effects of electric current on the human body are acceptable and which are unacceptable, and then choose electrical current limits for electrical equipment.
Concern for electric burn effects may arise with d. c. or at high frequencies (for example, above 30 kHz for 3,5 mA touch current). At lower frequencies, startle-reaction and letgo-immobilization will be the dominant considerations. Where there is such a concern, the unweighted r.m. s. value of touch current shall be measured (Figure 1), in addition to measurement for either startle-reaction (Figure 2), or inability to let go (Figure 3).
The network of Figure 2 shall be used for low level electric shock limits. This circuit is to be applied where the a. c. limit value in the product standard is up to 2 mA r.m. s. or 2,8 mA peak.
The network of Figure 3 shall be used for higher level electric shock limits. This circuit is to be applied where the a. c. limit value in the product standard is more than 2 mA r.m. s. or 2,8 mA peak.
IEC standard 60990 establishes nine connection diagrams for single-phase and three-phase electrical equipment for touch current measurements, which simulate its operation in low-voltage electrical installations corresponding to different types of system grounding. As examples, Figs. 4 and 5 are the two diagrams presented respectively in Figs. 6 and 11 of the international standard.
IEC standard 60990 establishes nine connection diagrams for single-phase and three-phase electrical equipment for touch current measurements, which simulate its operation in low-voltage electrical installations corresponding to different types of system earthing. As examples, Figs. 4 and 5 are the two diagrams presented respectively in Figs. 6 and 11 of the international standard.
Touch currents should be measured under normal conditions, when all test switches “I”, “n” and “e” are closed, and under several abnormal operating modes of the electrical equipment and its power supply. In particular, IEC 60990 stipulates that measurements shall be made under conditions of failure of a protective conductor connected to an exposed conductive part of class I electrical equipment, simulated by opening of switch “e”. The standard also specifies that measurements shall be made under conditions of a damaged neutral conductor, simulated by opening of switch “n”, and of line conductors, simulated by opening of switches “I”. For single-phase electrical equipment, measurements are additionally performed with different line and neutral conductor connections simulated with switch “p”.
IEC 60990 prescribes the measurement of touch currents under normal operating conditions of electrical equipment and under some fault conditions, which relate only to the supply system and the low-voltage installation and do not occur in the electrical equipment under test.
Touch Current Limits
Information on the effects of electric current flowing through the human body, from which its limit values can be derived, is contained in the technical specification of IEC 60479-1. However, despite this, and the fact that the specification or inclusion of certain touch current limits is not the scope of IEC 60990, Reference Appendix D “Choice of current limits” has been included in that standard. It gives examples of electrical current limits and their selection, which can be used by technical committees when they select touch current limits for a particular piece of electrical equipment.
Consider examples of electric current limits.
No limit assumed.
It is assumed that the limits chosen for touch currents will be well below the threshold for ventricular fibrillation.
Inability to Letgo-Immobilization
The method of measurement is specified in IEC 60990.
IEC TS 60479-1 assumes 10 mA r.m. s. as the approximate average threshold level of letgo-immobilization current, whereas 5 mA r.m. s. as proposed for IEC TS 60479-1, would include the entire adult population.
- startle and some reaction are acceptable as an indication of a first fault, when the letgo-immobilization limit is applied;
- men and women are estimated to have an average letgo-immobilization threshold of 16 mA r.m. s. and 10,5 mA r.m. s. respectively;
- some people have a lower threshold, for example the 99,5 percentiles of men and women have been reported as 9 mA r.m. s. and 6 mA r.m. s. respectively, and the threshold values for children are expected to be lower;
- certain single fault conditions may justify letgo-immobilization limits, with startle reaction limits applying for normal (non-fault) conditions.
The method of measurement is specified in IEC 60990.
The startle-reaction threshold given in IEC TS 60479-1 is approximately 0,5 mA r.m. s. for low frequencies. Various limits are in use between the thresholds for startle-reaction and letgo-immobilization.
Limits for startle-reaction and lower limits:
- need to avoid involuntary startle-reaction, where severe consequences may result (for example, fal l ing from a ladder or dropping equipment) ;
- the limit for startle-reaction is generally 0,5 mA r.m. s. or 0,7 mA peak for a sinusoidal current;
- a limit lower than 0,25 mA r.m. s. (0, 35 mA peak) is indicated where the user is particularly sensitive or at risk due to environmental or biological reasons.
Touch current can be perceived at levels as low as a few microamperes. Unless the current is high enough to produce involuntary startle-reaction that might result in harmful effects, these small touch currents are not considered hazardous and not usually measured by these methods.
The method of measurement specified in IEC 60990 can be used, unless otherwise specified in the applicable standard for the particular product.
0,25 mA r.m. s. (one half of the startle-reaction threshold) is used for class I I equipment in product standards such as IEC 60065, IEC 60335-1, IEC 60950-1 and IEC 62368-1.
Limits lower than 0,25 mA r.m. s. are specified for some medical applications. For such applications the method of measurement in this standard may not provide an appropriate body impedance model.
Appendix D of IEC 60990 also provides general guidelines for selecting electrical current limits, which are usually expressed as maximum DC and AC currents at frequencies up to 100 Hz. For electrical equipment with parts that can be grasped by hand, the highest electrical current limit is the threshold of let-go.
Between the response and release thresholds, there may be a side risk of injury due to involuntary muscle contraction. However, it is not normally expected that injuries to a person will be directly caused by an electric current flowing through their body. When a release current limit is applied to electrical equipment, such an electric current can be regarded as permissible under the conditions of a single fault, representing, for example, a defective connection when carrying out protective earthing.
Normative documents limit the maximum allowable touch currents for electrical equipment. For example, Table 5A “Maximum current” of IEC 60950-1, part of which is reproduced below, gives the maximum allowable contact currents for information equipment.
|Type of equipment||Maximum touch current mA r.m.s|
|Movable (other than Hand-Held, but including Transportable equipment)||3,5|
|Stationary, pluggable Type A||3,5|
|All other stationary equipment||3,5|
Equipment with Touch Current Exceeding 3,5 mA
Touch current measurement results exceeding 3,5 mA r.m.s. are permitted for the following equipment having a main protective earthing terminal:
- Stationary permanently connected equipment;
- Stationary pluggable equipment type B;
- Stationary pluggable equipment type A with a single connection to the AC mains supply, and provided with a separate protective earthing terminal in addition to the main protective earthing terminal, if any. The installation instructions shall specify that this separate protective earthing terminal be permanently connected to earth;
- movable or stationary pluggable equipment type A for use in a restricted access location, with a single connection to the ac mains supply, and provided with a separate protective earthing terminal in addition to the main protective earthing terminal, if any. The installation instructions shall specify that this separate protective earthing terminal be permanently connected to earth;
- stationary pluggable equipment type a with simultaneous multiple connections to the AC mains supply, intended to be used in a location having equipotential bonding (such as a telecommunication centre, a dedicated computer room or a restricted access location). A separate additional protective earthing terminal shall be provided on the equipment. The installation instructions shall require all of the following:
– the building installation shall provide a means for connection to protective earth; and
– the equipment is to be connected to that means; and
– a service person shall check whether or not the socket-outlet from which the equipment is to be powered provides a connection to the building protective earth. if not, the service person shall arrange for the installation of a protective earthing conductor from the separate protective earthing terminal to the protective earth wire in the building.
NOTE 1. In Finland, Norway and Sweden, touch current measurement results exceeding 3,5 mA r.m.s. are permitted only for the following equipment:
– stationary pluggable equipment type a that
• is intended to be used in a restricted access location where equipotential bonding has been applied, for example, in a telecommunication centre,
• has provision for a permanently connected protective earthing conductor, and
• is provided with instructions for the installation of that conductor by a service person;
– stationary pluggable equipment type B;
– stationary permanently connected equipment.
NOTE 2. In Denmark, touch current measurement results exceeding 3,5 mA r.m.s. are permitted only for permanently connected equipment and pluggable equipment type B.
One of the following labels or a label with similar wording must be affixed to the AC supply circuit near the connection point of equipment having a touch current of more than 3.5 mA (r.m.s):
HIGH LEAKAGE CURRENT
EARTH CONNECTION ESSENTIAL
BEFORE CONNECTING SUPPLY
HIGH TOUCH CURRENT
EARTH CONNECTION ESSENTIAL
BEFORE CONNECTING SUPPLY
List of References
- IEC 60050-195-2021
- IEC 60479-1
- IEC 60990
- IEC 60950-1