Leakage current

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A leakage current is an electrical current that flows in an undesirable current path under normal operating conditions ( Internationales Elektrotechnisches Lexis - IEV 195-05-15 ). A common cause are filter capacitors connected to the protective conductor .

On the other hand, currents that are caused by an insulation fault, such as an insufficient insulation resistance, or by a device fault, are not leakage currents, but fault currents . See also residual current circuit breaker .

In the medical another leakage current is introduced: the Patient .

causes

Leakage currents can be caused by the Y capacitors used in line filters and their capacitive coupling . Y capacitors are particularly safe capacitors connected to earth , which serve to suppress interference. The capacities of lines, windings or electrical heaters with metal pipe sheaths can also cause currents to flow to earth , even if the insulation resistance is in accordance with regulations .

In the event of an error, e.g. For example, if the protective conductor of a grounded device is interrupted or the insulation of a fully insulated device is damaged, the leakage current can create dangerous contact voltages.

However , due to their large winding capacity, electric motors also generate leakage currents to the laminated core and thus to the housing, which when operated with frequency converters can increase considerably and even damage the motor ball bearings. The capacity of long motor cables also leads to leakage currents through the shield.

consequences

If, in the event of a fault, a leakage current flows through the user of the device or another person as an increased touch current, it can trigger an electric shock .

Excessive leakage currents can also trigger residual current devices (RCDs). Impulse-proof residual current circuit breakers have a response delay so that the particularly high capacitive leakage current peaks, which only occur when devices are switched on, do not trigger a trip. Only RCDs that are suitable for universal currents are suitable for leakage currents with a direct current component.

If a system contains several devices that each generate a permissible leakage current, their leakage currents can add up to an impermissibly high value.

Measurement

The leakage current can be determined using various measurement methods.

Measurement of the protective conductor current

In devices of protection class 1, leakage currents are discharged via the protective conductor, so only the protective conductor current has to be measured.

Direct measurement of the protective conductor current

The device in question must be isolated down to the power line, i.e. This means that all conductive connections to earth (e.g. pipes, data or antenna cables) must be disconnected. Then the protective conductor of the device is disconnected, an ammeter is connected and the device is put into operation. The measuring device then displays the protective conductor current.

Indirect measurement of the protective conductor current / differential measurement

With indirect measurement, the total current of the incoming and outgoing mains current is measured with a special clamp-on ammeter . The differential current corresponds to the leakage current. With this type of measurement, the protective conductor must not be fed through the clamp meter. The device does not need to be isolated for this, so that all operational, metallic connections can be retained.

Measurement of touch current

Devices of protection class 2 (protective insulation) do not have a protective conductor, which is why only the touch current is considered as leakage current , since a current flow only occurs when there is contact with earthed people or objects. Despite the double insulation of these devices, currents can flow to earth via metallic housing parts (gears, shafts, handles, decorative strips, ...).

As with protective conductor current measurement, the possible current to earth can be measured directly or indirectly, which would flow if touched. The touch current is measured on the corresponding metal parts of the housing. The polarity of the mains voltage must be reversed during the measurement, with the higher current counting as the measurement result.

Substitute leakage current measurement

Since the measurements listed above are carried out with mains voltage, they involve a not inconsiderable risk potential. Therefore, a substitute leakage current measurement can also be carried out in which the measuring circuit is galvanically isolated from the mains and can also carry low voltage (from 25 V). If the measuring voltage is lower, the measured leakage current must be extrapolated to the operating voltage. When measuring with mains voltage, the measured substitute leakage current may be halved, because both connections are interconnected for measurement, but during normal operation the network either only puts one connection under voltage (asymmetrical network with P and N conductors) or both to half the voltage (symmetrical network ).

The substitute leakage current measurement may only be used if there are no "mains voltage dependent switching devices" (relays, contactors, switched-mode power supply, electronic controllers, ...) in the device to be tested. These components are not active during the substitute leakage current measurement and prevent the test voltages from reaching the subassemblies in the device - a large part of the device may in fact remain untested.

Varistors (surge arresters), on the other hand, have the same influence as with direct or differential current measurement.

Limit values

The measured protective conductor current or contact current must not exceed the following limit values ​​in accordance with DIN VDE 0701-0702:

  • Protective conductor current:
    • for general devices: 3.5 mA
    • for devices with switched on heating elements with a total output of more than 3.5 kW: 1 mA / kW to max. 10 mA
    • If these limit values ​​are exceeded, it must be determined whether other limit values ​​apply due to product standards or manufacturer specifications.
  • for the touch current (probe current): 0.5 mA

Leakage current in medical technology

Two further leakage currents are introduced in medical technology: the patient leakage current and the patient auxiliary current. The patient leakage current is determined in the same way as the housing leakage current, but only measured on the application part (i.e. the part that the patient must touch for the treatment or examination). The patient auxiliary current is the current required for operation between parts of the medical device, which flows over the patient and is measured between the applied parts.

Limit values

With the limit values ​​for the patient leakage current, a distinction is made between direct and alternating current (see also DIN 60479-1). The limit values ​​for direct current are significantly lower than for alternating current. A distinction is made between NC (normal situation from English normal condition ) and SFC (one-fault situation from English single fault condition ). Depending on the classification of the applied part, different limit values ​​must be observed:

Type B is the lowest protection level and is mostly implemented by earthing. NC: 10 µA DC, 100 µA AC; SFC: 50 µA DC, 0.5 mA AC

Type BF must be set up separately from earth (F stands for floating). NC: 10 µA DC, 0.1 mA AC; SFC: 50 µA DC, 0.5 mA AC

Type CF must be set up separately from the earth and is suitable for use on the heart (C stands for cardio). NC: 10 µA DC, 10 µA AC; SFC: 50 µA DC, 50 µA AC

(Values ​​taken from EN 60601-1 , 3rd edition, table 3)

Measures against excessive leakage current

Capacitors in line filters to earth, capacitances between the windings and to the core in transformers should be kept as small as possible. In the case of line filters, an increase in the inductance of the interference suppression chokes is necessary in order to still comply with the interference emission limit values. Multiple devices with high leakage currents have on different circuits, each with an optionally residual-current device are divided (Residual Current Device RCD) to false alarms to avoid.

In the case of high leakage currents, technical measures must be taken to prevent the leakage current from flowing through the human or animal body. These are the potential separation and the potential equalization. The latter is mainly used.

With devices of protection class I, leakage currents normally flow via the protective conductor to the neutral point of the electrical installation. However, if the protective conductor is interrupted due to a fault, the leakage current can flow through the human body.

With devices of protection class II (devices without a protective conductor), leakage currents can only flow through the body of the person touching the device or in any other way connected to it and must therefore, for example, due to the reinforced insulation required anyway or structurally reduced capacities to the housing or other accessible ones conductive parts are kept low. Typical examples are metal lights or home electronics devices. In the case of particularly high safety requirements (medicine, container construction), isolating transformers and other measures for safe electrical isolation are therefore also used.

In addition, in medicine, all device housings that are arranged after the isolating transformer are connected by a local potential-free equipotential bonding conductor, in which all other touchable parts are included. Circuits after an isolating transformer, IT network , are also monitored by insulation monitors.

For leakage currents that are generated by EMC filters on 3-phase devices with B6 bridge circuits (e.g. 3-phase frequency converters, power supplies, UPS, inverters, etc.), there is the option of active leakage current compensation. This compensation makes it possible to reduce leakage currents of the frequencies 150 Hz, 450 Hz and 750 Hz to a few milliamps. A 150 Hz discharge in particular often triggers RCD protective devices without a fault being present.

Web links

Individual evidence

  1. ↑ Teaching material of the DGUV (accessed on May 28, 2019)
  2. ↑ VDE 0800 part 1 . 1989.
  3. ↑ VDE 0107, heavy current systems in hospitals . tape 17 . VDE series of publications, ISBN 3-8007-2135-X , p. 18 .