The loop impedance ( English loop impedance ) or loop resistance is the sum of all the impedances of a closed current path, which is traversed in an insulation fault in an electrical operating means (body circuit) from the fault current. The residual current loop for determining the loop impedance consists of:
- the power source (e.g. transformer ) taking into account the internal resistance.
- the unearthed active conductor (outer conductor), in IT systems also the neutral conductor , from the power source to the point of failure.
- the return conductor to the power source. In TN systems the return conductor is always the earthed protective conductor , in TN-C systems the PEN conductor with its parallel connections via the earthing system (e.g. foundation earth ) or via the equipotential bonding system . In TT systems , the electrical equipment is directly earthed by means of protective earthing conductors, or connected to the system earth of the consumer system.
After the installation of a low-voltage system, a measurement of the loop impedance with mains frequency must be carried out to check compliance with the disconnection conditions. This measurement in accordance with DIN VDE 0100-600: 2008-06 is particularly required if protection against indirect contact is only provided by automatic shutdown using overcurrent protection devices. The measurement results must be recorded in writing in test reports and handed over to the operator of the system.
The loop impedance (Zs) is the impedance (sum of ohmic and inductive resistance at 50 Hz) between the external and protective conductors at the measuring location (e.g. socket), the internal network resistance or the network impedance (Zi) is the corresponding value between the normally live conductors (mostly outer and neutral conductors).
The loop impedance leads to contact voltage in the event of a short-circuit to frame (e.g. on the housing). The overcurrent protection (e.g. the associated miniature circuit breaker) must therefore trip immediately.
The internal network resistance , together with the lowest network voltage, results in a minimum short-circuit current, which in any case must be sufficient to trigger the circuit fuse (associated circuit breaker) before the lines become too hot.
Both values are influenced by the length and cross-section of the installation as well as by contact transition resistances and are usually similar in TN systems. In the TT system, the loop impedance essentially depends on the quality of the grounding .
If the values are too high, the error must be found or a new installation must be carried out. Alternatively, the value or type of the upstream fuse (i.e. the tripping behavior of the line circuit breaker ) can be adjusted.
Performing the measurement
The loop impedance (L-PE) and the network internal resistance (LN) are measured. In the TN system it is often assumed that both values are the same. Practice shows, however, that both the loop impedance and the internal network impedance can be higher in TN systems. During the measurement, the measuring device generates a small fault current and uses this to calculate the loop impedance and, from this value, with the currently applied voltage, the expected short-circuit current.
Here, often the upstream solves leakage circuit breaker (Residual Current Device, RCD) from. Good measuring devices therefore measure in such a way that the RCD does not recognize these currents and therefore does not trip.
The measuring location must always be selected at the end of the chain, i.e. in final circuits e.g. B. at the most distant socket to which a consumer is plugged. If necessary, this is the end of a cable drum. Since the value deteriorates with the length of the cable and the number of plug connections, it is not permitted to plug in several multiple sockets or extension cables one behind the other. This can increase the loop resistance to such an extent that the safeguards do not trip in the event of an overload.
Since several fuses are often used selectively in series, it is advisable to also measure at the connection of the downstream fuse in order to evaluate the previous fuse (size / characteristic) or the cables and connections up to the following one (at which one is measuring) to be able to evaluate separately - e.g. B. at the sub-distribution to assess the fuse in the main distribution as well as cables and connections in between.
Maximum permissible loop impedance
The loop impedance measurement is not primarily about an impedance (in Ω) that should be fallen below, but about the resulting (voltage-dependent) short-circuit current. Most measuring devices calculate this current and display it. For measuring devices, the measuring tolerance (deviation of up to max. 30% - measuring device according to EN 61557) must be observed. If the measured value is close to the permissible values, it should be checked whether and how the loop impedance can be reduced, especially if a significantly lower value was to be expected, despite compliance.
In the TN network, disconnection times in final circuits up to and including 63A (≤63A) and 230 V outer conductor (nominal voltage) to earth of 0.4 s or for nominal voltages up to 400 V to earth of 0.2 s are required.
In the TT network, disconnection times in final circuits up to 63 A of 0.2 s / 230 V and 0.07 s / 400 V must be observed.
For miniature circuit breakers - regardless of the type (Z, B, C, K, D) - this always corresponds to tripping by the electromagnetic quick release. The following example applies to type B16, which is mostly used in house installations:
The quick release of a circuit breaker B16 trips between 3xIn (48 A) and 5xIn (80 A) - a minimum current of 80 A (worst case) must therefore be assumed, which must flow so that the fuse is guaranteed to trip in the required time.
The mains voltage may be in Germany (according to EN 50160) . The lower voltage (207 V) is relevant for the rough calculation.
Theoretically, the loop impedance in this case (LS B16) must be below 2.6 Ω. Safety reserves are also taken into account.
Maximum permissible loop impedance in the TN system
The measured value of the fault loop impedance must not exceed 2/3 of the value given above , as the impedance measurement is only carried out at low currents at room temperature, while in the event of a fault a high current heats the conductor and thereby increases the impedance.
- - Nominal alternating voltage (rms value) to earth (star point).
- - Shutdown current, which causes the respective protective device to switch off automatically within a certain time.
Maximum permissible loop impedance in the IT system
In IT systems in which the electrical equipment is connected to one another with an earthed protective conductor, the maximum permissible loop impedance for the automatic shutdown of the power supply in the event of a double fault must be calculated as follows:
- - Nominal alternating voltage (rms value) between the outer conductors. In IT systems with a neutral conductor, the voltage between the outer conductor and the neutral conductor should be used instead.
- - Shutdown current that causes the respective protective device to switch off automatically within the time required by DIN VDE 0100-410: 2018-10 (Installation of low-voltage systems, part 4-41: Protective measures - protection against electric shock).
The loop impedance measurements may only be carried out with the measuring devices provided for this purpose in accordance with DIN EN 61 557-3 (VDE 0413 Part 3). These special measuring devices often offer additional functions, such as B. an insulation resistance measurement . The devices require approval for safety-relevant measurements.
The criteria for choosing a measuring device are:
- Portable version for mobile use
- Sufficiently high or low measuring current - depending on the system to be measured
- Two-wire or four-wire measurement
- Measurement while the system is in operation
- direct calculation of the expected short-circuit current
- DIN EN 61557-3 VDE 0413-3: 2008-02 Electrical safety in low-voltage networks up to AC 1000 V and DC 1500 V - devices for testing, measuring or monitoring protective measures; Part 3: loop resistance
- VDE 0100-410 2018-10
- DIN VDE 0100-410: 2007-06, section 411.4.4
- DIN VDE 0100-600: 2008-06 Setting up low-voltage systems - Part 6: Tests , Section: C.188.8.131.52 Measurement of the fault loop impedance: consideration of the increase in conductor resistance with increasing temperature (page 32)
- DIN VDE 0100-410: 2007-06 Setting up low-voltage systems, Part 4-41: Protective measures - protection against electric shock