How to turn off the circuit

Measurement internship: loop impedance and internal network resistance

  • (Comments: 20)
  • Check
4.4 / 5 stars (53 votes)

By performing measurements, the qualified electrician can prove the effectiveness of protective measures. One of the most common protective measures for electrical circuits is protection by disconnection. One of the most important measurements in this context is the measurement of the loop impedance.

The terms "impedance" and "fault loop impedance"

An impedance is understood to be the alternating current resistance of a circuit. This impedance is composed, for example, of all line, terminal and contact resistances in the circuit. In standardization (e.g. DIN VDE 0100-600 "Setting up low-voltage systems - Part 6: Tests") this impedance to be measured is also called fault loop impedance.

The fault loop impedance is made up of the total alternating current resistances from the power source (e.g. transformer) via the active conductor (outer conductor) as the outward path and the return conductors to the power source (e.g. protective or PEN conductor) of the circuit that acts in the event of a fault.

Loop impedance measurement therefore means the measurement of the resistance of the entire outward and return path of a current loop within an AC circuit, which occurs in the event of a fault (e.g. short circuit in the housing of an item of equipment).

The following figure shows such an error loop for a TN-CS system in the event of an error.

In the practice of a qualified electrician, the TN-CS system is probably the most frequently used network type. Voltages between 230 V and 400 V are almost always used. Typical areas of application for loop impedance measurement are socket outlet circuits and consumer circuits with rated currents up to 32 A.

Editor's tip

The inspector for electrical safety

over 500 test reports in Word

Perform your tasks as a qualified electrician, e.g. in the organization and implementation of tests of electrical work and equipment.

Better check now!

Switch-off times must be observed

According to DIN VDE 0100-410 ("Setting up low-voltage systems - Part 4-41: Protective measures - Protection against electric shock"), Table 41.1, in order to implement the protective measure "Protection by disconnection" for circuits up to max. 32 A, the protective device switches off the faulty circuit within 0.4 seconds.

The switch-off time of the protective device depends on the characteristics of the protective device and the rated current. For example, a circuit breaker with a rated current of 16 A and a type B characteristic switches off within 0.4 seconds if at least five times the rated current flows. An LS switch is built in such a way that it switches off safely at at least five times the rated current. The tripping time is shorter than the 0.4 seconds required by the VDE 0100-410 standard. For currents less than five times the nominal current, this is not always the case
ensured. In practice this means that with a fault current of 5 x INominal (5 x 16 A = 80 A) the switch-off time is normatively ensured by the circuit breaker described.

It should be noted here that circuit breakers with a type C characteristic require ten times the nominal current and those with type D characteristics require twenty times the nominal current in order to cause a shutdown within the same time. When replacing line circuit breakers (e.g. due to inrush currents at CEE sockets), the protective measure must therefore always be checked.

Line and contact resistances limit the fault current

In order for the protective measure "Protection by disconnection" to work in the event of a fault, it is necessary that the largest possible fault current can flow within the fault loop. However, the fault current is largely determined by the line resistance (e.g. due to the length and cross-section of the line). The longer the cable length and the smaller the cable cross-section, the greater the cable resistance.

The loop impedance (Zs) in accordance with DIN VDE 0100-410, Paragraph 411.4.4, must be dimensioned in a TN system in such a way that it does not exceed the quotient of the nominal voltage (Unominal voltage) of the outer conductor to earth (e.g. 230 V for a socket) and the required cut-off current (IShutdown) the protective device (e.g. 80 A for a type B 16 A miniature circuit breaker). Expressed in a formula this means:

ZS. ≤ Unominal voltage/ IShutdown

As a calculation and measurement example for the maximum loop impedance, the 230 V socket, which is protected by a type B 16 A LS switch, should once again serve:

Zs ≤ 230 V / 80 A

Zs ≤ 2.88 Ω

Apply a safety factor to the measured value

Theoretically, the maximum loop impedance should therefore be 2.88 Ω. However, this is a purely academic value. Measuring devices according to DIN EN 61557-10 VDE 0413-10 ("Electrical safety in low-voltage networks up to AC 1,000 V and DC 1,500 V - devices for testing, measuring or monitoring protective measures") may, even if they are calibrated, cause a measurement error (so-called. Operating measurement deviation) of 30%. This means that the qualified electrician should pay attention to a safety margin of at least 30% (factor 0.7):

Zs ≤ 0.7 x (230 V / 80 A)

Zs ≤ 2.0 Ω

In practice, this means that loop impedance measured values ​​should be achieved well below 2.0 Ω. Measured values ​​around 2.0 Ω should always be a reason for increased attention or correction.

Tips for recurring measurements

For the regular recurring tests (here you come to the protocol for repeat tests), the loop impedance does not necessarily have to be measured for all sockets in an area (e.g. a hallway). In practice, it is rather sufficient to carry out the loop impedance measurement at the most distant socket in a circuit (e.g. last socket in the hallway). This socket has the longest cable route. As a result, it must also have the highest impedance and thus ultimately also the worst measured value of the entire socket circuit in the hallway.

Difference: loop impedance and internal network resistance

In practice, there is often confusion between the terms “loop impedance” and “internal network resistance”. The loop impedance is measured between the outer and protective conductor.

The internal network resistance, however, between the external and neutral conductor. The measurement of the internal network resistance is therefore not a substitute measurement for the loop impedance. The measurement does not provide any direct information about the protective conductor. Rather, this measurement assumes that the impedance of the neutral and protective conductor is approximately the same.

Measurements in practice

In practice, the loop impedance measurement is carried out, for example, on sockets by plugging the measuring device with the appropriate adapter (e.g. socket adapter) into the live socket. Depending on the measuring device, there is a corresponding selection button for the loop impedance (e.g. "Zs" or "ZSchl.").

After the automatic measurement, both the impedance value (ZSchl.) And the theoretical short-circuit current (Itripping or Ik) calculated by the measuring device on the basis of the voltage measurement (UN) calculated at the same time can usually be read on the measuring device.

The following figure shows an insufficient loop impedance measured value for a socket outlet circuit (Ik ≈ Unrated voltage / Zs = 230 V / 3.2 Ω = 71.9 A).

Conclusion

The measurement of the loop impedance is important for checking compliance with protective measures. With the measured value determined, the qualified electrician can provide evidence that a circuit complies with the standardized switch-off conditions. The rated voltages to earth and the cut-off current of the protective device must be taken into account by the qualified electrician to comply with the cut-off times according to DIN VDE 0100-410 for the various network types. If the loop impedance is too high, the line length can be reduced, the line cross-section increased or the characteristics of the protective device can be made more sensitive to correct this.

References:

Beuth Verlag GmbH, Am DIN-Platz, Burggrafenstrasse 6, 10787 Berlin; DIN VDE 0100-600 VDE 0100-600: 2017-06; Erection of low-voltage systems - Part 6: Tests

Beuth Verlag GmbH, Am DIN-Platz, Burggrafenstrasse 6, 10787 Berlin; DIN VDE 0100-410 VDE 0100-410: 2018-10; Erection of low-voltage systems - Part 4-41: Protective measures - Protection against electric shock

Article from 2016, was checked and updated on August 6th, 2020

Author: Dipl.-Ing. (FH) Christoph Schneppe, B.A.

Editor's tip

The inspector for electrical safety

over 500 test reports in Word

Perform your tasks as a qualified electrician, e.g. in the organization and implementation of tests of electrical work and equipment.

Better check now!

Back