Automatic disconnection of supply during a short circuit – methods for verifying protection
The use of electrical installations and equipment may pose a risk of electric shock, particularly when a fault occurs in the supply circuit or in equipment connected to it. Protection of persons who are in the immediate vicinity of electrical equipment at the moment a short circuit occurs is achieved by the automatic disconnection of supply.
What does protection by automatic disconnection of supply involve?
To verify that this type of protection is in place, it is necessary to check whether the current generated under fault conditions exceeds the current required to operate the protective device installed in the circuit under test.
HD 60364-4-41 specifies that disconnection in the event of a short circuit should occur within 0.4 s (TN systems, 230-400 V AC). The fault current triggering automatic disconnection within this time depends on the time-current characteristics of the installed fuse links or other types of protective devices.
Fault current and the characteristics of overcurrent protective devices
For commonly used reusable overcurrent protective devices mounted on a TH rail, this is straightforward to determine. Devices with B, C and D characteristics are designed to operate at 5, 10 and 20 times their rated current, respectively, regardless of the manufacturer.
The actual fault current is calculated based on the measured earth fault loop impedance. In final circuits, this test is relatively easy and can be carried out with any earth fault loop impedance tester available on the market. The resulting fault current is obtained by dividing the nominal supply voltage by the measured loop impedance.
Comparing the values of the above currents makes it possible to determine whether this type of protection is effective.
Measurement issues in industrial distribution networks
In distribution systems, verifying this type of protection becomes considerably more difficult. In such installations, very high short-circuit currents occur, resulting in extremely low earth fault loop impedances at voltages higher than 400 V, for example 690 V.
Measuring instruments typically used for such tests offer a measurement range from approximately 0.13 Ω upwards, with a resolution of 0.01 Ω. This is insufficient for testing industrial distribution networks due to the maximum permissible measurement uncertainties specified in EN IEC 61557-1.
High-current Sonel earth fault loop impedance meters – an industrial solution
To meet these demands, Sonel S.A. has developed the unique MZC-320S, MZC-330S and MZC-340-PV high-current earth fault loop impedance meters.
To increase measurement resolution and, consequently, extend the measurement range, these instruments employ short-circuit measurement circuits capable of generating a forced short-circuit current of approximately 130 A at a mains voltage of 230 V (single-phase circuit measurement) and up to approximately 300 A at voltages up to 750 V (phase-to-phase loop measurement using a 2.5 Ω short-circuit resistor). This design allows measurements starting from 7.2 mΩ with a resolution of 0.1 mΩ.
Measurements are performed using the four-wire method, eliminating the influence of test lead resistance on the result.
Limitations of measurements at very high short-circuit currents and alternative criteria
In some cases, even very high metrological performance may prove insufficient. This is the case with circuits with very high short-circuit currents protected by high-rated protective devices.
Image 1. Measurement using the Sonel MZC-330S loop impedance meter
In such situations, determining the correct fault current may not be possible, or the condition for automatic disconnection of the power supply may not be satisfied. To address this, when performing earth fault loop impedance measurements, the Sonel MZC-320S, Sonel MZC-330S and Sonel MZC-340-PV meters can also measure touch voltage and shock voltage, allowing the installation to be approved for operation based on voltage criteria.
At present, the Sonel MZC-320S, Sonel MZC-330S and MZC-340-PV are unique on the market, as they can be used in networks with rated (line-to-line) voltages of 500 V, 690 V and up to 900 V.
| Meter | Sonel MZC-320S | Sonel MZC-330S | Sonel MZC-340-PV |
| Voltage measurement range | 0 V…550 V | 0 V…750 V | 0 V…900 V |
| 4-wire method – high-current impedance measurement in accordance with EN IEC 61557 (up to 300 A) | 7.2 mΩ…1999 mΩ | 7.2 mΩ…1999 mΩ | 7.2 mΩ…1999 mΩ |
| Measurement category according to IEC 61010-2-030:2023 | CAT IV 600 V | CAT IV 600 V | CAT IV 1000 V |
Measurement safety and cooling system for high currents
During testing, very high short-circuit currents are intentionally generated (up to 300 A), resulting in the dissipation of substantial amounts of energy. To address this, an innovative and highly efficient cooling system has been developed, allowing up to ten measurements per minute.
Image 2. Sonel MZC-320S and Sonel MZC-340-PV
Communication interfaces, ergonomics and environmental resilience of measuring instruments
As standard, measuring instruments are equipped with internal memory and data transmission functions, allowing communication with a computer via USB or wireless connection. The Sonel MZC-340-PV is fully controlled from any device with built-in Wi-Fi communication, such as a smartphone, tablet or computer.
Due to their intended application, the instruments are housed in case-type enclosures with an IP67 degree of protection. The MZC-320S and MZC-330S feature backlit displays that support intuitive and user-friendly operation. The Sonel MZC-340-PV, by contrast, has no display, as it is operated exclusively via an external device.
Application of Sonel meters in final and distribution systems
Thanks to their unique features, the MZC-320S, MZC-330S and MZC-340-PV meters are currently the only devices on the market capable of measuring very low earth fault loop impedances (below 0.13 Ω) in industrial networks operating at 500 V, 690 V and 900 V line-to-line voltages.
Their metrological and operational properties also ensure suitability for earth fault loop impedance measurements in all types of electrical systems, including both final circuits and distribution systems, regardless of the type of protective device used.
Authors:
Roman Domański
Wojciech Siergiej
Sonel S.A.
