What is an error loop
In many cases, such as in densely populated areas, in practice it is difficult or impossible to measure the grounding resistance with auxiliary electrodes due to the fact that the necessary space for the auxiliary electrodes cannot be found. Thus we resort to measuring the resistance of the phase-conductor fault error loop. Loop Error is the path through which said fault current flows to earth and which starts and ends at the defect purpose of this measurement is to measure the impedance of the loop will be created if a ground connection system TN or TT, occur negligible impedance fault between active conductors, or between a phase conductor and an exposed conductive part or a protection conductor.
The error loop formed in the event that the phase conductor, due to defective or worn insulation, comes in contact with the exposed conductive parts of a device is shown in the figure below.
If a phase conductor comes in contact with the ground conductor in an installation or in an appliance, the short-circuit current to be generated may be large enough to cause an electric shock or cause a fire. Under normal conditions the safety of the respective circuit or other protective device will be activated within tenths of a second by interrupting the supply. However, this will happen provided that the loop resistance of the circuit is low enough that the corresponding short circuit current generated is sufficient to activate the circuit protection.
Otherwise, that is, if in an electrical installation the loop resistance is high and the short-circuit current is relatively small, then the protection device will be slow to activate or not activate at all, thus endangering the user from electric shock or creating the conditions for the occurrence of fire.
What does the measurement of the error loop resistance show us?
The error loop resistance does not depend on the ground resistance in the neutral networks (TN). So measuring the error loop resistance does not show us if the ground resistance is correct. It shows us whether the protective devices we use (surge and short circuit fuses and leakage leakage relays ) will act in the foreseen time so that the life of the user of the installation is not endangered or there is no risk of fire.
Automatic power outage control in TN systems For the application of the protection method with "automatic power outage" in the case of TN neutral grounding system it is necessary to select a suitable protection device for overvoltage protection, which in case of error will automatically cut off the power supply within a predetermined time. The satisfactory response in the event of a fault of the protection device to be selected depends, in addition to the nominal characteristics of the device, on the quality of the loop in which the fault current will circulate.
A) Regarding the nominal characteristics of the protection device, the nominal intensity of the device and the characteristic time-current functions are crucial for the implementation of the protection with "automatic power outage".
In other words, an important role in the implementation of the method is whether, for example, we have chosen a fuse fuse or an automatic fuse to secure the circuit and even if the microautomatic has a function characteristic B or C, etc.
B) The quality of the error loop in which the error current will circulate is controlled by the value of impedance Zs that the loop displays.
The following should therefore be verified:
a) The suitability of the protection device selected for the automatic interruption of the supply
b) The suitability of the error loop by measuring the resistance Zs that it displays.
The purpose of measuring the impedance of the fault loop in the case of a TN supply system is to determine that in the event of a fault, a current sufficient to cause the protection device to be disconnected within the time specified by the standard, in order to do not develop a dangerous contact voltage. The estimated time is 0.4 sec for power supply lines of portable or mobile voltage 230V and 5 sec for distribution circuits (see table below).
Therefore, the maximum acceptable value for Zs that will be measured during the error loop control process must satisfy the relation: Zs <= 230V / Ia where:
Zs: error loop resistance which includes the source, the active conductor up to the point of the fault and the protection conductor between the fault and the source.
Ia: the current that causes the protection device to automatically disconnect at the specified time. Eg for MCB 10A Ia is Ia = 5Ion = 5 * 10A = 50A If a differential current protection device (DDE) is used, then Ia is the rated differential operating current of the device
Zs: error loop resistance which includes the source, the active conductor up to the point of the fault and the protection conductor between the fault and the source.
Ia: the current that causes the protection device to automatically disconnect at the specified time. Eg for MCB 10A Ia is Ia = 5Ion = 5 * 10A = 50A If a differential current protection device (DDE) is used, then Ia is the rated differential operating current of the device
Acceptable measurement results of the error loop impedance
The value of the measured error loop resistance Ζs depends each time on the protection device used in the controlled supply line. Indicative values for the maximum value of Zs are given in the following tables, for the different types of protection devices and for disconnection times of 0.4s and 5s.
Also knowing the maximum value of the Zsmax fault loop impedance can calculate the minimum required ikmin fault current, which will activate the protection device and cause the circuit to be cut off. The calculation of Iqmin is done from the relation: Ikmin = 230V / Zsmax
The following table gives both the values of the fault loop impedance and the minimum required value of the Ikmin fault current for the cases of power circuits of ordinary single-phase electrical appliances.
To determine the values in the table below, it was considered that the protection device selected is miniature type B, and the required time of the tripping miniature is 0,4s for supply voltage 230V.
The following table gives both the values of the fault loop impedance and the minimum required value of the Ikmin fault current for the cases of power circuits of ordinary single-phase electrical appliances.
To determine the values in the table below, it was considered that the protection device selected is miniature type B, and the required time of the tripping miniature is 0,4s for supply voltage 230V.
Note: In any case, in order for the protection to work with "automatic power failure", the value of the expected error current Ik must be greater than the current required to activate the protection device selected Ikmin .
When measuring with the certified instrument we will measure:
the error loop resistance Zs (should be <from the Zs of the fuse).
and the expected short-circuit current Ik that will leak the loop when an error occurs (should be> from Ik where the fuse will cut). The Zs and Ik of the fuses are taken from tables as we saw above.
and the expected short-circuit current Ik that will leak the loop when an error occurs (should be> from Ik where the fuse will cut). The Zs and Ik of the fuses are taken from tables as we saw above.
Comment: The error loop is tested for the control protocol on each line of the circuit. In the case of TT earthing system, the measurement of the fault loop impedance is accepted as a ground resistance value, knowing that it gives a value higher than the actual value of the earth resistance due to the fact that conductors participate in the whole device (ELOT HD384 paragraph 612.6.2, note 2 ).
Example: A power line is secured with a 16 A type B micro-automatic in an installation with a TN earthing system. For this case the expected disconnection time is 0.4s.
Based on the operation characteristic of this miniature to be activated within 0,4s time required fault current of at least five times the nominal (5 * 16 = 80 A).
So for this supply line the maximum acceptable value of the error loop impedance should be Ζs = 230/80 = 2,875Ω and respectively the minimum expected error current will be Ικ = 80Α.
Based on the operation characteristic of this miniature to be activated within 0,4s time required fault current of at least five times the nominal (5 * 16 = 80 A).
So for this supply line the maximum acceptable value of the error loop impedance should be Ζs = 230/80 = 2,875Ω and respectively the minimum expected error current will be Ικ = 80Α.
In the following example of measurement with the instrument MACROTEST 5035, in a socket line secured with automatic safety 16A the measured Zs should be less than or equal to 2.87Ω and the current that must circulate in the loop Iqmin to stop the safety at 0, 4sec be greater than or equal to 80.14A according to the above tables. We see that the measurements are acceptable.
In the following example of two measurements with Eurotest instrument we have a correct and an incorrect indication:
In a line secured with 16A type C automatic fuse, the measured Zs must be less than or equal to 1.44Ω and the current that must circulate in the Iqmin loop to stop the fuse at 0.4sec must be greater than or equal to 80, 1A according to the above tables. We see that the measurements are acceptable.
In a line secured with automatic fuse 10A type B the measured Zs must be less than or equal to 4.6Ω and the current to be circulated in the Iqmin loop to stop the fuse at 0.4sec must be greater than or equal to 50A according to with the above tables. We see that the measurements are not acceptable.
For the most accurate documentation of the error loop information, the measurements of the complex error value of the error loop Ζ s as well as the expected short-circuit current Ικ or Isc have the same weight in relation to the evaluation of results. It would be appropriate to duplicate the results. For many, the value of the expected short-circuit current is more important, due to the fact that we confirm the correct use or selection of the characteristic curve of the micro-automatic (B, C, D, etc.) in relation to the minimum short-circuit current of this I sclimit .
The closer the PPC transformer is to the building, the higher the value of the expected short-circuit current Ικ or Ι sc
The closer the PPC transformer is to the building, the higher the value of the expected short-circuit current Ικ or Ι sc
Important Note: The use of a test current of less than 15mA may in some special cases lead to misleading measurements or measurements with a very large value error for the impedance of the error loop. In order to obtain an indicative reference value of the ZS resistor, it is advisable to precede a measurement of the fault loop impedance at the input of the power supply to the installation and before the differential protection device is mediated, as shown in the figure below. In this way on the one hand a test current greater than 15mA can be used and on the other hand a safe
conclusion about the value of the impedance of the ZS error loop displayed by the main supply line of the electrical installation.
Then we can for the individual power supply circuits of the electrical installation which are protected by D.D.R. to use the test current of 15mA, as having obtained from the previous measurement a measure of comparison, we are able to reject measured values with a large deviation which are due to possible errors of the method.
conclusion about the value of the impedance of the ZS error loop displayed by the main supply line of the electrical installation.
Then we can for the individual power supply circuits of the electrical installation which are protected by D.D.R. to use the test current of 15mA, as having obtained from the previous measurement a measure of comparison, we are able to reject measured values with a large deviation which are due to possible errors of the method.
The fault loop in TT grounding systems
In the TT (direct ground) earthing system in the event of an insulation fault between a phase and the protection conductor or an exposed conductive part, the fault loop in addition to the conductors (active conductors and protection conductor) also includes a part of a ground path.
So the error loop in this case will only be created if there are earths on both sides or if these earths are connected (the connection is made through the conductive path of the soil where they are located).
Because the earthing resistors are interposed, the fault current between the phase and exposed conductors is less than the current of a solid short circuit (as in the TN earthing system), but it may have such a value that dangerous contact voltages can occur (such as eg when grounding is done by connecting to an extended metal water supply network).
You need to check the resistance of the loop error to be found that fulfills the requirement Automatic switching to the voltage contact not to exceed the 50V and will be discontinued in 5sec The protections against overcurrent is not suitable for protection against indirect contact in TT grounding system only if the earth electrode resistance is too low.
Examples of completing a protocol formIndicative size classes for ground resistance and for different NDF values of the differential current protection device are summarized in the following table:
Error loop measurement in TN- CS grounding systems ( points to watch out for)
If there are electrical consumptions in operation in the circuit to be measured they may affect the measurement result.
If there are leakage currents or foreign voltages in the protection conductor they can also affect the measurement result.
If there is a backup power supply in the installation ( eg generator) the error loop measurements must be repeated with the backup power supply in operation.
If there are electrical consumptions in operation in the circuit to be measured they may affect the measurement result.
If there are leakage currents or foreign voltages in the protection conductor they can also affect the measurement result.
If there is a backup power supply in the installation ( eg generator) the error loop measurements must be repeated with the backup power supply in operation.
Error loop measurement in TN- CS earthing systems ( results not acceptable) If the results of a measurement do not meet the standard (high values) it should first be investigated whether it is a measurement error, a supply problem ( eg PPC) or installation error.
In order to determine if it is a supply error ( eg PPC) or an installation error, measurements must be made very close to the point of supply.
If it is found that the prices there remain high then it is a supply error and the relevant body must be informed for its restoration.
A measurement error may be due to:
Poor loose connections
Small conductor cross sections
Defective electrical materials
Once the cause of the deviation has been identified and rectified, we repeat the measurement.
In order to determine if it is a supply error ( eg PPC) or an installation error, measurements must be made very close to the point of supply.
If it is found that the prices there remain high then it is a supply error and the relevant body must be informed for its restoration.
A measurement error may be due to:
Poor loose connections
Small conductor cross sections
Defective electrical materials
Once the cause of the deviation has been identified and rectified, we repeat the measurement.
Error loop measurements between phase-neutral or between phase-phase
These measurements are not required by the standard or the Legislation. But they are useful in practice.
In grounding system TN- CS comparing the fault loop measurements between phase-neutral and between phase-grounding conductor can evaluate the quality of neutralization .
An example in practice: In a house in Crete (TN- CS system ) the measurement of an error loop in the panel between phase-protection conductor gives 24.5Ω.
At the same point the phase-neutral error loop measurement gives 1.1Ω.
In another neighboring house fed by the same substation, the measurement of an error loop in the panel between phase-protection conductor gives 1.3Ω.
At the same point the phase-neutral error loop measurement gives 1.2Ω. Conclusion: The neutralization in the first building are problematic. If there was no differential power supply the installation would be dangerous. Because the
neutralization (PEN-PE binding) is in the meter, you should be requested to be there intervention of the responsible operator of the distribution network ( e.g. PPC) Labeling: In initial test has not become initial feed of the installation and when a re the supply has been interrupted, error loop measurements are not possible.
In these cases, it is recommended that these measurements be made after the installation has been fed and that the results be documented in a supplementary protocol.
If there is an alternative-backup power supply in the installation ( eg generator) the above measurements should be repeated with it and the results should be documented in the control protocol.
source: Most of the information is from Salevri-Hadjisofianou (New PSO and Electrical Installation control protocols)
