Earth Fault Loop Impedance

Earth fault loop impedance test :

Earth fault loop impedance testing is performed to measure the impedance of the earth fault loop in an electrical circuit. The purpose of this test is to ensure that the electrical installation is safe and can operate properly under fault conditions.
During the test, the circuit is energized and a test current is injected into the circuit. The voltage drop across the circuit is measured, and the impedance of the earth fault loop is calculated using Ohm's Law.
If the earth fault loop impedance is too high, it may result in a high fault current, which can cause the protective device, such as a circuit breaker, to trip. If the impedance is too low, it may not allow enough current to flow to operate the protective device, which can result in a dangerous situation.
During the test, the circuit may be tested with both trip and no-trip conditions. The trip condition involves the activation of the protective device, which is designed to disconnect the circuit in case of a fault. This test is performed to ensure that the protective device can operate correctly and within the required time.
The no-trip condition involves testing the circuit without activating the protective device. This test is performed to measure the impedance of the earth fault loop under normal operating conditions.
Both the trip and no-trip conditions are important to ensure the safety and proper operation of the electrical installation. The test results are compared against the values specified in the relevant standards or regulations to determine if the installation is compliant.

BS 7671, 411.5.4 States that, where an overcurrent protective device is used the following condition shall be fulfilled: Zs x Ia ≤ Uo x Cmin is a formula used in the standard to determine the Maximum earth fault loop impedance to achieve the disconnection time vary with the different types of protective device for a given circuit.

Its worth noting that these all values are measured values, not calculated one.

Zs = Measured impedance at furthest point of installation.

Ia = Operating Current for the protective device under specified time.

Uo = Measured Voltage of the system between Line-Earth

Cmin = 0.95 ( For Voltage variation considering worst case scenario.

Lets take an Example -1,

Type B circuit breakers are designed to trip if the current flowing through hits between three and five times the recommended maximum or 'rated load'. For 20A , it would be 100A max.

Suppose , Measured Zs = 0.31 and Measured Uo = 239 Volts

Then Ia = (239*0.95) / 0.31 = 732.4A

This value (732.4A) is 7.3 times the breaker maximum current.

If breaker of 20A with fault capacity of 4kA , it will safely operate and can resist 732A of Line-Earth fault and 1464.8A of Line-Line fault. Second , It will surely operate after max value of 100A within specified time say(0.3Seconds).

Similarly , Zs for given CB of 20A with maximum rating of 100A Should be ≤ (239*0.95) / 100A = 2.27Ω

Considering temperature factor , we should apply 80% rule.

The so called 80% Rule tells us to multiply all tabulated values by 0.8 to find the maximum permitted measured value. This permitted measured value is the ohms reading that we use on site. The actual value measured with our test meter should not exceed the maximum permitted measured value TABULATED VALUE × 0.8 = MEASURED VALUE

As temperature increases, resistance in the circuit also increases

The tabulated value of Zs is defined at 20°C,We must make allowance for temperature changes up to 70°C

To NOT exceed the tabulated value at 70°C we make the 20°C value even smaller by applying the 80% rule - multiplying by 0.8

If Zs(m) passes at 20°C then Zs will pass at 70°C

***Therefore Zs ≤ 2.27 * 0.8 ≤ 1.81Ω , This is minimum Condition for 100A or 20A B-Type Circuit Breaker ***

Example -2

For our 10 amp breaker , In * 5 times = 50A

Voltage = 230 * 0.95(Cmin) = 218.5V
Raw value = 4.37Ω

Applying the 80% rule to our 10 amp breaker

4.37 x 0.8 = 3.5Ω

3.5Ω is the maximum permitted value when comparing readings on your meter. So,3.5Ω is the maximum measured value.

Compare the Standard Value and On-Site Guide :

Tabulated value , Zs = 4.37Ω

Maximum measured value , Zs(m) = 3.50Ω

Conclusion : Measured impedance at furthest point in a system should be in a such a way that Circuit Breaker must trip , Means

1) Ia , Current should be greater than In(max). and

2) Should be less than Fault Current capacity say(4kA) of Circuit Breaker.

Example -1 (239*0.95) / 4000A ≤ Zs ≤ (239*0.95) / 100A

0.056Ω ≤ Zs ≤ 2.27Ω

0.044Ω ≤ Zs(m) ≤ 1.81Ω

Example -2 (230*0.95) / 4000A ≤ Zs ≤ (230*0.95) / 50A

0.054Ω ≤ Zs ≤ 4.37Ω

0.043Ω ≤ Zs(m) ≤ 3.5Ω