Earth Fault Loop Calculator

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Include Transformer Impedance

Include transformer impedance in calculation

Transformer MVA Rating

Transformer MVA rating

Transformer Impedance

Transformer impedance percentage

Insulation Type

Active Conductor CSA (mm²) *

Earth Conductor CSA (mm²) *

Cable Length *

Cable length in meters

Use Supplementary Earthing

Include supplementary earthing conductor

Supplementary Earth CSA (mm²)

Nominal Voltage (Uo) *

Nominal voltage in volts

Breaker Rating *

Circuit breaker rating in amps

Trip Curve Type

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Calculation No.: Calc-001
Revision: A
Description:
Created By: Engineer Name

Project: Project Name
Project No.: P001
Client: Client Name

Input Parameters

The following design parameters were considered for this earth fault loop impedance calculation:

Parameter	Symbol	Value	Notes
Cable Length	L	50 m	Main cable run
Active Conductor CSA	CSA_active	4	Phase conductor
Earth Conductor CSA	CSA_earth	4	Protective conductor
Nominal Voltage	U_o	230 V	Phase to earth voltage
Breaker Rating	I_n	32 A	Circuit breaker nominal current
Trip Curve Multiplier	k	7.5	Type C curve

Calculations

All calculations were performed in accordance with IEC 60364 and BS 7671 standards.

Cable Loop Impedance

The cable impedance is calculated from the active and earth conductor impedances: This represents the impedance of the cable between the source and the fault location.

Formula: $$ \begin{aligned} Z_{cable} &= \sqrt{R^2 + X^2} \\ &= \sqrt{(0.561)^2 + (0.0102)^2} \\ &= 0.561 \; \Omega \end{aligned} $$

Trip Current Calculation

The required trip current is calculated using: This is the minimum current required to operate the protective device.

Formula: $$ \begin{aligned} I_a &= I_n \times \text{Trip Multiplier} \\ &= 32 \times 7.5 \\ &= 240 \; \text{A} \end{aligned} $$

Maximum Allowed Impedance

The maximum earth fault loop impedance is calculated using: This is the maximum impedance that allows proper operation of the protective device.

Formula: $$ \begin{aligned} Z_{max} &= \frac{U_o}{I_a} = \frac{U_o}{I_n \times \text{Trip Multiplier}} \\ &= \frac{230}{32 \times 7.5} \\ &= 0.958 \; \Omega \end{aligned} $$

Total Earth Fault Loop Impedance (Zs)

The total earth fault loop impedance is the sum of source and cable impedances: This is the actual impedance of the complete earth fault loop path.

Formula: $$ \begin{aligned} Z_s &= Z_t + Z_{cable} \\ &= 0 + 0.561 \\ &= 0.561 \; \Omega \end{aligned} $$

Compliance Check

The calculated earth fault loop impedance Zs = 0.5610 Ω is less than the maximum allowed Zmax = 0.9583 Ω. The installation complies with the requirements.

Summary of Results

Below is the summary of calculated values for ease of reference:

Parameter	Symbol	Value	Notes
Trip Current	I_a	240 A	Required tripping current
Cable Loop Impedance	Z_cable	561 mΩ	Active + Earth conductor impedance
Total Earth Fault Loop Impedance	Z_s	561 mΩ	Measured/calculated impedance
Maximum Allowed Impedance	Z_max	958 mΩ	For proper protection operation
Compliance Status	Status	PASS	Circuit complies with requirements

Approval:

This calculation has been reviewed and approved by:

Created By
Engineer Name
Date: ..........................

Checked
By: .............................
Date: ..........................

Approved
By: .............................
Date: ..........................

Client Approved
By: .............................
Date: ..........................

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