Foretec FLIT — Fault Loop Impedance Calculator

1 Project, Client & Test Details

Client / Consumer name

Installation / Site

Report reference no.

Date of study / test

Carried out by

Instrument used (make / model / serial)

Calibration certificate no.

Calibration valid up to

Ambient / conductor condition

2 System, Earthing Arrangement & Source

Earthing system and reference voltage

Earthing system

Cl. 4.2.11.4 / .5 / .6 of IS 732

Current type

U₀ — line to earth V

Max. disconnection time t s — Table 1 IS 732

Installation type — Table 47 IS 732 (voltage drop)

Circuit use

Permissible voltage drop %

Apply the Table 47 note — beyond 100 m of main wiring the limit may be increased by 0.005 % per metre, up to 0.5 %

Source — transformer or generator

Source type

Rating MVA

Primary voltage kV

Secondary (LV) voltage kV, line-to-line

Impedance %Z

Source impedance Z_s Ω

%Z × kV² / (100 × MVA) — Eq. 7, IEC 60909-0

Source R Ω

Source X Ω

X/R ratio assumed

Splits Z into R and X for vector summation

Three-phase symmetrical fault current at source A

Full-load current of source A

Earth-fault current at source terminals A

3 Fault Loop — Circuit Levels (source → final circuit)

How to use. Add one level for each protective device in the loop, in order, starting at the main LV panel. Each level carries its own OCPD, line conductor and protective (PE) conductor. Impedances accumulate down the cascade exactly as in the fault loop. Enter measured values in section 4 once the installation is tested.

4 Measured Values & Instrument Accuracy (IEC 61557-3)

Total operating uncertainty of a loop-impedance meter shall not exceed ±30 % of the measured (fiducial) value — IEC 61557-3 with IEC 61557-1 Table 1. Enter the meter data sheet figures once; they apply to every measurement below.

Range — minimum Ω

Range — maximum Ω

Resolution Ω

Error — % of measured value

Error — digits × resolution

Injection current

Test point sequence

Enter measured Z_s per level in the table below

5 Results & Compliance Summary

Fault loop impedance build-up

Prerequisite compliance at each level

Recommendations

6 Client Report

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7 Reference Data & Basis of Calculation

Expressions used

Eq.	Expression	Source
1	Z ≤ U₀ / I_a	Cl. 4.2.11.4.4, IS 732:2019
1A	R_A × I_Δn ≤ 50 V	Cl. 4.2.11.5.3, IS 732:2019
2	Z_{max, allowable} = ⅔ × Z	Cl. 6.2.3.6.2, Annex MM, IS 732:2019 — allowance for conductor temperature rise during the fault
3	Z_source = %Z × kV² / (100 × MVA)	Eq. 7, IEC 60909-0
4	R = ρ × L / A ; R_ac ≈ 1.1 R_dc	Table 1, IEC 60287-1-1:2023
5	X = 2πf × 2×10⁻⁷ × [¼ + ln(D/r)] Ω/m	IEC 60909-4 — self + external inductance
6	I_b ≤ I_n ≤ I_z and I₂ ≤ 1.45 I_z	Cl. 4.4.4.1, IS 732:2019
7	u = b(ρL/S·cosø + λL·sinø) I_b ; Δu% = 100u/U₀	Annex Y, IS 732:2019
8	S_min = √(I²t) / k, for t ≤ 5 s	Cl. 4.4.5.5 and Cl. 5.4.3.1.2, IS 732:2019 (k from Table 11 and Annex EE Tables 58 to 62)
8A	S_p ≥ S for S ≤ 16 ; 16 for 16 < S ≤ 35 ; by calculation for S > 35	Table 12, Cl. 17.2.2.2, IS 3043:2018
8B	Separate PE ≥ 2.5 mm² with mechanical protection, ≥ 4 mm² without	Cl. 17.2.2.3, IS 3043:2018
7B	ρ_θ = ρ₂₀ [1 + α₂₀(θ − 20)]	Table 1, IEC 60287-1-1:2023 — temperature correction of conductor resistivity
9	B% = ±(B / fiducial value) × 100 ≤ 30 %	IEC 61557-3 with Table 1 of IEC 61557-1

Table 1 IS 732 — maximum disconnection times

Conductor resistance and reactance (IS 8130 / manufacturer data)

OCPD trip multiples — IEC 60898-1 / -2, IEC 60269-1, IEC 60947-2

k values — Table 11 and Annex EE Tables 58 to 62 of IS 732

Protective conductor sizing — Table 12 and Cl. 17.2.2.3 of IS 3043

Transformer minimum %Z — Table 1 of IEC 60076-5

Cable current ratings — Tables 20 and 21 of IS 732

Line inductance — derivation behind Expression 5

Engineering notes, assumptions and deviations from the source workbook

Foretec Electric India Pvt Ltd — Passion in Power Quality · Authorised Janitza Electronics GmbH Blue-Level Solution Partner
This tool assists the designer and the tester. It does not replace the judgement of a competent electrical engineer or the requirements of the licensing authority. Standards are cited for guidance; always work from the current licensed text.

FLIT — Fault Loop Impedance Calculator

1 Project, Client & Test Details

2 System, Earthing Arrangement & Source

Earthing system and reference voltage

Source — transformer or generator

TT / IT system — RCD condition (Cl. 4.2.11.5.3 of IS 732)

3 Fault Loop — Circuit Levels (source → final circuit)

4 Measured Values & Instrument Accuracy (IEC 61557-3)

5 Results & Compliance Summary

Fault loop impedance build-up

Prerequisite compliance at each level

Recommendations

6 Client Report

7 Reference Data & Basis of Calculation