Verification of earth fault loop impedance

Earth fault loop impedance testing is a common test undertaken every day all over the country. It is a staple of the electrical contracting industry. Most of the time the tests are simple to perform, and results are often easy to interpret. However, in some cases the measurements can be confusing, or give readings that seem to suggest the system is unsafe. Here, Curtis Jones, Technical Manager at ECA, provides some guidance about the methodology and interpretation of test results when undertaking earth fault loop impedance testing.

Key points to remember when performing the test and verifying the readings include the following:

• The method used may affect the accuracy of the reading.

• Test equipment with a low measurement range may be required when conducting tests near the origin of supplies or transformers.

• The temperature of conductors needs to be considered when verifying readings.

What is earth fault loop impedance?

The definition of ‘earth fault loop impedance’ can be found within part 2 of BS 7671 which states it is ‘the impedance of the earth fault current loop starting and ending at the point of earth fault. This impedance is denoted by the symbol Z_s.’

Essentially, it’s the level of impedance within a circuit opposing the current generated during an earth fault condition.

Example of an earth fault loop path in a circuit

The verification of earth fault loop impedance is crucial for each circuit within an installation to confirm automatic disconnection of supply (ADS) is achieved, when this is the protective measure provided against electric shock. Often this is achieved by the use of an overcurrent protective device. The device should disconnect the supply to the line conductor of the circuit or the equipment in the event of an earth fault condition, within a specified time. Table 41.1 of BS 7671 indicates maximum disconnection times and should be applied for final circuits within the scope of Regulation 411.3.2.2.

Table 41.1 – Maximum disconnection times from BS 7671

The figures presented in Table 41.1 rely upon a suitably low earth fault loop impedance path within the circuit, allowing sufficient fault current generation. This in turn will cause the protective device to operate within the specified time.

Methods of obtaining earth fault loop impedance values

There are two widely used methods for obtaining earth fault loop impedance values of a circuit:

1. The preferred method is often to perform a continuity test of R₁ + R₂ and then add the value obtained to the external earth fault loop impedance (Z_e) where the circuit is fed from the origin of the installation, or the (Z_db) where the circuit is fed from a sub-distribution board.

2. Direct measurement of total earth fault loop impedance (Z_s) can be made with an earth fault loop impedance tester.

It is important to select test equipment suitable for the purpose of the test, with measurement ranges that meet the requirements of BS EN 61557-3:2022. Performing a direct measurement is particularly useful when working within an existing installation, for example when carrying out periodic inspection and testing or maintenance activities.

Inaccuracies when measuring earth fault loop impedance

Modern multifunctional test instruments usually offer three methods for performing direct measurements of earth fault loop impedance. These methods can present certain limitations when trying to obtain an accurate reading that need to be considered.

The two-wire ‘high current’ test setting should always be utilised when possible. This method generates a large enough test current (typically in the region of 25 A) to create a measurable voltage drop and therefore a stable and accurate reading. However, if, for example, a residual current device is present, then either a three-wire ‘non-trip’ test setting or a two-wire ‘non-trip’ test setting would need to be utilised. The two-wire ‘non-trip’ setting is the most technically difficult for the instrument to perform and therefore more prone to errors. For this reason, this method should only be used as a last resort, for example at a passive switch where no neutral is present. The difficulty with the ‘non-trip’ test settings is that the test current is significantly smaller than the two-wire ‘high current’ test setting (generally not exceeding 15 mA) in order to prevent the residual current device within the circuit from tripping. Due to such a low level of current, the test does not create a significant voltage drop and readings are prone to a larger degree in variation.

Further factors such as external load switching, electrical noise, harmonics and other electronic components or devices will also create difficulties when obtaining a measurement. These factors can have a greater effect when using a ‘non-trip’ test method where they may further hinder the accuracy of the test results obtained. This emphasises the need to utilise the ‘high current’ test setting where possible.

Residual current devices (RCDs) can at times also present problems with test results if the internal impedance of the device itself is included. On occasion, this may be as high as an additional 0.5 Ω. Clearly there may be times when the addition of this number within the test result reading may indicate that the circuit doesn’t conform with the requirements of BS 7671.

This is commonly known as ‘RCD uplift’ and inspectors should be aware of this possibility, although it’s worth noting that this anomaly does not occur for all types of RCDs. If RCD uplift is suspected, then a simple measurement can be conducted on the supply side of the device and then repeated on the load side of the device to establish if additional impedance is observed. Some multifunctional test instruments are immune to the effects of RCD uplift and inspectors should consult manufacturers’ literature for their equipment.

Permissible measurement errors and effects on low impedance measurement accuracy

Measurements of earth fault loop impedance when close to a transformer or other large source of supply can present difficulties to off-the-shelf test instruments because the values likely to be obtained may be below the measurement range for the instrument. In this case, a high-resolution test instrument should be used to ensure more accurate and reliable test measurements.

It is worth remembering an enquiry may be made to the distributor to obtain external earth fault loop impedance and prospective fault current levels at the origin of an installation. For larger systems with a private dedicated supply, the organisation responsible for its commissioning may make these values available.

When establishing the suitability of an instrument to make reliable measurements, refer to the BS EN 61557 series for permissible values of measurement error for which test equipment should comply. For loop impedance testers, BS EN 61557-3:2022 allows for a permissible measurement error of up to 30%.

Permissible measurement error according to the BS EN 61557 series

When comparing this information with the manufacturer’s literature for the test equipment, the 30% permissible error looks quite large, and inspectors may initially believe their instrumentation is significantly more accurate than this requirement. However, there are a few terms we need to consider before an accurate assessment and verification of the test equipment can be established.

Common measurement terms

• Display range – the numerical value of the display. e.g. 2,000. Typically, a display will show values of 00.00-19.99 Ω, 199.9 Ω or 1,999 Ω.

• Resolution – the smallest change in the measured value that the displayed range can show. For a 2,000 numerical value display this would typically be 0.01Ω.

• Measurement range – where the manufacturer states that the possible error is not greater than that specified in BS EN 61557-3. For example, 0.20 Ω to 1,999 Ω.

• Instrument accuracy – this is specified and made up of two values.

– Analog error: normally expressed as a percentage of the measured value.

– Digital error: shown as an additional digit error or digit measurement value.

Accuracy is shown on a product datasheet as ±(% of m.V. + digit error).

For example, a typical multifunctional test instrument may have a numerical value display range of 0.00 Ω to 1,999 Ω for the earth fault loop impedance test setting. However, in reality, these types of instruments are likely to have a measurement range different to this. Measurement values below around 0.20 Ω in typical multifunction test instruments may be susceptible to significant errors and fall outside the 30% permissible measurement error range.

Example of a measurement range with 0.01 Ω resolution with an accuracy/error of ±(2% m.V. + 4 digits)

Manufacturers of test instruments are now required to provide information on the measurement range of the instrument, taking account of the permissible measurement error values. Previously, the information provided with such test instruments was often the numerical displayed value range, rather than the measurement range.

This information can be found within the manufacturer’s literature or may be displayed on the instrument itself, and could typically be 0.30 Ω to 1000 Ω with a resolution of 0.01 Ω. This would still satisfy BS EN 61557-3:2022 when used within its parameters, but this equipment may not be suitable for use where measurements fall outside of the measurement range specified by the manufacturer.

Example of user guide display and measurement range tolerances for a multifunction tester (Sonel MPI-540)

High resolution earth fault loop impedance testers

Many larger installations have private substations and transformers supplying the premises. In these circumstances, values of impedance near the origin are likely to be very low, and protective devices such as BS EN 60947-2 moulded case circuit breakers (MCCBs) are often used. Where live testing is performed, suitable test equipment should be employed in order to verify adequate disconnection of such devices. It would be wrong to assume a circuit fails to meet its disconnection time, if maximum values of impedance aren’t met, if the values fall outside the instrument’s measurement range taking into account the permissible measurement error values.

For such circumstances, a suitable high resolution earth fault loop impedance test instrument should be selected when performing tests. These types of instruments typically have increased measurement resolution of 0.1 mΩ and have a measurement range reading as low as 7.2 mΩ, whilst providing a test current in the order of 130 A to 300 A.

Example of a measurement range with 0.1 mΩ resolution with an accuracy/error of ±(2% m.V. + 2 mΩ digits)

The increased measurement range and instrument resolution enables more accurate readings to be obtained and thus the inspector can verify if the requirements of automatic disconnection are being satisfied. Inspectors should always consult manufacturers’ literature to confirm that test instruments are used within the parameters for which they were designed.

Example of a high-resolution earth fault loop impedance test instrument (image supplied by Sonel)

Maximum acceptable values of earth fault loop impedance

Values of earth fault loop impedance obtained should preferably be verified against maximum values stipulated by the manufacturer of the protective device, which are often less onerous than the values detailed within BS 7671. This information should be noted on the schedule of circuit details and schedule of test results when completing required documentation.

In the absence of manufacturers’ data, BS 7671 provides maximum earth fault loop impedance values for common fuses and circuit breakers. However, these values are based on the line conductors being at their maximum permitted operating temperature, and circuit protective conductors being at their assumed initial temperature when the measurement of impedance was obtained. For measurements made at ambient temperatures, Appendix 3 of BS 7671 provides guidance, and adjusted values for typical circumstances can be found within the IET’s Guidance Note 3 or the IET’s On-Site Guide. These are often known as the ‘80% Values’.

Many inspectors apply a blanket approach to verifying Z_s values against the values that have been adjusted for ambient temperature. However, in some instances this may be difficult to achieve due to the circuit constraints. For example, during periodic inspection and testing a load may have been momentarily shut down to allow a test to be performed. As a result, the circuit conductors will likely still be considerably warmer than that of ambient temperature in a vicinity. The circuit may also be installed within shared containment systems such as trunking, where other adjacent circuits operating under load conditions may cause heating effects for the circuit under test. As can be seen, verifying Z_s values obtained often involves applying sound engineering logic in order to confirm compliance with BS 7671.

Summary

Earth fault loop impedance testing, when done well, is a simple and easy method to verify the safety of circuits. In some cases, off-the-shelf test equipment may not always be accurate or specific enough to measure, with any degree of certainty, the earth fault loop impedance of the circuit.

It is therefore vital that not only should you have the correct competence of people using the equipment, but the equipment itself should be suitable for the function it is performing.

A simple off-the-shelf multifunction piece of test equipment will normally suffice for most projects, but on occasion specialised equipment may be needed.

FURTHER READING:

• ECA Guidance Note – Verification of Earth Fault Loop Impedance.

• ECA Guidance Note – Work on or near live LV electrical systems.

ECA would like to thank Rob Barker of Power Quality Expert and Sonel for their contributions to the ‘Verification of Earth Fault Loop Impedance’ Guidance Note available free to members through the ECA Members portal.

eca.co.uk

This article appeared in Electrotechnical News May/June 2025 edition – read it here