Loop impedance accuracy

Factors affecting the accuracy of loop impedance measurements

The accuracy of measurements taken with all types of test instrument is affected by various factors. This article looks specifically at earth fault loop impedance test instruments and the need to take account of factors affecting accuracy when using the instrument and when checking the test readings for compliance with the requirements of BS 7671.

Measuring range Loop impedance instruments generally have a number of ranges, typically 0-20 Ω, 0-200 Ω and 0-2000 Ω.

The most precise test result will be given using the lowest applicable range. So, if a loop impedance test result of say 12 Ω is obtained on the 0-200 Ω range, it would be advisable to retest, having changed to the 0-20 Ω range.

Test current setting Where circuits do not incorporate protection by an RCD or other device that would trip, such as a 6 A Type B circuit-breaker, earth fault loop impedance measurements should be made using the higher test current setting (up to about 25 A) of the instrument. A displayed test result less than about 0.2 Ω could be prone to significant errors. Such errors can significantly affect the calculation of prospective fault current.

On the low current, or “no-trip”, setting (such as 15 mA), displayed test results less than about 1.0 Ω could be prone to significant errors, which again can significantly affect Mains noise or disturbance Non-trip loop tests frequently use low test currents (15 mA) for testing RCD-protected circuits. These tests are susceptible to noise or mains disturbances, which may create variation in the results. If there is any concern about the result, the test should be repeated.

Supply and load Product standard BS EN 61557-3 indicates that the following conditions must be met while a measurement is being taken, in order for the loop impedance instrument to function within its intended operating uncertainty limits. If any of the conditions is not met, the validity of the test result will be reduced. • No installed load is connected to the circuit. • The voltage is not lower than 85% or higher than 110% of the nominal voltage. • The frequency is not lower than 99% or higher than 101% of nominal frequency. • The system voltage and frequency are maintained constant.

the calculation of prospective fault current.

Poor connections between the instrument and the circuit to be tested Poor connections between the instrument and the circuit to be tested can significantly reduce the accuracy of the measured loop impedance value displayed on the instrument. Causes of such poor connections can include, amongst other things: • probe contact resistance: this will depend on the condition of the probe tips and the material to which they are connected, and the pressure applied. • crocodile clips: the clips may be weak, or one side of a clip may have a lower resistance than the other, the hinge creating the higher resistance path. • condition of test lead connectors: poorly maintained, old or worn connectors can add significant error and variability to a result. Test leads do wear out. • leads other than those supplied with the instrument.

Note. Manufacturers may state slightly different conditions.

Proximity to a distribution transformer A loop impedance measurement taken within about 50 m of the distribution transformer can be very low, typically less than 0.1 Ω. Also, the measurement is liable to suffer an error (reading too low) due to the high inductive reactance of the transformer not being properly taken into account by the instrument.

As prospective fault current values are generally derived from the loop impedance measurements either directly by instruments or by manual calculation, small variations in the measurement of loop impedance values can result in significant differences in prospective fault current indications or calculations.

In such cases, it may be necessary to use a method other than a loop impedance test instrument to determine prospective fault current, such as calculation.

ONOFFON OFF Fig 1 – Measurement of maximum prospective fault current using a three-lead instrument Note: Refer to instrument manufacturer’s instructions for correct connections instrument with a digital display gives a test reading of 0.3 Ω for a certain circuit, and that the manufacturer has declared the following information about the instrument.

• Accuracy: ±3 % • Variation in least significant digit of display: ± 3 • Resolution: ±0.01 Ω

For a displayed test reading of 0.3 Ω, this means that the true value being measured lies somewhere between 0.278 Ω and 0.322 Ω, as follows: • ±3 % accuracy means that the true value is between 0.291 Ω and 0.309 Ω.

• ± 3 in the least significant digit means that the true value is between 0.288 Ω and 0.312 Ω. • ±0.01 Ω resolution means that the true value is between 0.278 Ω and 0.322 Ω.

Further information Information about the accuracy and consistency of test instruments generally, as well as about systems for confirming ongoing accuracy and consistency, can be found in Best Practice Guide No 7, published by the Electrical Safety Council. This can be downloaded free-of-charge at http://www.esc.org.uk/industry/ industry-guidance/bestpractice-guides/

Note: Refer to instrument manufacturer’s instructions for correct connections

Alternatively, a loop test instrument designed to operate at the appropriate system phase angle may be used (BS EN 61557-3 clause 4.1 refers). Advice about this should be obtained from test instrument manufacturers.

Instrument accuracy and resolution

Accuracy The accuracy of test instruments for use on installations is normally expressed as a percentage, typically plus or minus (±) a certain value. For instruments with a digital display, the accuracy may also be declared in terms of the variation in the least significant digit(s) of the display, or in terms of an ohmic value to be added to or subtracted from the indicated value.

4.60 kA

P-E P-N

PSCC 20kA

2000A 20Ω

LOOP

200Ω

2000Ω

TEST

200-260V 50/60HZ

Resolution The resolution of a measuring instrument is the smallest change in the quantity being measured that causes a perceptible change in the indication of the instrument. So, a resolution of 0.01 Ω allows measurements to be displayed in steps of 0.01 Ω, whereas a resolution of 0.001 Ω allows measurements to be displayed in steps of 0.001 Ω. Any variance less than the resolution step size will be rounded down.

The resolution may not be the same for the different measuring ranges of an instrument. Step sizes often become larger for the higher ranges.

Worked example Suppose a particular loop test