Bill Allan from NAPIT writes about the measurement of low Zs Values and gives some great advice from measuring Zs on large cables to the formula for the earth fault loop impedance.
When Your Meter Lies To You
(Measuring Low Zs Values)
by
Bill Allan
Introduction
Knowledge of the earth fault loop impedance (Zs ) is an essential factor where the most commonly used protective measure - automatic disconnection of the supply – is used (Fig. 1).
Fig 1 Earth fault loop impedance path
The formula for the Earth Fault Loop Impedance is as follows:
Zs = Ze + (R1 + R2) ohms
where,
Ze = the external impedance
R1 = the resistance of the line conductor
R2 = the resistance of the circuit protective conductor
Zs = the earth fault loop impedance
Regulation 612.9 of BS 7671 permits the Zs value to be determined by measuring it on a live installation with a suitable test instrument or by using an alternative method, such as calculation (Chapter 2 of IET Guidance Note 3 Inspection and Testing, explains this in detail). The value obtained is then compared with the appropriate tabulated maximum value of Zs as given in Tables 41.2, 41.3 and 41.4 of BS 7671, after making allowance for the 0.8 temperature correction factor as indicated in Appendix 14.
When measuring Zs, inaccurate measurements often occur where the Zs value being measured is very low. This generally occurs in two situations:
when the measurement is carried out close to the supply transformer
2. when the measurement is carried out on circuits which are protected by overcurrent devices with a rating in excess of 50 amps.
As the prospective fault current figure is based upon the Zs measurement (whether carried out as an internal calculation by the instrument or by manual calculation), it can also be affected significantly by small variations in Zs values.
This article will discuss the problem of inaccurate readings at low values of Zs and what can be done about it.
Factors affecting the accuracy of EFLI testers
Firstly, we will consider the inherent limitations of earth fault loop impedance (EFLI) test instruments. A number of factors affect the accuracy of the readings obtained. Such factors include variation of the mains voltage, mains noise (ie. interference), harmonics and various types of external influences which may be present during the brief test period. It is therefore recommended by IET Guidance Note 3, Inspection and Testing that the EFLI test is repeated at least once. (Fig. 2)
Fig 2 Earth fault loop impedance test
Another factor affecting the accuracy of EFLI test instruments, as mentioned above, occurs when measuring very low values of Zs. The accuracy and repeatability of the instrument’s readings decrease at very low values of Zs.
Resolution and accuracy
A person carrying out inspection and testing must be familiar with the manufacturer’s instructions to be able to use the test instruments to achieve reliable results. This will include being aware of the resolution and the accuracy of the specific instrument being used.
The resolution is the smallest unit that can be measured by that instrument. A typical resolution is 0.01.
Accuracy is the amount by which the instrument’s reading differs from the true value and a figure of 5% is common.
An instrument with a resolution of three digits e.g.0.01 and an accuracy of 5% will be accurate down to around 0.2 (0.01 divided by 0.05 = 0.2).
The resolution of digital instruments is often stated as ‘digits’. For example, let’s say a digital test instrument has an efficiency of 2% and is measuring 100Ω. As 2% of 100 is 2, this means that the instrument could read between 98 Ω and 102 Ω. If this instrument has a resolution of 2 digits, this means that it can display 2 digits on either side of 98 Ω and 102Ω, so that the lowest value it could read is 96Ω and the highest value it could read is 104Ω.
Table 1 shows the variation in readings that can be obtained when using a digital test instrument having an accuracy of 5% and a resolution of 3 digits. As the values of earth fault loop impedance are usually below 1 ohm, and the lowest reliable reading is around 0.2 ohms, this Table shows values from 0.9 down to 0.2. Notice that some of the readings have a negative value.
Positive % accuracy
Negative % accuracy
Difference 3 digits
Overall worst case
Overall worst case
Ohms
5.00%
5.00%
will make
positive
Negative
0.20
0.21
0.19
0.3
0.51
-0.11
0.30
0.315
0.285
0.3
0.815
-0.015
0.40
0.42
0.38
0.3
0.72
0.08
0.50
0.525
0.475
0.3
0.825
0.175
0.60
0.63
0.57
0.3
0.93
0.27
0.70
0.735
0.665
0.3
1.035
0.365
0.80
0.84
0.76
0.3
1.14
0.46
0.90
0.945
0.855
0.3
1.245
0.555
The standard which EFLI instruments are required to conform to is BS EN 61557-3 and instruments conforming to this standard usually have a resolution of 0.01 Ω (10 mΩ) and an efficiency of 5% and therefore are generally accurate for values down to about 0.2 ohms. This means that EFLI instruments are accurate for circuits with values of Zs higher than 0.2 ohms, that is, circuits rated up to about 50A. However, the Zs value of circuits with higher current ratings (and lower values of Zs) should be obtained by calculation (Zs = R1 + R2 ).
Example 1
We will now use an example to illustrate the variation in values that can be obtained at very low values of impedance and the effect on the prospective fault current.
An EFLI test is carried out on a circuit having a Zs value of 0.1 ohms. The test instrument has an accuracy of +/- 5% (0.05). As 5% of 0.1 is 0.005, the reading obtained could vary from:
0.1 + 0.005 = 0.105 ohms to
0.1 – 0.005 = 0.095 ohms
The prospective fault current calculated from these values could vary from:
230
If = 0.105 = 2.19 kA
to
230
If = 0.095 = 2.42 kA
Hence the need to obtain the Zs value by an alternative method.
Test current
The level of test current used by EFLI testers is also significant in determining the accuracy of such instruments. Again, the figure of 0.2 ohms is generally regarded as being the lowest reliable value when using instruments having higher test currents (up to about 25A). A value below 0.2 ohms must be regarded as being potentially in error. Testers which have a low test current to avoid tripping RCDs (eg. 15 mA) are generally accurate down to values of around 0.1 ohms.
Measuring Zs on large cables
The impedance of the fault loop path comprises of inductive reactance (XL) as well as resistance but most loop testers measure only the inductive reactance. The value of inductive reactance becomes significant in larger cables with cross-sectional areas of over 16mm2 , which leads to a reduction in accuracy when measuring Zs.
Measuring Zs near a supply transformer
When measuring the Zs value near to a supply transformer, the Zs values can be very low, typically less than 0.1 ohms, creating a problem with accuracy. In addition, the high inductance of transformer windings affects the accuracy of the measurement recorded by most loop testers. In such situations, the Zs value should be obtained by calculation using the manufacturer’s information. The following example shows how the fault current and the impedance close to a transformer can be calculated.
Example 2
Calculate the fault current and the impedance value of a transformer having the following characteristics:
3 phase, 11kV primary / 415V secondary (315 kVA rating)
Percentage impedance voltage (from name plate), Vz = 4.8%,
where Vz is the percentage impedance voltage (ie. the voltage applied to the primary which results in full load current when the secondary is short-circuited)
Apparent power, S = √3 Vl Il
S
Load current, Il = √3 Vl
315000
I fl = √3 x 415 = 438 A per phase
I fl x 100
Short-circuit current, I sc = Vz
438 x 100
= 4.8
= 9.125 kA
V
Impedance, Z = I sc
240
= 9125
= 0.026 ohms
Conclusion
In common with other test instruments, EFLI instruments must be regularly checked and recalibrated as specified by the instrument manufacturer. Check boxes are available which allow the accuracy of test instruments to be monitored. Check boxes are not used instead of annual calibration. They are intended to be used on a regular basis to ensure that inaccuracies do not occur. Annual calibration of both the test instrument and the check box should be carried out, either by the manufacturer of the test instrument or by an authorized dealer.
Leads must also be regularly checked.
A competent person carrying out inspection and testing of an electrical installation must be aware of the limitations of the test instruments being used and be able to accurately interpret the readings obtained and, where necessary, be able to obtain accurate test results by an alternative method.