The electrical tests part 2.
8.6.1 - Testing earth electrodes
The earth electrode, where used, is the means of making contact with the general mass of earth. Thus it must be tested to ensure that good contact is made. A major consideration here is to ensure that the electrode resistance is not so high that the voltage from earthed metalwork to earth exceeds 50 V. Where an RCD is used, this means that the result of multiplying the RCD operating current (in amperes) by the electrode resistance (in ohms) does not exceed 50 (volts). for normal dry locations, or 25 (volts) for construction sites and agricultural premises.
If a 30 mA RCD is used, this allows a maximum electrode resistance of 1,666 Ω, although it is recommended that earth electrode resistance should never be greater than 200 Ω .A maximum value of 100 ohm is proposed in a draft amendment of BS 7430, Code of Practice for Earthing.
There are several methods for measurement of the earth electrode resistance. In all cases, the electrode must be disconnected from the earthing system of the installation before the tests commence.
Fig 8.13 - Measurement of earth electrode resistance with a dedicated tester
1. - Using a dedicated earth resistance tester
The instrument is connected as shown in {Fig 8.13} with terminals C1 and P1 being connected to the electrode under test (X). To ensure that the resistance of the test leads does not affect the result, separate leads should be used for these connections. If the test lead resistance is negligible, terminals Ci and P1 may be bridged at the instrument and connected to the earth electrode with a single lead.
Terminals C2 and P2 are connected to temporary spikes which are driven into the ground, making a straight line with the electrode under test. It is important that the test spikes are far enough from each other and from the electrode under test. If their resistance areas overlap, the readings will differ for the reason indicated in {Fig 8.14}. Usually the distance from X to Y will be about 25 m, but this depends on the resistivity of the ground. To ensure that resistance areas do not overlap, second and third tests are made with the electrode Z 10% of the X to Y distance nearer to, and then 10% further from, X. If the three readings are substantially in agreement, this is the resistance of the electrode under test. If not, test electrodes Y and Z must be moved further from X and the tests repeated.
The tester provides an alternating output to prevent electrolytic effects. If the resistance to earth of the temporary spikes Y and Z is too high, a reduction is likely if they are driven deeper or if they are watered.
Fig 8.14 - Effect of overlapping resistance areas
a) resistance areas not overlapping
b) resistance areas overlapping
2. - Using a transformer, ammeter and voltmeter
The system is connected as shown in {Fig 8.15}. Current, which can be adjusted by variation of the resistor R, is passed through the electrode under test (X) to the general mass of earth and hence to the test electrode Y. The voltmeter connected from X to Z measures the volt drop from X to the general mass of earth. The electrode resistance is calculated from:
| voltmeter reading (V) |
| ammeter reading (A) |
As in the case of the dedicated tester, the test electrode Z must again be moved and extra readings taken to ensure that resistance areas do not overlap. It is important that the voltmeter used has high resistance (at least 200 Ω/V) or its low resistance in parallel with that of the electrode under test will give a false result.
Fig 8.15 - Measurement of earth electrode resistance
with a transformer, ammeter and voltmeter
3. - Using an earth fault loop impedance tester
The tester is connected between the phase at the origin of the installation and the earth electrode under test as shown in {Fig 8.16}. The test is then carried out, the result being taken as the electrode resistance although the resistance of the protective system from the origin of the installation to the furthest paint of the installation must be added to it before its use to verify that the 50 V level is not exceeded. If an RCD with a low operating current is used, the protective system resistance is likely to be negligible by comparison with the permissible electrode resistance.
Fig 8.16 - Measurement of earth electrode resistance using an earth-fault loop tester
It is most important to ensure that earthing leads and equipotential bonds are reconnected to the earth electrode when testing is complete.
8.6.2 - Measuring earth-fault loop impedance and
prospective short-circuit current
The nature of the earth-fault loop and its significance have been considered in detail in {5.3}. Since the loop includes the resistance of phase and protective conductors within the installation, the highest values will occur at points furthest from the incoming supply position where these conductors are longest. A measurement within the installation will give the complete earth-fault loop impedance far the point at which it is taken (Zs), or the earth-fault loop impedance external to the installation (Ze) may be measured at the supply position. Internal loop measurements should be taken at points furthest from the intake to give the highest possible results.
In simple terms, the impedance of the phase-to-earth loop is measured by connecting a resistor (typically 10 Ω) from the phase to the protective conductor as shown in {Fig 8.17}. A fault current, usually something over 20 A, circulates in the fault loop, and the impedance of the loop is calculated within the instrument by dividing supply voltage by the value of this current. The resistance of the added resistor must be subtracted from this calculated value before the result is displayed. An alternative method is to measure the supply voltage both before and whilst the loop current is flowing. The difference is the volt drop in the loop due to the current, and loop impedance is calculated from voltage difference divided by current.
Fig 8.17 - Simple principle of earth-fault loop testing
Since the loop current is very high, its duration must be short and must be limited to two cycles (or four half-cycles) or 40 ms for a 50 Hz supply. The current is usually switched by a thyristor or a triac, the firing time being controlled by an electronic timing circuit It is very important to have already checked the continuity of the protective system before carrying out this test. A break in the protective system, or a high resistance within it, could otherwise result in the whole of the protective system being directly connected to the phase conductor for the duration of the test. Commercial testers are usually fitted with indicator lamps to confirm correct connection or to warn of reversed polarity. {Fig 8.18} shows a typical earth-fault loop tester connected to a socket outlet so that its loop impedance can be measured. If the circuit to be measured includes socket outlets, the tester is connected as indicated in {Fig 8.18}. Special leads for connection to phase and to earth are provided by suppliers for all other circuits.
Fig 8.18 - Earth-fault loop tester connected for use
Before testing, the main equipotential bonding conductors are disconnected (BUT NOT THE CONNECTION WITH EARTH) to prevent parallel earth return paths and to ensure that there is no reliance on the service pies for gas and water for effective earthing, (REMEMBER TO RECONNECT THE MAIN EQUIPOTENTIAL BONDING AFTER THE TEST).
Tests must be carried out at the origin of the installation, at each distribution board, at all fixed equipment, at all socket outlets, at 10% of all lighting outlets (choosing points farthest from the supply) and at the furthest point of every radial circuit. The test should be repeated at least once to allow for the effect of transient variations in the supply voltage.
A modified version of the earth-fault loop tester, which effectively measures the phase to neutral impedance and calculates then displays the value of the current which would flow if the supply voltage were applied to this impedance are readily available. The principle of such a PSC tester is described in {3.7.2}.
Since the test result is dependent on the supply voltage, small variations will affect the reading. Thus, the test should be repeated several times to ensure consistent results. The test resistor will be connected across the mains for the duration of each test. and will become very hot if frequent tests are made. Some testers will then 'lock out' to prevent further testing until the resistor temperature falls to a safe value.
The earth fault loop impedance measured as described will be for installation cables at ambient temperature, unless the circuit concerned has been in use immediately before the test, when it will be the impedance at normal operating temperature. Under normal operating conditions, cable temperature will rise, and so will the resistive component of the impedance. This effect is difficult to calculate, and a practical alternative is to ensure that the measured values of earth fault loop impedance do not exceed three quarters of the maximum values shown in {Tables 5.1, 5.2 or 5.4} as appropriate.
A circuit protected by an RCD will need special attention, because the earth-fault loop test will draw current from the phase which returns through the protective system. This will cause an RCD) to trip. Therefore, any RCDs must be bypassed by short circuiting connections before earth-fault loop tests are carried out. It is, of course, of the greatest importance to ensure that such connections are removed after testing. One manufacturer supplies a patented loop tester which does not require RCDs to be short circuited and which will not cause them to trip
when the earth-fault loop test is made. Some instruments limit the test current to 15 mA so as not to trip RCDs with ratings of 30 mA and above. Whilst such tests may often be useful, they do not test the integrity of the system under fault current conditions.
When loop testing at lighting units controlled by passive infrared detectors (PIRs), there may he damage to the associated electronic switches unless they are short-circuited before testing.
An alternative to the use of a dedicated earth-fault loop impedance tester is to measure the combined resistance of the phase and protective conductors from the incoming position to the point for which earth-fault loop impedance is required (this is R1 + R2 - ) and to add to it the external earth-fault loop impedance (Ze) which can be obtained from the electricity supplier. All earth-fault loop impedance test results should be carefully compared with the data in [Tables 41B and 41D], adjusted to allow for ambient temperature, or with figures provided by the designer. To ensure that ambient temperature is taken into account, the results should never exceed three quarters of the values given in [Tables 41B and 41D].
8.6.3 - Testing residual current devices (RCDs)
Residual current devices should comply with BS 4293 and are described in (5.9), from which it will be seen that they are provided with a built-in self test system which is intended to be operated regularly by the user. BS 7671 requires that correct operation of this test facility should be checked, and that other tests are also carried out. The time taken for the device to operate must be measured, so the old type of 'go, no-go' tester is no longer adequate. {8.7.1} gives test instrument requirements.
RCD tests are carried out with a special tester which is connected between phase and protective conductors on the load side of the RCD after disconnecting the load {Fig 8.19}. A precisely measured current for a carefully timed period is drawn from the phase and returns via the earth, thus tripping the device. The tester measures and displays the exact time taken for the circuit to be opened. This time is very short, in most cases being between 10 and 20 ms, although it can be much longer, especially for S-types which have delayed operation
Fig 8.19 - Connections for an RCD tester
1. - General purpose non-delayed RCDs
This is a general purpose type of RCD which is intended to operate very quickly at its rated current. Three tests are required:
a) - 50% of the rated tripping current applied for 2 s should not trip the device,
b) - 100% of rated tripping current, which should not he applied for more than 2 s, must cause the device to trip within 200 ms (0.2 s), and
c) - where the device is intended to provide supplementary protection against direct contact, a test current of 150 mA, applied for no more than 50 ms, should cause the device to operate within 40 ms.
2. - Time-delayed RCDs
In {5.9.2} we discussed the need for discrimination between RCDs. This type is deliberately delayed in its operation to make sure that other devices which are connected downstream of it will operate more quickly. A 3:1 discrimination ratio is required between two RCDs which are connected in series, and this must be verified before testing. It means that the delayed RCD must have an operating current at least three times that of the non-delayed type. For example, to discriminate properly with a 30 mA device, a second connected on the supply side would need to have an operating current of at least 90 mA (in practice, a 100 mA RCD is likely to be used).
The test for the time-delayed RCD consists of applying 100% of the normal rated current, when the device should trip within the time range of:
For example, an RCD with a rated tripping time of 300 ms should trip within a time range of:
An RCD tester is an electronic device which draws current from the supply for its operation. This current is usually of the order of a few milliamperes which is taken from the phase and neutral of the supply under test, and will have no effect on the measurement of single-phase systems. However, if a three-wire three-phase system (there is no neutral with this supply) is being tested, the tester must be connected to a neutral conductor to provide the power it needs for operation. Thus, its operating current will flow through a line conductor and return through the neutral, giving a basic imbalance. A 'no-trip' test must also be carried out, during
Fig 8.20 - RCD tester connected for use
which the RCD must not operate when 50% of the rated tripping current is applied for 2 s. The extra current to power the tester, which adds to the test current, may then cause operation. It is necessary in this case to obtain from the RCD manufacturer the value of this current and to take it into account before failing a device on the 50% test.
The RCD tester is connected to the device to be tested by plugging it into a suitable socket outlet (see {Fig 8.20}) or by connecting to phase and neutral with special leads obtainable from the instrument supplier.
| (150 + 200) ms = 350 ms and |
| (300+200) ms = 500 ms |
| 50% of rated time delay plus 200 ms, and |
| 100% of rated time delay plus 200 ms. |
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To achieve compliance with the legal requirements of the Electricity at Work Regulations 1989 requires proof that an electrical system is safe, which involves amongst other things, proper inspection and testing of a system by competent people and the creation and maintenance of records.
Electricity at Work Regulations 1989 is law in the United Kingdom.
No person shall be engaged in any work activity where technical knowledge or experience is necessary to prevent danger or, where appropriate, Injury, unless he possesses such knowledge or experience, or is under such degree of supervision as may be appropriate having regard to the nature of the work.