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6  Loop impedance
tests

17

Disconnect the instrument from the circuit under test before
operating the function switch.

To select the loop testing range select ‘LOOP’.

6.1 Voltage Measurement

When the tester is set to the loop test function, mains voltage is displayed
as soon as the instrument is connected for test. This voltage display is
automatically updated five times every seconds. The voltage function
operates whenever the test button is in the up position.

6.2 What is earth fault loop impedance?

The path followed by fault current as a result of a low impedance fault
occurring between the phase conductor and earth is called earth fault
loop. Fault current is driven round the loop by the supply voltage, the
amount of current depending on the voltage of the supply and on the
impedance of the loop. The higher the impedance, the lower will be the
fault current and the longer it will take for the circuit protection (fuse or
circuit breaker) to operate and interrupt the fault.

To  make sure that fuses will blow or that circuit breakers will operate
quickly enough in the event of a fault, the loop impedance must be low, the
actual maximum value depending on the characteristics of the fuse or the
circuit breaker concerned. The IEE Regulations (BS 7671) provides tables
showing the maximum permissible values of loop impedance in circuits
protected by various fuses and circuit breakers. Every circuit must be
tested to make sure that the actual loop impedance does not exceed that
specified for the protective device concerned.

6.3 Automatic over-temperature cut-out

During the short test period the instrument operates at high power. If
frequent tests are conducted over a prolonged period of time, the internal
test resistor will overheat. When this happens, further tests are
automatically inhibited and the over-temperature symbol   appears in the
display. The instrument must then be left to cool down, after which testing
may be resumed.

6.4 The loop impedance test

Since the earth fault loop is made up of conducting path which includes the
supply system back to the supply transformer, it follows that loop testing
can only be carried out after the mains supply has been connected. In
many cases, any RCD in the circuit will be tripped by this test, which draws
current from the phase and returns it through the earth system. The RCD

Summary of Contents for KT62

Page 1: ...KEWTECH KT62 digital multi function tester Instruction manual ...

Page 2: ...17 6 1 Voltage measurement 17 6 2 What is earth fault loop impedance 17 6 3 Automatic over temperature cut out 17 6 4 The loop impedance test 17 6 5 Loop impedance at 3 phase equipment 19 7 Prospective Short Circuit Current PSC tests 21 7 1 What is Prospective Short Circuit 21 7 2 Testing Prospective Short Circuit Current 21 8 RCD tests 23 8 1 Purpose of RCD Test 23 8 2 What does the RCD test real...

Page 3: ...talwork may become live for the duration of the test 5 Never open the instrument case except for fuse and battery replacement and in this case disconnect all leads first because dangerous voltages are present Only fully trained and competent electrical engineers should open the case If a fault develops return the instrument to Kewtech for inspection and repair 6 If the overheat symbol appears in t...

Page 4: ...ntinuity test and an insulation test is to follow disconnect the test leads from the circuit before moving the selector switch 15 Do not rotate function dial when test button is depressed If the function switch is inadvertently moved to a new function when the test button is depressed or in lock down position the test in progress will be halted 16 Always check the test lead resistance before carry...

Page 5: ...D Phase switch Polarity switch Continuity null switch RCD rated tripping current switch Function switch Wiring check LED LCD display Test button LCD display Test Lead with IEC Connector Test Lead for Continuity and Insulation Testing Fig 1 ...

Page 6: ...obe for loop testing In the insulation resistance testing mode the instrument provides a rated current of1mA as required in BS 7671 IEE Wiring Regulations and BS EN 61557 2 1997 In the continuity testing mode the instrument provides a short circuit current of 200mA as required in BS 7671 IEE Wiring Regulations and BS EN 61557 4 1997 Continuity and insulation resistance functions have the following...

Page 7: ...rther measurements Phase angle selector The test can be selected from either the positive 0 or from the negative 180 half cycle of voltage This selector is used in the RCD mode to obtain the maximum trip time of an RCD for the test selected DC Test Allows testing of RCDs which are sensitive to DC fault currents Auto data hold Holds the displayed reading for a time after the test is complete Auto p...

Page 8: ... 0 1mA or greater 250kΩ as per BS7671 20 200MΩ Auto Ranging 1 5 rdg 3dgt 500V 30 0 1mA or greater 500kΩ as per BS7671 20 200MΩ Auto Ranging 1 5 rdg 3dgt 1000V 20 0 1mA or greater 1MΩ as per BS7671 20 200MΩ Auto Ranging 1 5 rdg 3dgt Insulation Resistance 230V 10 15 50Hz 230V 10 15 50Hz 230V 10 15 50Hz Loop Impedance 25A 15mA 15mA 3 rdg 4dgt 3 rdg 8dgt 3 rdg 4dgt 20Ω 200Ω 2000Ω 230V 10 15 50Hz Prosp...

Page 9: ...100 300mA Note on x5 range max current that can be generated is 1A 50ms T rip Time 1 rdg 3dgt To prevent wrong connection of test leads and to maintain safety the dedicated terminals used for continuity and insulation tests are automatically covered when using the terminals for loop impedance PSC and RCD tests Instrument dimensions130 X 183 X 100mm Instrument weight 1130g including batteries Refer...

Page 10: ...est is correct The reverse lamp is lit when P and N are reversed Auto data hold In the loop impedance PSC and RCD test functions the LCD reading is automatically frozen for 3 seconds after measurement Display The liquid crystal display has 3 1 2 digits with a decimal point and units of measurement Ω MΩ A kA V and ms relative to selected function The display is updated approximately five times per ...

Page 11: ... any test lead resistance You should only use the test leads supplied with the instrument Proceed as follows 1 Select the continuity test by rotating the function dial 2 Connect the ends of the test leads firmly together see Fig 2 and press and lock down the test button The value of the lead resistance will be displayed 3 Operate the Continuity Null button this will null out the lead resistance an...

Page 12: ...e test lead resistance already subtracted if the Continuity null function has been used 7 Note that if the circuit resistance is greater than 20Ω the instrument will autorange to the 200Ω if it is greater than 200Ω it will autorange to the 2000Ω range Note If the reading is greater than 2000Ω the overange symbol OL will remain displayed The KT62 is provided with a facility to change the polarity o...

Page 13: ... as leakage current This current is made up of three components which are 1 capacitive current 2 conduction current and 3 surface leakage current 5 1 2 Capacitive Current The insulation between conductors which have a potential difference between them behaves as the dielectric of a capacitor the conductors acting as the capacitor plates When a direct voltage is applied to the conductors a charging...

Page 14: ...ved for the connection of conductors and so on current will flow across the surfaces of the insulation between the bare conductors The amount of leakage current depends on the condition of the surfaces of the insulation between the conductors If the surfaces are clean and dry the value of the leakage current will be very small Where the surfaces are wet and or dirty the surface leakage current may...

Page 15: ...urrent falls to zero and a steady insulation resistance reading indicates that the capacitance of the system is fully charged The system is charged to the full test voltage and will be dangerous if left with this charge The KT62 provides an automatic path for discharging current as soon as the test button is released to ensure that the circuit under test is safely discharged If the wiring system i...

Page 16: ...s by switching to continuity test and shorting out the lead ends A high reading will indicate that there is a faulty lead or that the fuse is blown 4 Make sure the circuit to be tested is not live A warning lamp is lit if the instrument is connected to a live circuit but test the circuit as well 5 4 Insulation resistance measurement The KT62 has a selectable triple test voltage of 250V 500V and 10...

Page 17: ... 7 3 If the mains warning lamp lights and or the buzzer sounds do not press the test button but disconnect the instrument from the circuit Make the circuit dead before proceeding Fig 8 4 Press the test button when the display will show the insulation resistance of the circuit or the appliance to which the instrument is connected 5 Note that if the circuit resistance is greater than 20MΩ the instru...

Page 18: ...nd the live circuit warning buzzer will sound CAUTION Never turn the function dial whilst the test button is depressed as this may damage the instrument Never touch the circuit test lead tips or the appliance under test during insulation testing Always release the test button first after testing before removing the test leads from the circuit This is to ensure that charges stored in the circuit ca...

Page 19: ...h in the event of a fault the loop impedance must be low the actual maximum value depending on the characteristics of the fuse or the circuit breaker concerned The IEE Regulations BS 7671 provides tables showing the maximum permissible values of loop impedance in circuits protected by various fuses and circuit breakers Every circuit must be tested to make sure that the actual loop impedance does n...

Page 20: ...he third red LED is OFF 4 Note the mains voltage displayed by the instrument 5 Press the Press to test button The value of the measured loop impedance will be displayed with the appropriate units The test will start at the sound of a bleep Whilst the test is being conducted the display will show a series of moving square symbols see below When these stop the measurement value will be displayed 6 I...

Page 21: ...e external earth probe socket ensuring the probe is held with fingers behind the finger guard The red LED will switch on as well as the two green LEDs remaining on The digital display will change to LO Initially the buzzer will sound indicating that the probe is plugged in When this probe is connected to an earth point the red LED will switch off and the display will revert to voltage mode If the ...

Page 22: ...m has no neutral then you must connect the black neutral probe to the earth i e the same point as the green earth clip This will only work if there is no RCD in this type of system Note 200Ω Loop range This range uses complex sampling and averaging routines Therefore if the circuit under test changes in value during the period when the unit is plugged in for test the resulting measurement displaye...

Page 23: ...d neutral or at a socket outlet between phase and earth If testing at a distribution board proceed as follows 1 Select the 200A 2000A or 20kA range 2 Connect the distribution board lead to the IEC socket on the instrument 3 Connect the red phase probe of the lead to the phase of the system the black probe to the neutral of the system and the green crocodile clip to the neutral of the system 4 Ensu...

Page 24: ... under test changes in value during the period when the unit is plugged in for test the resulting measurement displayed will be the average of all values If the reading between successive test button presses changes significantly the resulting displayed value may be inaccurate Note PSC function has a power factor correction of 0 84 PSC V x 0 84 A ...

Page 25: ...aximum 8 2 What does the RCD test really do The RCD is designed to trip out when the difference between the phase current and the neutral current this is called the residual current reaches the tripping value or rating of the device The tester provides a carefully preset value of residual current depending on its setting and then measures the time lapse between the application of the current and t...

Page 26: ... the lamps are lit press the test button to apply full rated tripping current and the RCD should trip the tripping time being shown on the display If the RCD has tripped the PN and PE LEDs should be off Check this is so 13 Press the phase selector switch to indicate 180 in the display and repeat the test 14 Make sure to keep clear of earthed metal during the operation of these tests 8 4 Testing RC...

Page 27: ...eration of this test Note If the RCD does not trip the tester will supply the test current for a maximum of 2000ms on the x1 2 and x1 ranges The fact that the RCD has not tripped will be evident because the PN and PE LEDs will still be on The test current will flow for nominally 50mS on the x5 range 8 6 Testing DC sensitive RCDs The KT62 has a facility to test RCDs that are sensitive to DC fault c...

Page 28: ...es Replace with eight 8 new 1 5V AA batteries taking care to observe correct polarity Replace the battery cover The continuity test circuit is protected by a 600V 0 5A HRC ceramic type fuse situated in the battery compartment together with a spare If the instrument fails to operate in the continuity test mode first disconnect the test leads from the instrument Next remove the battery cover take ou...

Page 29: ...that the instrument will be serviced and returned to you more quickly Return the instrument to Service Department Kewtech Corporation Limited 76 St Catherine s Grove Lincoln LN5 8NA Regular re calibration is recommended for this instrument Kewtech recommends that with normal use the instrument be calibrated at least once in every 12 month interval When the instrument is due for re calibration retu...

Page 30: ...KEWTECH Kewtech Corporation Limited 76 St Catherine s Grove Lincoln LN5 8NA www kewtechcorp com Distributor 92 1619 04 03 ...

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