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About Live Test
About Live Test
After completed the DEAD tests on complete wiring installation the next step is to connect supply and perform LIVE test sequence. Live testing is also known as temporary supply connected test.
Types of LIVE testing
Polarity Test
Earth fault loop impedance Test
RCCB/RCD Tripping Time Test
Phase Sequence Test
Last Point Trip Test (with Socket Tester)
Polarity Test
Polarity Test
Polarity under LIVE test condition is simple.
Tools for perform simple LIVE polarity tests:
use electrician tools such as test-pen. To do the test simply point the tip on L label of every terminal or equipment connected with supply switch ON.
using a socket tester
using multi-meter - set to voltage range to measure the voltage supply.
test set such as earth fault loop impedance test set and RCCB/RCD tripping time test set also provide polarity check.
test lamp - 100 watt incandescent lamp are used to perform this test.
Earth Fault Loop Impedance (Zs)
Earth Fault Loop Impedance (Zs)
This test is done during LIVE supply is energized and to perform to check if a fault occurs in an electrical installation, sufficient current will flow to operate the fuse or circuit breaker protecting the faulty circuit within a predetermined time.
The value of the earth fault loop impedance acceptable shall be below 100 ohm/Ω (Zs < 100Ω).
Formula Earth Fault Loop Impedance
Zs = Ze + ( R1 + R2 )
whereZs = is the system loop impedance
Ze is the external loop impedance (from the suppler side of the consumer unit back to the transformer)
R1 = resistance of phase conductor R2 = circuit earth resistance ( cpc )
Refer to figure on the right is the earth fault loop path.
When Zs value is more than requirement value Zs >100Ω
The following conditions where Zs is greater and it affects on the electrical system.
RCD or RCCB may not trip and cut off supply according to standard tripping time. (< 40ms )
this indicates poor earthing system on site and needs attention.
Conductor poor termination or wiring improperly done.
conductor poor quality.
Solution:
check and measure again the earth resistance.
Increase the depth of the earth electrode copper rod (if found less than 3 meter).
check earth chamber and all the accessories ensure it properly done.
check all protective conductors (c.p.c) connected ensure proper securing the earth terminal.
How the Earth loop impedance Testing is done.
Method 1:
Figure above showing the earth fault loop impedance is been tested at consumer unit (distribution board). The test readings shall be below 100 Ω.
Method 2:
Figure method 2 test.
Commonly the earth loop impedance testset is plugged into any socket outlet and then prest TEST button to obtain the value about 0.4Ω to 0.6Ω
Referring to table below, the maximum value of Zs is indicated.
IEE Table on Zs
How to find fault current
Formula; If = Vn ÷ Zs
Where;If - fault current leakageVn - nominal voltage suppliedZs - value of earth fault loop impedance
Example calculations
Given the value of Ze is measured 0.4Ω, resistance of R1 is 4.2Ω, and R2 is 1.8Ω. Find the total earth fault loop impedance, Zs ?
Solution: Zs = Ze + ( R1 + R2 )
= 0.4Ω + ( 4.2Ω + 1.8Ω ) = 6.4 Ω (Acceptable, Zs< 100Ω )
If the supply connected is measured 240 volts, calculate the fault current, If ?
Solution: If = V ÷ Zs
If = 240 v ÷ 6.4 Ω = 37.5 A (Acceptable, this low current can quickly flow to earth. )
RCD/RCCB Tripping Time Test
RCD/RCCB Tripping Time Test
This test requires to ensure the protection device able to operate in specific time to quickly disconnect the incoming supply.
Standard test value is:
< 50 milliseconds (IEE standard)
< 40 milliseconds (Malaysia standard)
Test method: Set test cycle button to 0° and 180° deg repeat all test below 3 point.
No trip test (Time taken shall be not more than 2000 ms.)
Rated Trip test (RCD/RCCB should trip during this time, take value) Acceptable results should be <40ms (MS)
Fast Trip test (RCD/RCCB should trip during this time, take value) Acceptable results should be <40ms (MS)
RCD test can plug into a switch socket outlet (SSO).
Warning:
Precaution when using this test equipment, lack of knowledge using the equipment may damage the test set.
Phase Sequence Test
Phase Sequence Test
The phase sequence test is need to ensure the three phase supply order is correct, which is clock wise direction.
Phase sequence meter is used for detecting the sequence of the supply in three-phase electric circuits. Since the direction of rotation of three phase electric motors can be changed by changing the phase sequence of supply.
Correct phase sequence is clockwise rotating direction. (Correct order : R-Y-B-R )
If any of three phase line is reversed, the test set will shows anti-clockwise rotating directions.
Picture above is the example of rotating disc type (left) phase sequence tester and the LED lamp type (right) phase sequence tester.
Figure above is an old method of perform phase sequence test. This test methods has obsolete over the years due to many modern test equipment are available in the market today.
Figure above showing how phase sequence tester is been used and clip on to an isolator terminal.
Last Point Trip Test
Last Point Trip Test
This test is optional, basically is to ensure the RCD/RCCB will be able to trip due to earth fault from the furthest circuit or point from consumer unit. This way any sudden fault may be able detected by the protection device.
Items use to perform the test:
1no. 100 watt lamp bulb, two(2) x long pole insulated (or wood pole).
or socket tester (refer to test equipment topics).
Lamp bulb method:
ensure using insulated long pole or wooden pole with red and black cable,
this is done for lighting point, point one red cable on Live (L) terminal and black cable to the Earth (E) terminal, this will cause earth fault and trip the RCD.
Figure above is another test methods by using two long pole to perform test trip with a 100w bulb. This test should be performed on lighting terminal or ceiling rose.
Using Socket tester to perform last point tripping test.
Refer to right picture is the socket tester with three indicating lamps and a red color test trip button.
How use socket tester
plug in the socket tester to switch socket outlet (SSO)
switch on the socket switch
Look at all the three indicator light, if polarity is correct then all LED should lite up.
To perform tripping test simply press the red button , this should cause the RCD/RCCD to detect a fault and causes supply to cut off.
Relate topics : Basic Inspection Basic Dead Test Consumer circuit
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