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MEGGA TESTING CABLES - Cable Construction
A typical cable used for burying in the ground will look like the sketch below. There will be an outer ‘water proof’ cover
Usually HDPE (not PVC), inside that will be ‘Steel Wire Armor’ and on then inside that will be single ‘Insulated’ wires
bundled together as shown.
Cross Section of a Three Core Cable with Steel Wire Armor
Live Neutral and Earth
Cable Rolled on Drum
Between these points
Steel Wire Armor
If the insulation around the conductors
is damaged or water has got into the
Cable due to poor storage conditions
then the cable is useless.
We use megga testing to check the
Insulation is in good condition.
Other section showing cable ends ‘stripped off’
Itchy Did This
Megga Testing works by sending a very high voltage down the cable and checking to see if any of the electricity leaks
across the insulators. The principle is that very high voltages will cause insulators to break down – It’s a bit like
over inflating a car tire to check for air leaks.
Safety Note: Megga Testers work at very high voltage, they can give you a nasty electrical shock. Do not touch the
connectors or wires when megga testing a cable. Never test a cable if it is connected to anything, always disconnect and
Strip-Off both ends.
Before you buy the cable, and before the cable is cut off the drum
get both ends of the cable stripped off (as shown on page one)
Connect the Megga Tester to cores A and B as shown,
now make sure you are not touching the leads or wires
Press Test and record the reading.
Now move the black wire to C and test between A and C
Repeat until you have tested between A and all other points
Now move the red wire to B and test
between B to A , B to C and B to D
Then move to C and repeat.
All reading should be infinity ∞ resistance
or at least Megga Ohms MΩ
Any reading recorded below 1M Ω
suggests the cable Is damaged and useless.
Repeat these tests after you have put the cable in the ground, but before you back fill the cable trench. This way you
Don’t have to dig up the cable again if it is damaged during installation
Itchy Did This