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Continuity Testing Of Electrical Circuits

Electrical Cables and Continuity Testing

It is essential that our electrical circuits are well designed and correctly installed. We need to use the correct specification of good quality cable.

After installation we can check whether the wiring is safe by carrying out what is know as a continuity test which will show up any faults or the use of poor quality cable.

If our circuits are not safe we risk fires and electrocution.

house fire

Electricity is dangerous

The most dangerous things in our houses are earthquakes, fires, and electric shocks. Earthquakes don’t happen very often, thank goodness, fires and electric shocks are usually caused by faults in electrical systems.

So what can we do to protect ourselves.

In the case of Earthquakes we can make sure the structure of our houses are properly designed and built, a topic we have covered in other dusty annals of Fixed Abode.

In the case of electricity we can make sure that our electrical installations are safe. If they are properly designed and installed they will have safety mechanisms built into them that will cut off the supply of power if there is a problem.

It is most important to make sure the wiring in our houses is properly designed and correctly installed.

Let us start off with electrical cable.

Standard Electrical Cable

Standard electrical cable used for house wiring in Indonesia is NYY and NYM cable, these are recognised international world standard cables. NYM is designed for internal use, it should not be used in places exposed to direct sunlight or outdoors. NYY is a heavier duty cable that can be used outdoors, embedded in concrete, in water or buried in the earth but should not be used where mechanical damage is to be expected. (It is advised that all electrical cable is installed in rigid plastic pipe known as conduit).

These cables come in two and three core versions, the “cores” are copper wires wrapped in coloured insulating PVC sheathes and all enclosed in an outer sheath. There are different thicknesses of copper wire used depending on the amount of electricity being carried, for normal household installations we use cable with copper cores with a cross sectional area of 2.5 square millimetres.

Two core cable (NYY-O and NYM-O) is used for lighting circuits where no earth is required and has two wires: a brown one which is the live wire and a blue one which is neutral.

Three core cable (NYY-J and NYM-J) is used for plug sockets and any other circuits that require earthing. This cable has a brown wire for the live, a blue wire for the neutral and a third wire which is yellow and green striped and is for the earth.

The best quality of cable is generally regarded to be “Supreme”, you will find the name is printed in black friendly letters along the cable. Next best is “Focus“, lower quality cable is Eterna and Extrana and then you get into the “also rans.”

It is wise to take note that the price of cable reflects its quality and the average electrical installation contractor is alway keen to save some money.

For safety reasons power circuits need 3 core cable

Along with reinforced concrete structures the electrical installation is the part of building construction that you should never skimp on. It is frightening to see how contractors with little integrity will compromise a valuable property just to save a relatively small amount of money.

I was once asked to investigate a case where a little girl nearly died having been electrocuted in a major public facility. It turned out that the contractor had saved a bit of money by installing two core cable instead of three core cable for the plug sockets. The neutral (blue) wire had been connected as both the neutral and the earth. One day there was a fault in a lighting system that had been plugged into an offending plug socket, a steel marquee frame became live and a 3 year old girl nearly died.

Continuity Testing Of Electrical Circuits

So how do we know if our electrical installations are safe?

We can test a wiring installation by means of a continuity test. For this we use a continuity tester, a meter that sends a pulse of high voltage into the cable and measures how much leakage there is. We measure the resistance between the different cores in the cable. The test tells us if there are any short circuits, if the PVC insulation is working correctly and if the joints in the cable are properly connected. We don’t want any leakage of electricity from the system so when we test the circuits we want a high electrical resistance, anything up to 2,000 megohms. An ohm is a unit of measurement of how much something resists the flow of electricity, it is named after German Georg Simon Ohm.

Continuity testing ensures electrical wiring is safe

Unfortunately electrical circuits may fail continuity tests. Only a small fault such as a break in the insulation, a nail through the cable or a faulty connection will cause the test to fail. There are, however, two basic reasons for failure. The first is the quality of the cable used, poor quality cable leaks electricity.

The second is the standard of the installation particularly the wiring connections. It is a good idea to watch your electrician work. You may find that to connect cables he will bare the ends of the copper wires, twist them together then wrap a bit of plastic tape around the join. This is the standard method used in many parts of the world but is simply not good enough.

Cable connections should use small plastic caps which are screwed down onto the two copper ends. The plastic cap does two things: firstly it holds the two twisted copper ends tightly together and secondly it covers the joint to insulate it. The whole joint should then be contained inside a junction box.

Copyright © Phil Wilson August 2013
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