Insulation resistance – what & why ?
Insulation resistance testing is simply testing the insulation that covers cables, basically the test tells you if the insulation is in good condition and is still acting as a good insulator. All cables are covered in some basic form of insulation from paper, cloth,rubber, plastic all with one purpose in mind to stop people, objects & other cable conductors from coming into contact with each other.
As cables normally run under floors in attics or below ground it simply isn’t practical to look at them physically, you can however visually inspect portions where they are exposed and this is normally where the most cable damage occurs. Under floors & in attics cables are susceptible to rodent damage, outside to mechanical damage & UV from the sun, all these factors degrade the cables insulation & of course time also plays a part.
Above are a couple of examples of insulation failure & one of rodent damage, all have the potential to cause electric shock & fire propagation which is why insulation testing is of paramount importance
So how do I test a cables insulation properties
For this test there are standalone insulation resistance testers or multi function testers which have the same abilities & functions to undertake this successfully ? (pics below)
The test is performed by sending a high voltage & low current down two conductors, the test instrument measures the resistance between the two conductors & the value obtained indicates if the resistance is acceptable and the insulation is good or bad.
In the case of external cables especially run in the ground a low or bad reading can sometimes be obtained, this does not always indicate a poor cable but can indicate that a cable joint has let in water as this will indicate a low reading.
In alignment with BS7671 (18th Edition of the wiring regulations & IET GN3) the lowest acceptable reading is 1 Meg-Ohm (or 1 million Ohms) and the desirable test voltage is 500 Volts.
I personally would want to investigate further if my readings were going below 10 Meg-Ohms as to me would indicate there may be issues starting to surface.
Actually undertaking the test – how do I do it
To fully ensure that the cable you are testing is deemed fit for purpose there is a simple strategy to adopt when testing, you need to ensure that all the conductors have been tested between each other and to a ground or reference conductor.
I will for simplicity pick a standard domestic twin & Earth cable, this has Three cores, two line conductors (L&N) and a none insulated bare copper Earth conductor (CPC) running through the middle, this should be fitted with a Green/Yellow sleeve to indicate it is the Earth conductor.
To correctly perform the test all loads should be removed form the circuit especially sensitive electronic devices as these can be damaged due to the 500 Volt DC test voltage.
Once all loads and all supplies are disconnected (again test with approved voltage tested to ensure cables are voltage free & isolated) connect your tester to the Live & the neutral conductors, set the meter to test at 500 Volts, most have a test button to start the test, the result will be almost instantaneous and will give you a reading in Meg-Ohms, note this down.
Repeat the test between the Earth & the Live and then between the Earth & the Neutral noting down each result. If all are above the minimum acceptable value of 1 Meg-Ohm then the cable is considered fit for purpose, I however would never be happy with a reading below 10 Meg-Ohms as a new installation should see 500 Meg-Ohms and beyond if satisfactory.
There are additional test methods if sensitive loads cannot be removed or you think there is a risk of damaging anything that may be hard wired or not possible to remove from the circuit, this is achieved by connecting together the Live & Neutral and testing between the L&N joined & the Earth conductor, this should be done at a reduced voltage of 250 Volts, the minimum acceptable reading is still 1 Meg-Ohm.
Please be aware that high voltages are generated during these tests, although they will not kill as they are of a low current they still let you know they are there, in essence it bloody hurts.
Also be aware that some cables especially on long runs can store some of the test voltage as they have a capacitance characteristic and if you touch the cable after testing it can deliver quite a shock, again low current but you will still be aware of it, in these situations I always connect each conductor to a ground source to discharge any stored voltage that may have built up.