Trisan S8 - Page 9

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TRISAN S8 Important Read General description
Definitions and explanations If in doubt ask! Contact us to provide further explanations
Work hardening copper: A mechanical process of continual flexing or vibrating soft copper until the material be-
comes brittle and fractures. This is the process which occurs in leaded appliances. By observation a lead becomes stiff
and has a memory of shape rather than pliable with a soft lay. These leads become dangerous, the multi stranded cop-
per become hard and brittle eventually piercing the insulated cord. The result will cause a fire or create carbon that
will conduct around the surface of the break, leading to possible electric shock. This is quite a common occurrence in
older appliances which is why inspection and testing has become so important.
Body resistance:
The voltage necessary for electrocution depends on the current through the body and the duration of the current.
Ohm’s Law states that the current drawn depends on the resistance of the body. The resistance of human skin varies
from person to person and fluctuates between different times of day. Under dry conditions, the resistance offered by
the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms,
adding to that high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance
to 500 Ohms.
Insulation: A material that does not conduct electrical current. However, these materials can become conductive by
contaminates such as salts, carbon and small fragments of metal particles etc.
Electrical cords and appliances behave in a similar way to body resistance, given the environmental conditions
(temperature, humidity, mechanical movement and wear, conductive particles and acids in the atmosphere will vary
the dielectric strength of the insulation resistance. Only by applying sufficient electrical strength over time across the
insulation barrier will degrading dielectric material be measured. The higher the electric voltage applied for several
seconds will induce potential weaknesses in an insulated body. With reference to standards these testing voltages lie
between 250 to 500VDC or 240 to 1500VAC ( high pot isolation testing can be as 3750 to 10,000VAC).
If there is an insulation weakness the high electric field ionises this zone, creating localized heating. Any partially
conductive contaminates will break down the resistance even further to a point of flash over. (Total insulation break-
down). Relatively low test voltages as 250 or 500VC will produce similar results but not as severe and resistance of
the insulation may fall to several hundred ohms. Reduction below 1M ohm is a failure according to AS/NZS 3760.
Lower test voltages will not produce the electrical strength required to impart the effect as described above. Substi-
tute leakage current testing is no better than using a resistance measurement on a multimeter (test voltage 40–
50VAC). Breakdown of insulation is non-linear not linear as determined by this test. Multiplying a measured leak-
age by a factor of 6 does not equate to the same leakage determined by an electric potential of 250VDC. The behav-
iour of an insulation breakdown would follow an exponential law but the characteristic nature of the breakdown will
determine the end result.
Substitute leakage current testing does not guarantee an electrical A - N continuity circuit or a measure of isolation
between input and output. No recognized electrical standard governing substitute leakage current testing.
For this reason appliance testers are specified instead of general purpose resistance meters. Appliance testers with ap-
plied high test voltages to equipment will accelerate breakdown of possible or potential flaws in insulation as de-
scribed above.
Be cautioned some testers on the market use extra low voltage to test for insulation breakdown. (check specifications
before purchase). There are no standards which prescribe this type of testing. Portable appliance testers whose fea-
ture’s list like a multimeter’s description should be avoided. Competent operators other than electricians/engineers
have no training in insulation materials and their behaviour, guidance is generally from the retailer and the little infor-
mation received from training courses.
The testing of in-service electrical appliances is about finding the potential onset of hazards, not just the obvious
breakdown that substitute leakage current testing will only find. Using this method of testing 99.9% of the appliances
tested will pass. Reliance on visual inspection becomes most important.