Amprobe MEGATEST-5000 Megohmmeter - Manual (Page 7)

2.1 Purpose of the device
This instrument can be used to measure the insulation of installations, equipment,
insulating material and so on.
Testing must be performed with no other voltage present, and after having disconnected
the parts not being measured or which are unable to withstand the testing voltage.
This instrument is also able to measure the voltage present before the test, to display it
and to inhibit starting up of the measurement in case the tested element should be live.
The pre-set measures are, in most cases, sufficient for using the instrument without any
additional programming, employing the settings that were prepared at the factory.
2.2 Measuring principle
The instrument applies the requested voltage to the object being tested (material or
equipment or installation), measures the circulating current and calculates the relevant
insulation resistance.
While measuring, the instrument simultaneously detects and displays the resistance,
making it possible to calculate its variation through time.
In certain situations (e.g. very large electrical installations or measures on condensers),
there can be a stray capacity parallel to the resistance to be measured. This stray capacity
is due to the presence of conductors running next to each other, and can sometimes be
quite high. In such a case, the value of the resistance displayed can start out from a very
low value and reach - in no short time - the real value of the insulation resistance.
In this case, therefore, the measurement is deemed to have been completed only when
the displayed resistance value stabilises at less than a certain tolerance (normally 10%).
2.3 What is insulation resistance
We are used to thinking of insulation as a total inhibition of the passage of electrical
current. This approximation is valid in most cases.
However, when we have to measure an insulation resistance, this approximation no longer
holds true: we can imagine an inhibition that is not total, but simply very great.
Insulation is known to depend on the physical features of the material involved and on its
dimension, and from an electrical point of view appears as a resistance, of course of a
very high value. The higher the resistance the better the insulation.
Every kind of material has its maximum permissible voltage, depending on its thickness.
When this value is exceeded, the insulation may be perforated (discharge). This voltage
value is called insulating strength, and its value is always related to the thickness of the
material. When measuring insulation, one must never exceed the maximum permissible
voltage (insulating strength x thickness of the material) since it can lead to deterioration of
the material (perforation).
2.4 Possible fields of application
This instrument is most frequently used to make sure the insulation of electrical
installations or equipment complies with the relevant regulations.
Measurements taken in the "PROGR" mode make it possible to increase the voltage
slowly. This way the efficiency of an insulator can be checked out, the insulation
resistance should stay more or less the same even when the voltage is increased. Should
the resistance diminish as the voltage is increased, this will mean that the insulator is not a
good quality one. Continuing the test might lead to an electric discharge and the
perforation of the material.
-5-