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Tech Talk
Getting technical
with Chris Turl
Comparing the
cord. Appliances fitted with electronic mains
switches or RCD plugs cannot be tested in this
manner as it is not possible to close the mains
alternatives for
switch (as they require mains to be present).
An insulation resistance test does not power
up the appliance and though this can be
insulation testing
regarded as an advantage (reducing the time
taken to test and eliminating the danger of
moving hazardous parts) extra care should be
taken that the equipment switch is still put into
Chris Turl of Seaward looks at the relative merits of
the ‘on’ position to make the test meaningful.
different electrical safety tests used to assess the
In some cases, sensitive electronic devices
and particularly older IT equipment which does
integrity of insulation in electrical appliances.
not comply with BS EN60950 may be damaged
by 500V. However, in practice, this may not be a
As part of any routine electrical safety testing All have their relative merits and their place in significant issue as BSEN 60950 has been
programme there is a requirement to assess the periodic testing provided the different limitations around a lot longer than most IT equipment
integrity of the insulation between live parts of of each test method are understood. currently in common use.
an appliance and parts accessible by the user.
Conventional insulation resistance Insulation resistance Surge protection and filtering
measurement (insulation test) has been the Insulation resistance is normally checked by Some electrical equipment components
traditional method of carrying out this applying 500V dc between both live conductors connected to the line/neutral conductors for
requirement, with protective conductor current (line and neutral) connected together and EMC filtering or surge protection (MOV) can
or touch current as a complementary test for protective earth when testing a Class I appliance significantly influence insulation resistance
use when an insulation test cannot be or a test probe applied to accessible metal parts measurements. Referring to figure 2, the current
performed or gives questionable results. The when testing a Class II appliance. The i
may be relatively large whilst the capacitors in
recent publication of the third edition of the IEE insulation resistance is calculated from the the EMC filter are charged by the insulation test
Code of Practice for In-service Inspection and applied voltage and any fault current returning voltage. Similarly, the current i
may be
Testing of Electrical Equipment has introduced via the protective conductor or test probe significant if the insulation test voltage is higher
two new additional test methods as potential (see figure 1). than the threshold voltage of the MOV. When the
alternatives. The result is compared with the minimum test voltage is applied to the insulation,
The new IEE Code recognises that testing acceptable value to assess pass or fail current is drawn from the test voltage source
insulation resistance at 500V dc may be conditions. and so the apparent insulation resistance,
problematic in certain circumstances and as an With all measurements of insulation resistance, calculated from the applied voltage (500V) and
alternative suggests the use of either a reduced the appliance under test must be switched on return current (i
+ i
), may indicate an
voltage (250v) dc test, a protective before performing the test otherwise the test erroneous failure of the test (see figure 2).
conductor/touch current measurement or voltage does not pass beyond the mains switch, Reducing the test voltage to 250V, as specified
substitute/alternative leakage current test. in which case we are only testing the mains in the IEE Code of Practice, overcomes the
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