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Electric
Strength Testers
Moe
Lamothe, P.Eng, Lamothe Approvals Inc.
The
intention of the electric strength test (also known as dielectric strength
test) is to determine if there is an unintentional path to ground from
line or neutral. The test is conducted by connecting one lead to the hot
and neutral line and the other lead to the ground connection. Intended
paths to ground are okay. Varistors by their nature cause problems with
this test and it is standard practice to remove them before the test is
conducted. For production line testing, a jumper can be used or in some
cases, a sampling test program can be used.
The
allowable leakage current is subject to much debate and most standards do
not indicate a limit, only that there be no breakdown. Typical testers
have trip limits of 10 to 95mA. Typically, AC testers have a higher trip
limit to compensate for capacitive leakage. DC testers need a lower limit
to prevent continuous arc damage in a breakdown situation.
A
Word About Electric Strength Testers
There
are a wide variety of electric strength testers available to suit various
needs. They are sometimes referred to as earth testers or earth strength
testers. There is also a group of test equipment often referred to as
Meggers® (the correct name is megohmmeters). They are used to
test insulation resistance, typically line-to-line or line to chassis.
They are not a suitable alternative to electric strength testers.
Electric
strength testers can have either AC and/or DC output. The DC output is
used when there is excessive leakage from line to ground due to filter
capacitors. The voltage in the standards is usually stated in Vrms
but if you use DC, the value is multiplied by 1.414 to get the DC test
value.
The
test is conducted by connecting one lead to the hot and neutral line and
the other lead to the ground connection. Intended paths to ground are
okay. Varistors by their nature cause problems with this test and it is
standard practice to remove them before the test is conducted.
The
allowable leakage current is subject to much debate and most standards do
not indicate a limit, only that there be no breakdown. Typical testers
have trip limits of 10 to 95mA. Some standards such as UL 1459 require the
limit to be adjustable. Typically, AC testers have a higher trip limit to
compensate for capacitive leakage. DC testers need a lower limit to
prevent continuous arc damage in a breakdown situation.
Ground
continuity tests are often required and it is helpful to have this feature
included in the electric strength tester. It is strongly recommended that
each test be conducted only once. In prototype testing, a one-minute test
is standard.
Our
recommendation is that you acquire a tester with the following
specifications:
CSA
Certified or UL Listed, adjustable output up to 5kV (2.5kV minimum),
adjustable leakage trip limit, AC or DC output selectable, front panel
receptacle for the equipment under test and with built-in ground
continuity checker. Various testers feature automatic programmed
sequences, actual trip voltage display, etc. The table on the next page is
intended to give you some contacts. We do not recommend any specific make
or supplier nor do we maintain a price list. Costs will likely vary from
$1,000 to $5,000 depending on features.
CAUTION:
-
The
electric strength test employs high voltages and they can give you a
severe burn or shock. They need to be treated with respect.
-
The
electric test and the 25A ground impedance are potentially damaging to
the equipment being tested. It is strongly recommended that each test
be conducted only once. This is particularly true of the electric
strength test. In prototype testing, a one-minute test is standard.
For production line testing, you can test for one second with a
voltage 20% higher than used for prototype testing. THIS IS STRONGLY
RECOMMENDED.
Moe
Lamothe is the president of
Lamothe Approvals Inc.
(Georgetown, ON, Canada). He can be reached at
info@lamotheapprovals.com.
The
copyright for all of the text, tables and illustrations remains with Lamothe Approvals Inc. Permission is granted to print or reproduce
this document provided that it properly attributed to Lamothe Approvals Inc.
Prepared
May 2003
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