Procedure: Measurement
of the nominal varistor voltage (the voltage across the metal oxide varistor
with 1 mAdc through the device) identifies the voltage rating of the MOV
used in each design. Changes in the voltage can indicate the degradation
of a device after testing. This parameter is measured according to the
IEEE definition of varistor voltage (ANSI/IEEE C62.33-1982).
The sample must exhibit
the characteristics of an MOV with a voltage rating of at least 150V,
but no higher than 175V.
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Temporary Overvoltage (TOV)
Procedure: Because of
neutral and/or connector problems, which cause voltage shifts on the residential
120V legs, the temporary overvoltage (TOV) characteristics of the device
requires measuring. Voltage is applied individually to each leg, starting
at 150Vac. The applied voltage is raised in 5Vac increments at 5 minutes
Per step. For this test, a Tektronics model 5103N oscilloscope mainframe
equipped with a 5A21N Vertical Amplifier and a P6021 AC Current Probe
is used to measure/monitor the MOV current.
The voltage step below which
thermal runaway occurs is considered the TOV capability point, provided
that the device demonstrates constant standby current and thermal stability
for five minutes. A TOV capability of 185Vac or higher is required.
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Surge/Multiple Surge Withstand
Procedure: two types of surge
withstand test are performed. The first consists of the application of
one 8/20 usec. current surge with an amplitude of 40kA. Performing this
test with 120Vac applied is currently under consideration. The surge arrester
shall be able to withstand a single 8/20 usec. current surge of 40kA.
After this surge, the varistor voltage may not change more than 10% from
the initial value and the disconnecting device (i.e. fuse) shall not have
operated. Any arcing, whether between two points in air or on a circuit
board, is also considered a failure.
The second test is a multiple
surge withstand test, performed at a level of 800J per surge, with a modified
cable fault locator or "thumper". Each arrester section is surged individually,
with 120Vac applied continuously before, during and after the surge. The
cable "thumper" is modified to provide a Combination Wave, 13KV-1.2/50
usec. open circuit and 5.5.kA - 8/20 usec. short circuit. A total of 100
surges at six-second interval are applied to the arrester.
The surge arrester must
survive without damage, show no signs of arching between components as
mentioned before, not fault the ac line, have less than a 10% change in
varistor voltage as measured in the paragraph on varistor voltage, and
the disconnect device (i.e. fuse) must not have operated. Also, the discharge
voltage across the device shall not exceed 2kV during the test.
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End-of-Life Failure Mode
Procedure: Similar to the fault
withstand test in ANSI/IEEE C62.11, one MOV leg of the arrester is first
put into thermal runaway using an overvoltage ac power source. Then, both
legs are energized simultaneously, applying full available fault current
to the device at full rated voltage (120/240Vac). Two levels of fault
current re used in this test. Values of 3kA and 7kA are applied to independent
samples. After the failure mode test, a 10kVdc withstand test is performed.
Performing an impulse withstand test with ac bias in lieu of the dc withstand
test is currently under consideration.
The test circuit is fed by a
37.5kVA overhead distribution transformer with a 240/120V low side. This
transformer then feeds a 600V, 600A class fused disconnect equipped with
600A slow blow, renewable fuses. Following the disconnect is a 600V, 2400A
contactor which is used to initiate and determine the test. This contactor
will remain closed for at least ten seconds, provided catastrophic failure
of the sample does not occur. Following the contactor is a standard 200A
service entrance meter enclosure. The device under test is hardwired to
the meter-base. In order to energize all components in the device, meter-shorting
bars are installed in the jaws.
The internal fusing of the arrester
must clear the fault without significant damage to the device, or meter
enclosure, and without phase-to-phase or phase-to-neutral arcing.
After the test, the damaged
leg (side) of the sample must withstand the application of 10kVdc for
5 minutes, without removing any part(s) from the device or damaged fuse
element. Also, the undamaged leg (side) of the sample must withstand the
application of 10kVdc for 5 minutes, with its fuse element removed (any
wires/leads/connectors to the fuse element must retain). Any arcing during
the 10kVdc applications, whether between two points in air or on circuit
board, is also considered failure.
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Copyright © 1998 TESCO. All rights reserved.
Revised: May 23 , 2000.
