Electrical Test Instruments MAC-21
Section III
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MAC-21
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Copyright(C) 1994-2004 Electrical Test Instruments, LLC All Rights Reserved
Page III-7
SERVICE INFORMATION AND DOCUMENTATION
MAINTENANCE AND CALIBRATION OF THE MAC-21
The MAC-21 is manufactured using solid state components that should not
require extensive maintenance. However, the accuracy of the MAC-21 is critical
to the testing of circuit breakers and is dependent upon the output of an air-
core current sensing coil, which could change due to movement caused by
shock or vibration encountered in transporting the test set. Other factors
which may affect calibration are contact resistance in the control cable,
unusual magnetic fields, insulation leakage, and aging of electronic
components in the MAC-21. Therefore, proper operation and calibration should
be checked at regular intervals and adjusted if proper equipment is available.
The inherently delicate nature of electronic circuitry and metering make it
generally inadvisable to leave the MAC-21 fastened into the breaker test set
while it is being transported, and subject to shock and vibration. The standard
rack-mount hardware and twist-lock connector make it simple to remove the
MAC-21 and carry it separately.
Field calibration of the MAC-21 may be performed as follows:
1.
Obtain a current measuring calibration standard capable of reading TRUE-
RMS current up to at least 1000 Amperes to an accuracy of at least 0.25%.
This may consist of a shunt or current transformer, in conjunction with a
digital multimeter or laboratory grade AC ammeter.
2.
Set up the breaker test set for normal breaker testing. If adjustments are
anticipated, remove the MAC-21 from the test set.
3.
Connect the calibration standard to the desired output tap. If cables are
used, it is recommended that they be twisted so as to minimize radiation
and pickup of stray magnetic fields.
4.
Apply power to test set and allow at least one minute for circuits to
stabilize.
5.
Check ammeter zero in all ranges. If reading on current meter is greater
than 1% of range, internal zero adjustment may be required.
6.
Set MAC-21 Range to 1000 A and adjust output of breaker test set to exactly
800 Amperes. If Coarse Tap is 1, and Output Control Vernier is less than
50%, additional resistance must be added to output circuit. This is
