Electrical Test Instruments MAC-21
Section III
–
MAC-21
==============================================================================
==============================================================================
Copyright(C) 1994-2021 Electrical Test Instruments, LLC All Rights Reserved
Page III-6
SERVICE INFORMATION AND DOCUMENTATION
MAINTENANCE AND CALIBRATION OF THE MAC-21
The MAC-21 is manufactured using solid state components that should not re-
quire 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 in-
tervals and adjusted if proper equipment is available.
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 stabi-
lize.
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 im-
portant, because waveform distortion is more prevalent at lower levels.
7.
If the MAC-21 reading differs from the standard by more than rated accu-
racy, adjust the gain potentiometer on the analog board for proper read-
ing. If an error of more than 5% is noted, and unit has been in service, the
accuracy and validity of previous tests may be questionable; otherwise,
