Section 3
TM9230A-C1-002 November, 2021
2 November, 2021
3-1
3.0 OVERVIEW
3.1
General
The Guildline Model 9230A series of four terminal DC current shunt standards are designed for precision
measurement of direct current and for use as high accuracy low ohm value resistance standards. They
are constructed from specially selected materials supported on an insulating base for mechanical
stability and covered with a perforated metal cover to allow air cooling while providing physical
protection for the resistive elements.
Particular design considerations take into account the effects of power dissipation levels of up to 125
watt from introducing excessive self-heating effects that would exceed the specifications. The types of
materials selected for the resistive elements have a very low temperature coefficient of resistance. The
size and number of individual element wires or plates for each model provide an optimum surface area
to dissipate the specified full scale current.
The performance of the 9230A series of DC current shunt standards can be dramatically improved by
operating them in a temperature controlled environment or immersing them in a temperature
controlled stirred oil bath where the power dissipation requirements are below 10 watt. The optional
Guildline Model 92310 forced air convection unit is essential for thermal dissipation levels above a
nominal 25 watt up to 125 watt and also can be used at lower power levels to improve measurement
stability. Performance specifications for use above a nominal 25 watt are based on the use of forced air
convection. Where forced air convection is not used additional uncertainties apply above the 25 Watt
power dissipation level.
3.2
Design Considerations
The
Guildline Model 9230A series of four terminal DC current shunt standards provide a number of
features for the purpose of minimizing the uncertainties of measurement with respect to environmental
factors, cable connection variations and long term variations in resistance values. Shifts in resistive value
due to ambient temperature changes, aging of the materials and other effects due to travel shock are
reduced by careful selection of the materials and manufacturing processes. Stress introduced in the
manufacturing processes is relieved through artificial aging of the completed assembly before final
calibration at the time of shipment.
The terminations for connection cables of the measurement system are dsigned to keep thermal EMF
potentials within acceptably low levels, to reduce errors that may be introduced by variations in cable
connections and to ensure that the applied current is distributed through the resistive elements in a
uniform manner. Binding post type test current connections are provided for lower current models
below a 1000 A full scale rating while hex head bolts are provided for models with a full scale rating of
1000 A and higher. Five way binding posts are provided on all models for potential measurement
connections.
For models below a 1000 A full scale rating a single cable connection point is provided such that
measurement values are independent of cable orientation. For models with a full scale rating of 1000 A
and higher the terminal design is such that multiple cable connections may be made in a symmetrical
manner that minimizes resistance value dependency on the connecting cable arrangements. Guidance
is provided in this manual with respect to optimum multi-cable connection arrangements.
Summary of Contents for 9230A Series
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