Overview
The SA380-IT may be used to measure the isolation between two independent circuits by wiring the device as shown below.
Wiring the Power supply to the unit into input number, one, via the "HI Z" terminals provides the benefit of measureing resistance to earth of this circuit, as well as the nominal voltage of the main supply circuit, which is likely the B12 circuit.
- At present, the SA380-IT measures the degree of isolation between circuit circuits as a standing voltage between the two circuits being measured. A table (at the end of this document) must be utilised to determine the possible level of insulation resistance.
Detectable Faults
A standing voltage on the SA380-IT inputs will be observed if any of the four fault conditions shown below are true.
Fault 1 and Fault 2 cause a voltage to present across both of the SA380-IT inputs. In this scenario the voltage may be as high as the nominal circuit voltage.
The equation for translating the voltage into a resistance reading is shown below.
EQUATION 1
For Reference, a table is provided at the end of this document to enable the operator to quickly look up a value.
Fault 3 and Fault 4 cause a voltage to present across one of the SA380-IT inputs only. In this scenario, the voltage may be as high as sum of the two circuit voltages.
The equation for translating the voltage into a resistance reading is shown below.
EQUATION 2
Fault 3 and Fault 4 cause a larger voltage to be presented to the SA380-IT than Fault 1 and Fault 2. All four faults can also present in any combination, however the voltages presented in the case of Fault 1 and Fault 2 represent a "worst case"
Equation 1 should therefore be used for the selection of alert levels and used to inform intervention decisions. The value yielded by equation 1 should there be read as "Resistance greater than or equal to X"
Equation 1 also assumes a nominal circuit voltage. A voltage of 11.4 volts has been used to produce the table below. If it is credible that the cell voltage could fall further, yet the circuit remains operable, then a different nominal voltage should be used.
Alternatively Inputs 4 and 5 can be wired directly across the terminal of each circuit to provide remote cell voltage monitoring. A wiring diagram showing such a solution is shown below
Configuration
Fault Resistance Table