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.

Detectable Faults

Low Nominal Circuit Voltage

Monitoring the circuit voltage allows abnormally high, or abnormally low battery voltage to be detected. This is well documented in the existing user-guide.

• Detect failing battery cells or chargers
• Warning that inter-circuit isolation may be lower than suspected in the event of a low nominal voltage

Isolation from Earth

The SA380-IT is primarily designed to ensure supply circuit that are earth-free by design remain so, Connecting both B12 and XB12 in the normal arrangement ensures that this is true.

This is well documented in the existing user-guide.

Inter-Circuit Isolation

Device misoperation is also possible due to cross-coupling of circuits where the fault path is not via an earth connection.

By wiring SA380-IT inputs "cross-pole" between two circuits, a standing voltage on the SA380-IT inputs will be observed if any of the four fault conditions shown below are true.

The wiring diagram above shows this "cross-pole" arrangement on inputs 2 and 3.

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: 

Where V_A is the nominal circuit voltage (12 V)

and V_ISO is the standing voltage between circuits measured by the SA380-IT

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: 

Where V_A is the nominal circuit voltage of the 1st circuit (12 V)

and  V_B is the nominal circuit voltage of the second circuit(12 V)

and V_ISO is the standing voltage between circuits measured by the SA380-IT.

Configuration

If the wiring diagram above has been followed, the SA380-IT inputs should be configured as per the table below:

Fault Resistance Table

Use the look up table below to convert the voltage present on either the "ISO 1" or "ISO 2" channels into a worst-case level of inter-circuit isolation.

The table is populated using Equation 1, using a nominal circuit voltage of 11.4 V

The resistance value stated is "worst-case" the true level of isolation resistance may be greater and will be dependant upon how the fault is manifest.