TIDUEZ8 May 2021

In an unearthed power distribution system, the isolation barrier protects the user and components sitting on the low-voltage side by preventing high currents flowing to protective earth. The isolation barrier is expected to be of a resistive nature. Nevertheless, some factors such as improper earth connection or humidity may increase the isolation capacitance to earth of the system.

In this system, under proper operation or asymmetrical fault of the isolation barrier, this static capacitance to earth forces a delay in the settling time of the isolation voltage when the resistive branch is switched in. A certain measurement time is expected.

By allowing higher currents across the isolation barrier through our switched resistive branch, faster settling times are expected and smaller errors on the isolation barrier resistance calculations are achieved. Assuming an asymmetrical fault on the negative resistor, where RisoN is small and RisoP big, the settling time of the isolation voltage is given by Equation 11:

Equation 11.
$\mathrm{\tau}=(\mathrm{R}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{P}//\mathrm{R}\mathrm{s}\mathrm{t}\mathrm{P})\mathrm{\times}\mathrm{C}\mathrm{i}\mathrm{s}\mathrm{o}\mathrm{P}\mathrm{}\left(\mathrm{s}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{s}\right)$

Specify the maximum isolation capacitance to protective earth, normally in the order of 2 μF to 5 μF.

As per standards, currents of up to 12.5 mA are allowed for less than one second through the isolation barrier into the switched resistive branch. Hence, consider the tradeoff between faster settling times and power dissipation when designing a resistive divider branch. Further details on the implementation in this reference design are found in Section 2.3.