Product safety testing per IEC 61010 calls for leakage current measurement of exposed metal parts in normal operation and fault situations. These measurements require the use of filtering networks per IEC 60990 and one more unique for IEC 61010. Such networks can be build fairly easily using some resistors and capacitors and the guidance given in IEC 60990. As usual with circuits presented in IEC standards, the actual implementation requires a bit more thought, especially for circuits that can be exposed to significant fault voltages.
We created a combined solution with shares the primary sensing network (resistive voltage divider) across all the networks because this involves carefully matched ceramic composition power resistors. The various network configurations can be selected using the rotary switch on the frontpanel.
The circuit is protected by a PTC element in the ground connection of the voltage divider. Because of this, the circuit will output most of the open-circuit voltage when the protection has tripped, thus preventing false negatives.