I am designing an embedded industrial controller that must have 11 separately isolated 4-20mA two-wire loop inputs. The power for each loop and the isolated circuitry is supplied by its own isolated 24V DC-DC converter. Line-to-line protection for the current-loop inputs is easy, but I don't see how to provide line-to-protective earth protection. The accumulated charge of multiple EFT strikes during the testing will accumulate on the isolation capacitance of the 24V DC-DC converter until reaching its isolation capacitance breakdown voltage. Further, the current that flows during each transient is in the neighbourhood of 25 amps, although only for some 15 nsec, but then it is about 15 amps for about 50 nsec.
So I have the following concerns:
1. How do I bleed the charge off the isolation capacitance without violating the isolation requirement? I can use a bleeder resistor to protective earth, but that has to be small enough to discharge the capacitance in the time between successive EFT strikes during the IEC 61000-4-4 test. With a maximum isolation capacitance of 120 pF, the resistor has to be less than about 15K to get to 5 time constants. To provide tolerance to mis-wiring during installation for this 240 VAC system, the resistor would need to have a 15 watt rating.
2. Is the large current spike a problem for the isolation capacitance?
3. Is there a better way to do all this?
Thanks for your time,
Peter