I'm needing to design a 3 phase ground fault detection system for a 480V system. It will need to be able to trip when there's ground fault current >1A.
I have done some reading and it sounds like there are two main methods. The first is to have one large current transformer (CT) and run all 3 phases through it and look for current in the loop.
The other method is to use 3 small CTs and monitor each individual phase. The output of these 3 CTs are then connected in parallel and then this loop is monitored for a current.
In either case if there is current in the monitored loop it indicates a ground fault.
Here's a picture of the two schemes:
I need to go with the second scheme with the smaller transformers.
I was thinking of monitoring the current loop using an Allegro ACS714 (SENSOR CURRENT 5A 5V 180 ~ 190 mV/A -40°C ~ 85°C) 620-1258-2-ND (or similar) and monitor it's output voltage with a microprocessor. This ground fault isn't for safety but just to protect the equipment and flag maintenance. I only need to trip when there's about 1A of current that's unaccounted for - although if I could trip at a lower current it wouldn't necessarily be a bad thing.
Initially for the CTs I've been looking at the CR8400 Series from CR Magnetics - Specifically the CF8420 as the size of the hole will allow larger wires. However, could I get away with using the CR8420-1000 (as that has a maximum current of 50A) instead of ther CR8420-1000-G? I know the 2nd one is rated specifically for ground fault but it's only rated to 20A (and perhaps I'm missing something) but isn't a current transformer a current transformer?
I'm also looking for any comments/suggestions of other things I should be on the lookout for as I haven't worked with ground fault detection much.