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:

enter image description here

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.

  • \$\begingroup\$ What is the peak/average load current you're dealing with here? \$\endgroup\$
    – Dave Tweed
    Apr 15, 2013 at 13:56
  • \$\begingroup\$ Peak current should be 43.3 A / phase \$\endgroup\$
    – user16105
    Apr 15, 2013 at 14:22
  • 1
    \$\begingroup\$ Probably the Ir limit is based on core saturation. When you run all phase lines through the same core, there is only the ground fault current magnetizing the core. When you run one phase per core, the whole 43A is driving each core. So the 20A rated cr8420-1000-G is not a good choice to put on a single 43A line. \$\endgroup\$ Apr 15, 2013 at 14:41
  • \$\begingroup\$ And watch out for current transformers. You can get 'pretty high voltages' if there is no load on the secondary. \$\endgroup\$ Apr 15, 2013 at 14:45
  • \$\begingroup\$ @ Bobbi yes, that's why I asked about using the CR8420-1000 (which is rated up to 50A) instead of using the CR8420-100-G (which is only rated for 20A - but states for GroundFault) Perhaps the ground fault just means it produces a lower secondary voltage so it's more adapt for a microprocessor? \$\endgroup\$
    – user16105
    Apr 15, 2013 at 14:50

1 Answer 1


A current transformer is just like a regular transformer. The ratio of current in the primary to current in the secondary is just the turns ratio. In this case 1000. So for a current of 1A in the primary, you will get 1mA in the secondary. You need to drop this across a known resistance to get a voltage readout.

In the three phase circuit you have shown, the secondary currents add and give you just what you want, the total current through all three lines, which should add to zero if there is no ground fault.

The Allegro ACS714 in the secondary will give you 180 ~ 190 mV per Amp in the secondary, that is 180 ~ 190 uV per primary Amp. I do not think that is what you want. You will want something to give you 1 Volt per mA. Maybe just a 1K resistor.

  • \$\begingroup\$ Commercial breakers are readily available. I would highly recommend you go to Mike Holt's site and see what they have to say about this and Arc Flash, you could be a crispy person in a few nanoseconds. You can develop several thousand amps with a ground fault in a 3 phase 480V system. You have to size the relay to properly open under a fault condition. Check the National Electrical Code for ground-fault protection of equipment in accordance with the provisions of 230.95. I am assuming this is a commercial system. \$\endgroup\$
    – Gil
    Mar 10, 2021 at 21:17
  • \$\begingroup\$ Wow, what a sleeper! And, revisiting this question, it is absurd. Even though this person says it is for monitoring only, it is pretty crazy to put a current transformer on each leg. Maybe, maybe if you wanted to monitor total equipment power or something, but whow. Gil's admonishment is totally apt. \$\endgroup\$ Mar 11, 2021 at 23:22

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