Measuring appliance earth leakage

[John Rumm]

On a TT system one would traditionally place such circuits on the 100mA trip time delayed RCD along with the lights etc.

[Ron Lowe]

I think 'entirely unsafe' is somewhat of an overstatement. TT systems were used for years before RCDs existed. I could accept 'less safe', but as I said, 'entirely unsafe' is somewhat of an overstatement.

No, it goes beyond 'not convenient'. You'd need to be out of your tiny little green mind to rip open all IT gear coming into your house, open the SMPS, rake around to find the filtering and disable it. I'm an electronic engineer, and could do that if I chose. But it is simply impractical.

Hmm. Just doodling it out here.

Looking at the secondary side.. Under normal ( no earth leakage ) circumstances, it matters not a jot if the appliance earth ( and incoming true earth ) is connected to the N side, or to CT.

Now, looking at leakage: With an RCD on the secondary, you will still have an imballance between L+N, regardless if the leakage returns to the N leg of the transformer, or the CT. The leakage does not flow through the sense coil, so the imballance remains, and the RCD trips.

What's the benifit of the CT arrangement in terms of helping with earth leakage, other that the usual site-power-tools minimise-voltage-to-earth deal?