Feasibility of Realtime Measurement of Earth Leakage Current

Is it feasible to measure, in realtime, using OEM / Emoncms, the measurement of earth leakage currents in the range of say 100uA (maybe too low but some commercial hand-held products start here) up to say 500mA?

I already have OEM & emoncms products for home energy monitoring and I’d like to bring leakage monitoring in too.

Having done some initial research, including looking at commercial hand-held leakage current clamp meters, and what I’d like to achieve looks potentially feasible. My challenge, I think, is to use an appropriate current transformer that will work at this low level and of a physical size to clamp around larger cables.

Has anyone else done this? Any suggestions, corrections to my thinking etc. welcome.

I look forward to your responses.


In a rural setting I have a domestic septic tank system that comprises a submersible electric pump of some 250W, a 15W fan that runs 24/7, a Grasslin timer to cycle the pump on & off complete with associated contactor, mcb, rotary isolator etc.

All is contained in a waterproof enclosure with transparent lid. The fan is housed in a separate enclosure but the fan is open to the elements although, as you’d expect, the cabling / connections in to the control enclosure are waterproof.

Single phase mains power is fed via swa cable from my neighbour - they pay for the electricity & I maintain it including having re-engineered / wired the 32 year old system.

From time to time we have replaced the pump due to normal wear & tear. The control system and rotary isolator replacement was necessary because the original enclosure seals were failing and the control housing lid wasn’t see through.

Recently we’ve had some RCD tripping which is impacting my neighbour. Upon investigation I found that a 30mA RCD is protecting both of my neighbour’s two ring mains (up & downstairs), likewise with lighting AND the septic tank system. The neighbour’s distribution board was replaced sometime in the last ten years or so which is when this RCD was [probably] introduced.

Whilst my background is small power / audio / RF electronics rather than electrical engineering my opinion is that the septic tank circuit should be separately protected. Furthermore the septic tank manual states that a [dedicated] RCD should not be less than 300mA. I’ve read this spec of RCD is common in rural installations, mainly to protect against fires rather than protection of life.

I have engaged a smart qualified electrician friend to conduct testing of the septic tank circuit / equipment and the neighbour’s installation and probably separate the septic tank on to its own mcb then a 300mA RCD.

You need an earth leakage clamp meter to measure earth leakage e.g Earth Leakage Clamps | test-meter.co.uk

The CTs that we use to measure current are designed for much larger currents.

Totally correct, it needs to be on it’s on RCD / RCBO to provide selectivity.

Thanks Glyn,
I’ve done some further reading / research this evening and now realise that if I’m to continuously monitor earth leakage current and bring it in to say Home Assistant via say an ESP32 I will need to amplify the current with something like this chip fed from what’s called a Zero Current Transformer: