I agree if the RCD trips immediately given a good supply and nothing connected to its output terminals, then the unit is duff.

Presumably this is an industrial environment - is there any other electrically  "noisy" equipment about? I'm just wondering about something putting large spikes on the mains - and type B RCDs will be packed with far more electronics than a traditional coil-and-solonoid AC type of yore. Any SPDs about?

     - Andy.

I appreciate your feedback. I used a earth leakage clamp meter to measure the current of the seven motors that make up the machine. When I clamped all three phases (Supply) together, the current was 45 mA. When I clamped all three phases and the CPC together, the current was 390 mA (fluctuating 400 +) I measured each motor separately. The current of the three motors that were connected to inverters was around 15mA when I clamped only the phases, but it increased to around 60 mA when I clamped the CPC as well. The feeder motor, which had a power rating of 1.1kW and was supplied by an inverter, had a different current pattern. It had a low current of 7.7mA when I clamped only the phases, but a high current of 140mA when I clamped the CPC as well.

When I clamped all three phases (Supply) together, the current was 45 mA. When I clamped all three phases and the CPC together, the current was 390 mA (fluctuating 400 +)

Are there any parallel earth paths from the machine (parallel to the c.p.c. that is) - e.g. any direct connections to metallic water pipes, structural steel or other machines with their own c.p.c.s. I'm just wondering if the c.p.c. currents are a bit of a red herring - either leakage originating from other parts of the installation and finding a path through your c.p.c. or even currents from outside the installations (e.g. diverted PEN currents from TN-C-S distribution systems).

In any event RCDs just see the currents in live conductors, so clamping around just those and ignoring the c.p,c. should be sufficient to see what's happening.

   - Andy.

Hello, yes. The machine has a lot of structural steel, steam pipes etc attached to it.

Hello, yes. The machine has a lot of structural steel, steam pipes etc attached to it.

So if I can disregard the CPC readings the overall leakage current shouldn’t be a problem. I suspect I’m not being told the full story leading up-to the RCD failure. Currently I have the machine locked off, I Will reenergise tomorrow. Thanks Andy