Dealing with >30 mA Residual Current due to an industrial servo drive in a 'man cave' scenario on residential electrics? UK Based Location.

Hi All,

Long story short, I have a man cave that runs off my home's main board. The main board was recently upgraded and is 18th reg compliant. My workshop is fed with a 32A RCBO which then goes into a sub DB which has a Wylex Isolator and then feeds into another 32A RCBO.

I can run most of my machines without issue and I create 3 phase using VFDs. One specific Servo Drive however needs 3 phase and is fed from a VFD which then in turn powers the Servo Drive (so VFD into VFD). This trips my RCBO.

The manual suggests that the Servo Drive (7.5KW/ 10 HP) should sit behind a 200mA RCD, otherwise its likely to trip. This is their exact wording.

"Cause: the leakage protection switch trips after the servo spindle start-up. Reasons and countermeasures: 1.A plain leakage protection switch with a leakage protection value of 200mAis recommended; otherwise candle the leakage protection switch; 2.Use the specified leakage protection switch dedicated to servo(or transducer) with a leakage protection value of 30mA; 3.Add an isolating transformer between the plain leakage protection switch and servo driver."

What are my options in the UK to deal with this?

Thanks in advance.

  • is that above 32A there is some tolerance / leeway in the requirement for 30mA RCD sensing?

    If you're going down the isolating transformer route, this is probably superfluous, but just for completion...

    In general 30mA is for additional protection The rating of the circuit doesn't really come into that. As Graham mentioned, it the fact that it supplies sockets (rated ≤32A) that triggers the requirement, or any domestic lighting circuit, or soft sheathed cables concealed in walls, or any bathroom circuits.  On TT systems (and occasionally elsewhere too) you'll need RCDs for ADS - but that doesn't necessarily mean an 30mA device - 100mA, 300mA or 500mA devices could equally be used if the circumstances are right (Ra etc) - time delayed too if you like.  So if the machine is hard-wired, and the cabling is either surface mounted or is armoured (or similar) the 30mA requirement could well not apply.

    If you are using an isolation transformer, keep and eye on switch-on (magnetising) surges - something large enough to supply 7.5kW might well nuisance trip the usual domestic B-type 32A MCB/RCBO.

    200mA is a very odd recommendation - it's not one of the preferred values for an RCD and I've never come across such a beast. If that's got lost in translation, I wonder what else has? (I've no idea of what they mean by "candling" the leakage protection switch).

       - Andy.

  • Yep, agreed. The 200 mA is probably a 'global' figure of what they believe is required, not set on anyone's standards as such. Candling is Chinglish ... no idea either.

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