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View Full Version : VFD supplied by RCBO vs MCB?


mattnedgus
01-07-2016, 09:01 PM
Is it wise to run the CNC including VFD etc from behind an RCBO or is an MCB better?

I read in the VFD manual something about it not wanting any potential disruption to power and got the impression this included any kind of RCD...

At the moment the 2.2kW VFD, 4 x NEMA23 stepper motors, PC and monitor (and 1kW CamVac) are all coming off the 32A ringmain that supplies the downstairs sockets but I am planning on installing another circuit breaker dedicated to the CNC equipment to reduce the risk of tripping. (I switch on two linear power supplies and VFD one at a time (with a switched extension lead) to avoid tripping the MCB with the inrush current but I'm looking at buying a new dust extractor that requires a 16A feed!)
m_c
01-07-2016, 11:21 PM
Power being cut cleanly isn't an issue, what is an issue is brown outs/surges/spikes in the incoming power.
I certainly would still want a RCD of some form in the supply.

An issue you may face though is the RCD/RCBO tripping due to the way the filtering in VFDs work. For single phase installations you really want Type A.
magicniner
03-07-2016, 06:26 PM
what is an issue is brown outs/surges/spikes in the incoming power.

How does a Residual Current Device help with voltage drop, surges and spikes?
m_c
03-07-2016, 06:41 PM
How does a Residual Current Device help with voltage drop, surges and spikes?

It doesn't. I was clarifying what the manual meant by potential disruption to the power supply.
magicniner
03-07-2016, 07:00 PM
It doesn't. I was clarifying what the manual meant by potential disruption to the power supply.

Ah, that wasn't clear from the first paragraph, it seems to link the issues.
I have overload only breakers on workshop equipment as no RCD I've found deals with my MIG welding equipment, it's all inductive kit, none of your modern switch mode stuff, and it deals better with fine work than any switch mode stuff I've yet tried.
mattnedgus
08-07-2016, 12:43 PM
Power being cut cleanly isn't an issue, what is an issue is brown outs/surges/spikes in the incoming power.
I certainly would still want a RCD of some form in the supply.

An issue you may face though is the RCD/RCBO tripping due to the way the filtering in VFDs work. For single phase installations you really want Type A.

Thank you for helping to clarify, I think what I'll consider doing is adding an A-type 40A RCBO and running a 16A socket for the cyclone extractor and two more UK-3-pin sockets - that way, I can try the VFD in the RCBO and if there's a problem, switch at least "it" back to the MCB!
mattnedgus
08-07-2016, 12:47 PM
Ah, that wasn't clear from the first paragraph, it seems to link the issues.
I have overload only breakers on workshop equipment as no RCD I've found deals with my MIG welding equipment, it's all inductive kit, none of your modern switch mode stuff, and it deals better with fine work than any switch mode stuff I've yet tried.

Yeah this was kind of my thinking - that just overload-only protection might be more reliable. At the same time I want to be protected from any shorts that might develop so an RCD would be some kind of peace of mind. I think if I put in an RCBO and keep the two sockets still to the overload-only MCB it gives me options. At the very worst I can just swap the RCBO back to an MCB at some point in the future!

I'm sure I'm making it more complicated than it needs to be but I like to deal with consumer units as little as possible :-)