Rcd tripping

[aebersold]

Hello All,
I’ve been using a lathe on a single phase supply which has a vfd 1.5hp and it worked fine, but if I then started my pillar drill (also has a vfd) while the lathe vfd was on, the main Rcd would trip. I’ve recently upgraded my lathe to a vicmarc which has an integrated vfd 2hp and that trips out the rcd on start up. The manual does specify the need for a type A rcd, but I’ve discovered that there is another type A version that incorporates a time delay. If anyone has the electrical knowledge to advise on whether the time delay is needed or not, or if I’m missing something, I’d much appreciate the advice please ?
Thanks,
Alex

 

[Doug B]

Hi Alex I had exactly the same problem when my planer, lathe & pillar drill were supplied via vfds, the only solution I found was to isolate one when the other two were switched on I'm afraid.
I did speak to the vdf supplier who told me it was something to do with earth leakage if I remember correctly.

 

[Spectric]

I did speak to the vdf supplier who told me it was something to do with earth leakage if I remember correctly.

The RCD works on an imbalance between current flowing to and from a load, so if there is a path to earth that causes the difference to exceed a given value then the RCD will trip. Now this leakage can be due to a fault but there are other situations that will also cause a larger current to flow to ground, one of these are filters used in many power supplies and the vfd being an invertor will have large capacitors to filter out ripple on the Dc side.

The question is why are you running the lathe and a pillar drill at the same time because running multiple machines is going to increase any earth leakage.

but I’ve discovered that there is another type A version that incorporates a time delay

You cannot use an RCD with a time delay because the regulations state that the RCD must disconnect within a given time and safety has to be the higher priority.

 

[Sideways]

Earth leakage (normally small) is a feature of switch mode power supplies and inverters.
By the time you get to a dozen or so laptop and similar power supplies on the same circuit with a 30mA RCD, you have a risk of the RCD tripping.
VFD's are the same, maybe more so, as @Spectric explains.

Just turn the unused one off or plug it into a different circuit with it's own RCD.

RCDs come in different versions suited to loads which include an element of DC current along with the AC current.
Type AC for no DC at all.
Type A for a little DC.
Type B for more DC.
Because of the way switching power supplies work, their leakage often includes some DC current as well as AC.
With the profusion of switch mode power supplies in homes, no one whould be fitting type AC RCDs any more, type A should be the default, and the installation instructions for your VFDs if they are decent and not a single sheet of chinglish, probably say you should be using type B RCD to protect the VFD circuit.

 

[aebersold]

Hi Alex I had exactly the same problem when my planer, lathe & pillar drill were supplied via vfds, the only solution I found was to isolate one when the other two were switched on I'm afraid.
I did speak to the vdf supplier who told me it was something to do with earth leakage if I remember correctly.

Thanks Doug, so far that’s what I’ve been doing, switching one off before using the other, bit of a faff if using both machines alternately, but probably something I’ll need to live with.

 

[aebersold]

The RCD works on an imbalance between current flowing to and from a load, so if there is a path to earth that causes the difference to exceed a given value then the RCD will trip. Now this leakage can be due to a fault but there are other situations that will also cause a larger current to flow to ground, one of these are filters used in many power supplies and the vfd being an invertor will have large capacitors to filter out ripple on the Dc side.

The question is why are you running the lathe and a pillar drill at the same time because running multiple machines is going to increase any earth leakage.


You cannot use an RCD with a time delay because the regulations state that the RCD must disconnect within a given time and safety has to be the higher priority.

That’s clarified the time delay aspect Roy, thank you. Just to clarify, I’m not running both machines at the same time, they are just powered up.
Here’s an excerpt from the lathe manual :

General Maintenance and Operating Hints

1. Important: The power supply to your power point must be earthed. If you have a circuit breaker fitted to your house the electronic variable speed unit could trip your circuit breaker. Electronic units are designed to have an earth leakage therefore your circuit breaker will sense this leakage and trip out. For those who do not have circuit breakers installed, the above mentioned will not apply.

Another excerpt from the vfd manual :
2. RCD (Residual-current-operated protective device) for the single
-phase 200V input inverter

In case of using RCD, as for the single-phase 200V input inverter (FVR+x-7***), RCD of type A or RCD of type B is allowed on the power supply side of this EE. (Electric equipment)

 

[aebersold]

Earth leakage (normally small) is a feature of switch mode power supplies and inverters.
By the time you get to a dozen or so laptop and similar power supplies on the same circuit with a 30mA RCD, you have a risk of the RCD tripping.
VFD's are the same, maybe more so, as @Spectric explains.

Just turn the unused one off or plug it into a different circuit with it's own RCD.

RCDs come in different versions suited to loads which include an element of DC current along with the AC current.
Type AC for no DC at all.
Type A for a little DC.
Type B for more DC.
Because of the way switching power supplies work, their leakage often includes some DC current as well as AC.
With the profusion of switch mode power supplies in homes, no one whould be fitting type AC RCDs any more, type A should be the default, and the installation instructions for your VFDs if they are decent and not a single sheet of chinglish, probably say you should be using type B RCD to protect the VFD circuit.

Sideways, thanks for your response. I have been turning one off before using the other but thought it may be an issue that could be overcome. The type B is an added layer of uncertainty. The AC rcd I read somewhere is becoming illegal. I think I’ll probably try the type A without the time delay. Thanks

 

[Spectric]

I’m not running both machines at the same time, they are just powered up.

Could you clarify this because it might be an issue as a VFD has to be connected to the load permanently and you cannot have a switch between the VFD and the load.

As for types of RCD, as said type AC will just become obsolete because it is not suitable for modern equipment with high earth leakage and can become inoperative due to Dc saturation so type A will become mainstream for most domestic purposes. Type B can be used for both single and three phase systems so could be used but not sure if there is anything to be gained. What happens with type A & B RCD's is that they let Dc pass through, remember an RCD uses a small coil which only works on Ac but can be saturated by Dc.

 

[aebersold]

To clarify, I plugin the machine to activate the vfd, then press the push button on the lathe to set in motion. My electrical knowledge is pretty limited as you are probably aware, but learning as I go along. Thanks for your patience !

 

[Duncan A]

Some VFDs have a small RF filter which can be a source of earth leakage, and may be designed for disconnecting by the simple removal of a screw. Obviously, this needs to be explained in a decent manual before inserting the screwdriver!
Your lathe may not have a proper VFD manual but, if you can identify the VFD, it is often possible to find a comprehensive manual on line.
FWIW, I've got a 3hp lathe and a 1.1kW drill, both with VFD's and never had a moments bother despite often leaving them powered up (but not rotating) at the same time.
The RCD's are standard - no idea offhand exactly what, but definitely not B's.
Duncan

 

[Paul Hannaby]

Pretty much all VFDs have filters which cause some earth leakage. That's how the filters function. Too much leakage or too many devices with leakage can lead to nuisance tripping which is what it looks like you are experiencing. From what I understand, the filters in Vicmarc lathes are renowned for high leakage which contributes to this issue.
One solution might be to replace the RCD with one with a higher tripping current. The standard is 30mA but you can get ones with higher rating. You would need to talk to an electrician to confirm what would be permissible by the current regulations in your application.

Incidentally, I have four variable speed lathes in my workshop and even when all four are running, I have never had a problem with nuisance tripping.

 

[Spectric]

The standard is 30mA but you can get ones with higher rating

There is a reason why they trip at 30mA within a set time, it is so they stop people getting electrocuted. As you increase the disconnection time the amount of current needed to stop a heart falls. 10mA for three seconds can be fatal but 30mA for less than 40ms is ok. Anything above 75mA will be very painful and probably fatal.

The problem here is probably a case of multiple leakage currents all adding up, so RCBO's really help because now your leakage currents are not adding together. An outbuilding can be supplied via a cable not RCD protected and then within a local board you have RCD protection on all circuits so if your machines were on seperate radials the issue would not exist.

 

[Dabop]

Definitely sounds like a 'cumulative load' of multiple earth leakages all adding up to trip the RCD... (especially if you have a lot of things plugged in on multiple spurs or if the workshop is connected to the house ring, as many are...)

One advantage of being an Aussie- ring circuits aren't used here at all unlike back in the UK...- every circuit is a radial (with or without spurs, depending on usage and the outlets it is feeding...)

A '10A powerpoint' can have up to a dozen outlets off the one breaker (each MCB has to be a combined circuit breaker and RCD these days here) but 15A or 20A single phase outlets (and a 20A or 32A 3 phase if your place has 3 phase run to it) all have to be on their own individual breaker/RCD

So here, you would have a 'shed supply' breaker in the main fusebox (no RCD) rated for the cable going to the shed from the house- its only job is protecting that spur supply cable- nothing else)- then a subboard at the shed, with the 'workbench' powerpoints on one (or more) RCD's, the lighting circuit on a second, and then each machine on its own RCD protected 15A or 20A powerpoint...

(another advantage with the Aussie GPO's is that they are 'plug interchangable'- a 20A outlet looks like a 15A or 10A one- and indeed a 10A plug or 15A plug will plug right into a 20A even, but not vice versa- same with a 10A plug will go into a 15A powerpoint, but not the other way round)

10A/2400W ie 'standard household powerpoint'- note all the 'pins' are the same width/height, only the earth one is longer (for safety- first in, last out)

15A/3600W (individual RCD for each outlet at the shed subboard required by law here)- larger earth pin stops the plug going into a 10a GPO but 10a plug still goes in safely into the 15a outlet)

20A/4800W (all three 'pins' are now the size of the 15A earth one- the 10A and 15A plugs still go in and are firm due the correct spacing of the holes, one side and one edge of the pin is still firm against the plastic to support them and guide them in when plugging it in)

Very handy in a workshop shed, as if a 10A powerpoint isn't handy, just use a 15A or 20A one nearby instead to plug in a hand tool like a drill or whatever...

With each 15A or 20A on its own RCD, cumulative leakage isn't an issue and 'false triggering' is pretty much a non issue...

That might be a way forward, not too sure on the current BS standards, (been a long time since I looked at them LOL) but is a MCB at the main consumer unit (fusebox in Oz) and a subboard with seperate RCD's in the workshop acceptable still there??? (ETA appears it is)- split those outlets off and have independent outlets each on their own RCD for the bigger machines acceptable practice there???
ie same feed from the house, but each of the big machine outlets on its own spur on its own RCD from a subboard located at the shed...

Would cut down on 'excessive' cumulative loads making the RCDs trigger early

It would have been acceptable in 'ye olden days' back before RCDs became the norm when I regularly checked the BS standards but like I said, its been a while since they had any application to me personally lol

 

[Spectric]

One advantage of being an Aussie- ring circuits aren't used here at all unlike back in the UK

You are so lucky in not having ring mains, they can be a right pita when it comes to testing and people bodging.

each MCB has to be a combined circuit breaker and RCD these days here

Yes we are going that way, we call them RCBO's (Residual Current Breaker with Over-Current) but many still do not use them as they are / were more expensive.

but is a MCB at the main consumer unit (fusebox in Oz) and a subboard with seperate RCD's in the workshop acceptable still there

Yes you can have just a protective device for the cable to an outbuilding providing in that outbuilding all the circuits are protected by an RCD, ideally an RCBO on each radial circuit. A problem you see is people hook into a ring main to feed the outbuilding because it might be easier and then problems begin rather than a dedicated outbuilding supply.

 

[aebersold]

Some VFDs have a small RF filter which can be a source of earth leakage, and may be designed for disconnecting by the simple removal of a screw. Obviously, this needs to be explained in a decent manual before inserting the screwdriver!
Your lathe may not have a proper VFD manual but, if you can identify the VFD, it is often possible to find a comprehensive manual on line.
FWIW, I've got a 3hp lathe and a 1.1kW drill, both with VFD's and never had a moments bother despite often leaving them powered up (but not rotating) at the same time.
The RCD's are standard - no idea offhand exactly what, but definitely not B's.
Duncan

Hi Duncan, I did speak to ‘Inverter Drives’ where I bought one of the units from and they did mention the screw you refer to. I’ll look into that and give it a go, thanks.
It’s also good to know that you have no issues having both inverters on at the same time, gives me confidence in solving the issue, thanks.

 

[guineafowl21]

The VFD for my mill required disconnection of the EMC filter if the power source had asymmetrical grounding. This is what we have for single phase in the UK (bottom left in the pic), so I did.

It means the voltage between each live conductor and earth is not the same.

Symmetrical earthing is where the centre of the transformer is tapped to earth, and the power is taken from the outer taps, like in the US (eg phases S and T in top right).

Of course, if the VFD is fed with UK 3 phase, it will also have symmetrical earthing.

 

[dgs2001]

Silly question, but have you had anybody test the RCD?

Any electrician with a testing ticket and an MFT can test to make sure the RCD is not faulty, and causing nuisance tripping

 

[aebersold]

Pretty much all VFDs have filters which cause some earth leakage. That's how the filters function. Too much leakage or too many devices with leakage can lead to nuisance tripping which is what it looks like you are experiencing. From what I understand, the filters in Vicmarc lathes are renowned for high leakage which contributes to this issue.
One solution might be to replace the RCD with one with a higher tripping current. The standard is 30mA but you can get ones with higher rating. You would need to talk to an electrician to confirm what would be permissible by the current regulations in your application.

Incidentally, I have four variable speed lathes in my workshop and even when all four are running, I have never had a problem with nuisance tripping.

Thanks Paul, I have 3 outbuildings each with their own consumer unit and earth spikes, which feed into a separate consumer unit in the house with a 100ma Rcd which is the one that trips out, not the workshop one (with the inverters) for some reason which has a 30ma Rcd. Good to know, like Duncan that there should be no issues have more than one on simultaneously.
Workshop Rcd :
House dedicated outbuilding CU Rcd :

 

[Spectric]

the house with a 100ma Rcd

Not required, the supply cable to each outbuilding does not need an RCD only short circuit protection in the form of a fuse / MCB and maybe you do not have a problem.

 

[Sideways]

That wiring is OK.
30mA rcd's at the sub boards to protect people.
100mA rcd at the main board to protect the distribution cables to those sub boards, provided the cables are installed in a way that there's no risk of somone putting a screw into them. They should be in conduit or run in steel wire armored cable. (But agree with Roy, an RCD is not required on that circuit, but it is reasonably sized and if it's not defective, it is pointing to an issue).

Still, that is a lot of cumulative leakage setting off the 100mA RCD.
The obvious next steps to my mind are to have the 100mA RCD tested with an electrician's multifunction tester (and do the 30mA ones while he's at it)
Then to check the AC earth leakage at numerous points with a high sensitivity, earth leakage clamp meter.
Depending on what you find, you may need to look for DC leakage current after that.