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Inverter Tripping RCD

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Steve Skelton 108/04/2020 12:03:46
56 forum posts
2 photos

Hi, you are going to struggle to find one at a reasonable price - I came across this when searching. We have a business selling obsolete electrical control gear and we very rarely come across them.

id-65-2998:dev-c&gclid=CjwKCAjw7LX0BRBiEiwA__gNw3ejZT-uMTXxgyXoU1ZlkIqVIa8yutDq4oPUFsE5rrAdDbPebByruRoCB7EQAvD_BwE">https://www.rapidonline.com/Schneider-Electric-A9Z51240-40A-Single-Phase-2-Pole-30mA-Type-B-RCD-65-2998?IncVat=1&pdg=pla-340003444795:kwd-340003444795:cmp-757438067:adg-98613163406:crv-428566073244id-65-2998:dev-c&gclid=CjwKCAjw7LX0BRBiEiwA__gNw3ejZT-uMTXxgyXoU1ZlkIqVIa8yutDq4oPUFsE5rrAdDbPebByruRoCB7EQAvD_BwE

Cheers

Steve

Steve Skelton 108/04/2020 12:06:41
56 forum posts
2 photos

Alternatively Proteus do a slightly lower cost one

**LINK**

Steve

Steve Skelton 108/04/2020 12:25:09
56 forum posts
2 photos

Last one - on eBay but is not a known brand to me, but much lower cost!

**LINK**

Cheers

Phil Whitley08/04/2020 12:35:18
avatar
1145 forum posts
142 photos

Ouch, that sounds like a lot, are you looking at a 3 or 4 pole RCD when all you need is a 2 pole one? A brief reading of the backstory on this thread seems to show that the understanding of what the filter does in the VFD is incomplete, it prevents EMI and voltage spikes in both directions, in order to protect the VFD from voltage spikes incoming from the mains supply, which would damage the VFD. It does this using a capacitance bridge type circuit which dumps any spike to earth, and depending on the value of the charge that is dumped, plus any residial leakage from other equipment, you arrive at the above mentioned cumulative leakage to earth, and if the leakage current exceeds the tripping current of the RCD, then it will trip, but it may be that the tripping only occurs when other equipment, lighting etc is in use as well, and the cumulative current is greater than the RCD rating, hence a situation where tripping is intermittent. The easy way to solve this is have a non RCD circuit (MCB protected) for the VFD only, or to protect that circuit with its own 60ma RCD (about £35). The other way, and I would recomend this anyway, is to have all the equipment including the VFD circiuts and equipment, checked (including lighting) for residual leakage. Always make sure that all the metalwork of the machine is earthed to the earth point provided by the supply authority. If an RCD trips, it is either sensing a leakage current beyond its tripping threshold, or it is faulty, there is no arcane magic! What often happens to confuse the unwary is that either a spike is discharged as a pulse large enough to cause tripping, but once discharged, does not reappear for some time, or that other equipment on the same RCD has small amounts of leakage, and the small amount of normal current leakage from the filter in the VFD( in other words, the filter doing what it is designed to do) causes the tripping, and the VFD is blamed for the tripping, whereas the tripping is actually caused by the addition of lots of small leakages from other equipment, and switching on the VFD was the "last straw". Leaving the VFD unprotected from spikes by removing or otherwise disabling the filter means that the first mains spike will destroy it, simply doesnt make sense.

Hope this makes sense, and helps with the problem!

Phil.

Edited By Phil Whitley on 08/04/2020 12:36:03

Edited By Phil Whitley on 08/04/2020 12:38:19

SillyOldDuffer08/04/2020 12:40:56
5605 forum posts
1153 photos
Posted by Barnabas Taylor on 08/04/2020 11:37:12:

Hey guys, sorry to get in on an old thread but I have just taken delivery of a 1.5Kw motor and VFD and I am concerned about tripping problems.

...

Have you actually got a tripping problem, or just concerned there might be one?

If the latter, I'd try plugging it into whatever you've got and trying it. No trip = no problem.

Otherwise, it's possible for several small leaks to accumulate enough to trigger a shutdown, even though the devices are all individually OK. (The trip can't tell the difference between a single big fault leak and a combination of small deliberate leaks due to EMC filters as fitted to many household devices.) A new appliance might be the straw that breaks the camels back rather than root cause.

If a new device causes a trip, it's worth unplugging everything else and reconnecting in various combinations. Possibly one or two of them are the big problem, not the new VFD.

I'm reluctant to recommend removing EMC filters - although the radiation is silent and invisible, it's anti-social. Bypassing the RCD with a rusty 6" nail isn't done either!

Dave

Barnabas Taylor08/04/2020 12:41:06
29 forum posts
2 photos

Cheers Steve, those are a bit more reasonable! It still amazes me why this is not a simple (cheap!) solution. Do most VFD users simply not replace their RCDs? Or do they just pay a huge electrician bill and never mention it when they say VFDs are great? Did I make a huge mistake buying such a thing as I now cannot afford to install it and use it? If I was less conscientious, I would never have even realised I was supposed to worry about such things because it rarely comes up on the knifemaking forums, where VFDs are very common...

I have a further problem because the slightly sketchy electrical set-up to the garage (left over from the previous owner) has the supply coming off the house DB, to a DB in the shed with an RCD to a DB in the garage with simply a breaker and MCBs. I think this means I need to replace both the RCD in the shed DB and the RCD in the house DB which is then a lot of money!

Would it be cheaper to get a man in to rewire the garage to an earth spike? are there problems with such a set-up? I remember my grandfather telling stories of him rigging up his own earth spike from a fence pole when he was about 12... He went on to have a successful career in the power distribution industry!

Barnabas Taylor08/04/2020 12:46:13
29 forum posts
2 photos

From what I gather from my own research, the problem with a VFD is not so much the earth leakage but that you need an RCD that will protect against more than just an AC current. Hence the silly priced versions.

this how-to blog is very handy but doesn't sold the cost issue!

How-to-Use-Inverter-Drives-with-RDC-Earth-Leakage

 

The type B ones still seem to trip at 30mA so how that helps with VFD leakage trips I am not sure, whether you have to just have them on a separate circuit which is yet more cost!

Edited By Barnabas Taylor on 08/04/2020 12:47:41

Steve Skelton 108/04/2020 13:02:37
56 forum posts
2 photos

Hi Barnabas,

Yes you are right , it is the DC component that causes the problem. You can use a basic AC type RCD which will trip on a pure AC fault but if the DC component is too high it may not trip. I am not an expert on VFD systems but the same problem can occur with charging systems for electric car charging systems and solar power installations.

It is no consolation for you but the price of Type B RCD's will drop as the demand increases.

I imagine that most people do not fit Type B units - probably from a lack of understanding they fit AC types.

My advice would be to do exactly as the manufacturer recommends as they are the experts but I would have thought that it is only the supply to the VFD that needs the Type B RCD and not the rest of the house.

Steve

duncan webster08/04/2020 13:17:39
avatar
2533 forum posts
49 photos

When I first got a VFD it was the MCB which kept on tripping, not the RCD. I cured it by fitting a fuse instead, it coped with the inrush current

Barnabas Taylor08/04/2020 13:21:00
29 forum posts
2 photos
Posted by Steve Skelton 1 on 08/04/2020 13:02:37:

Hi Barnabas,

Yes you are right , it is the DC component that causes the problem. You can use a basic AC type RCD which will trip on a pure AC fault but if the DC component is too high it may not trip. I am not an expert on VFD systems but the same problem can occur with charging systems for electric car charging systems and solar power installations.

It is no consolation for you but the price of Type B RCD's will drop as the demand increases.

I imagine that most people do not fit Type B units - probably from a lack of understanding they fit AC types.

My advice would be to do exactly as the manufacturer recommends as they are the experts but I would have thought that it is only the supply to the VFD that needs the Type B RCD and not the rest of the house.

Steve

Cheers Steve, I shall probably just get a sing RCB then. I shall think about the relative merits of the RCDs you sent, then I shall stop lying to myself and the bank manager and buy the cheap one! I'll fit it just to the one DB, that way I am protected and I shall just have to see if the VFD trips the 30mA earth fault. If so, it is back to the drawing board!

Clive Brown 108/04/2020 13:39:12
399 forum posts
11 photos

I purchased a Schneider VFD for a mill about 6 months ago. Tripped the domestic RCD straight away. Not something i'd experienced with the Teco VFD on my lathe After a brief investigation into type B RCDs, I read the Schneider manual carefully and found a warning about just this problem and its solution. The EMI filters can be disconnected, an internal, removable bridging link is provided for the purpose. Problem solved with no knock-on effects.

Then looked into Teco manual, there is similar provision, which I've not needed to apply. Fingers crossed!

My other VFD, a cheapo Ebay jobbie, gives no trouble. I doubt it has EMI filters.

Barnabas Taylor08/04/2020 13:53:00
29 forum posts
2 photos
Posted by Clive Brown 1 on 08/04/2020 13:39:12:

I purchased a Schneider VFD for a mill about 6 months ago. Tripped the domestic RCD straight away. Not something i'd experienced with the Teco VFD on my lathe After a brief investigation into type B RCDs, I read the Schneider manual carefully and found a warning about just this problem and its solution. The EMI filters can be disconnected, an internal, removable bridging link is provided for the purpose. Problem solved with no knock-on effects.

Then looked into Teco manual, there is similar provision, which I've not needed to apply. Fingers crossed!

My other VFD, a cheapo Ebay jobbie, gives no trouble. I doubt it has EMI filters.

Correct me if I'm wrong, but if you disconnect/bridge the filters, isn't the whole thing still unsafe unless you fit a type B RCD? (Not to mention the potential 'noise' in the electrical circuits) I am going to wire things up today (probably, if I can find some suitable wires in the box!) and see if it causes trips, if it doesn't, I shall get a type B RCD and fit that. if it does trip, then back to the drawing board! Either I shall have to remove the filters, or I shall have to run a dedicated line just for it!

Phil Whitley08/04/2020 14:05:27
avatar
1145 forum posts
142 photos

I am taking on board what is being said here, but a VFD without an EMI filter would not last long before a mains spike would take out its electronics. they are there to protect the VFD from the mains, and the mains from any EMI The VFD priduces. EMI filters are capacitive, and therefore BLOCK DC current An RCD is tripped by current flow imbalance, and therefore "you would think" that the AC/DC thing should make no difference, as it is current imbalamce between line and neutral which causes the trip, unless there is some part of the RCD circuitry that reads a DC component in the line as more current than it actually is? If this is the manufacturers explanation, and it works, then the only thing we can argue about is the price, which does seem to be an awful lot for what you actually get. I am lucky, I have "real" three phase, and only have RCD protection on the single phase installation.

Phil

Clive Brown 108/04/2020 14:39:20
399 forum posts
11 photos

For disconnecting the Schneider filters, the caveats in the manual are that to safeguard the VFD the drive switching frequency should not be set above 4kHz and also EMC compliance is not guaranteed.

Barnabas Taylor08/04/2020 18:59:25
29 forum posts
2 photos

A quick update, after some fiddling around with wires, I have got the VFD running, as yet I cannot 'load' the motor as I am waiting on a drive wheel but it hasn't tripped the house yet! This bodes well for just having to purchase an RCD for safety and being able to keep the filters intact on the VFD. Fingers crossed I am only out of pocket by another £100 and change...

Lynne09/04/2020 00:51:09
80 forum posts
27 photos

An interesting discussion. Having read the F_____ manual, it tells me that all vfd's have a leakage current to earth, the levels of which are affected by motor cable length and type, effective switching frequency, earth connection used, and type of RFI filter installed. If an ELCB is used, it must be fitted with a type B device, The vfd I have has an external screw which can be 'removed' if tripping is an issue. Now logic suggests to me, that no mfg. would tell one to do this if transient spikes from whatever source are going to destroy the vfd. There has to be some form of protection built into the drive. But hey ho, I am no electronics buffa. Regards, Lynne

Phil Whitley09/04/2020 12:53:06
avatar
1145 forum posts
142 photos

Thats interesting Lynne! I wonder what the screw actually does? If not dissconecting the filter entirely, it must perhaps change the way the filter works? The leakage curret to earth is the filter dumping what charges it has filtered out of the supply, and also any harmonics that the vfd emits back to the supply Maybe it is unwise to ponder on anything beyond finding a solution to the immediate problem?

Phil

Andrew Johnston09/04/2020 19:57:19
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5399 forum posts
621 photos

Some of the confusion arises from the fact that both RCD and overload functions use letters to determine the details, even though the functions are different and may be different parts.

If we look first at the overload function there are three common classes B, C and D. Class B will trip at 3 to 5 times rated current and is normally used in domestic properties. Class C trips between 5 and 10 times the rated current and is normally used in commercial properties. Class D trips between 10 and 20 times the rated current and is normally used in industrial properties. Class C and D are used where larger inrush currents are expected. All the breakers in my fairly new consumer unit are type B overload curve.

Confusingly the RCD function also uses letters to determine characteristics. The three most common are AC, A and B! Type AC only responds to AC residual currents. Type A also responds to pulsating DC currents and type B further responds to smooth DC currents. So in theory a VFD should be running from type B. But this makes it more likely the RCD will trip as it responds to more types of inbalance. Of course that might be seen as a good thing. The RCDs in my consumer unit are type AC and I run two VFDs on my CNC mill with no problems. Never had an RCD or overload trip.

Both VFDs run via EMI filters which contain a common mode choke and common mode and differential mode capacitors, but do not contain MOVs or transzorbs to catch mains spikes. Of course the filter components will attenuate mains spikes to some extent, but fast spike attenuation is left to the user, at least on the VFD I installed. I live in a rural area with overhead 3-phase feeders and fingers crossed I haven't had a spike related blow up yet.

Andrew

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