RCD protection for portable equipment with Inverters

[gerard]

Hi,

I know this has been covered before and I don't mean to repeat questions. I am looking to get some opinions on a problem I have encountered in the past and have experienced recently. I have a machine with two 3ph motors which are controlled by Inverters. They are 1ph - 3ph Inverters. The problem is they keep tripping the RCB. I tried putting a 100mA RCBO on the circuit, with no success. For this reason, I have taken the RCD out of the circuit and is now only protected by a MCB. I feel very uncomfortable with this and I am looking for similar problems you have encountered and how you solved the problem? The regs do not cover such equipment with Inverters and their protection.

Thanks

 

[netblindpaul]

Check Manuf' instructions.
Why are you so uncomfortable?
RCD's are THE most unreliable protection device that we use.
This is proven by independent authoritative research.

 

[gerard]

I looked at the manufacturers info and I am none the wiser. I feel uncomfortable because I am afraid the machine might become live and give the operators
a smokin' perm! The circuit is only used for this particular machine and not for appliances like Hoovers and floor scrubbers, also, the machine in question is stationary. So, you reckon all is fine and I have nothing to worry about then - no need for a higher rated RCD? What would you do in a similar situation?

 

[Knobhead]

RCD’s and inverterers don’t like each other. I think your on a hiding to nothing, you just have to accept there’s going to be leakage and arrange the supply to suit.

 

[gerard]

RCD’s and inverterers don’t like each other. I think your on a hiding to nothing, you just have to accept there’s going to be leakage and arrange the supply to suit.

Ok, so would you just protect the circuit with an MCB, make sure all connections are sound and forget about it?

 

[netblindpaul]

I feel uncomfortable because I am afraid the machine might become live and give the operators
a smokin' perm!

Why, is there no fundamental protection in the installation in the way of ADS (ex EEBADS?).
You ARE NOT ALLOWED TO RELY ON ADDITIONAL PROTECTION.
An RCD IS ADDITIONAL PROTECTION.
IF you are relying on an RCD, for basic protection then you are non compliant with 7671 & almost certainly EAWR & PUWER.

 

[gerard]

I am only relying on the MCB for protection

 

[netblindpaul]

I am only relying on the MCB for protection

Then your problem is?

 

[gerard]

Well do I have a problem and would you be happy with this?

 

[netblindpaul]

Gerard,
What is your problem, why are you unhappy, I can say anything from here and it would mean nothing.
I am asking these questions for a reason, and trust me please, it is not to be horrible, there is another reason, but there is also the same validity to my asking the questions as to why I am not answering them...
Please think about this and answer my question, I will help, but I can't give you the answer.

 

[gerard]

The machine is plugged into a 16A 1ph socket without RCD protection because of the reasons above. As the machine will not operate with an RCD, I am just concerned about the machine becoming live and injuring (or worse) an operator - that's all.

 

[netblindpaul]

You have perfectly functional and sound ADS on the supply circuit that complies with the current version of BS7671 do you not?
Excluding the RCD for the moment.

 

[gerard]

You have perfectly functional and sound ADS on the supply circuit that complies with the current version of BS7671 do you not?
Excluding the RCD for the moment.

Yes I do.

 

[netblindpaul]

So why do you want additional protection?

 

[Engineer54]

What do you think equipment and machinery relied on for more years than you can remember, or come to that, ...i can remember?? There was no RCD protection available then, RCDs are a relatively new form of protection, that far too many these day's rely on. While they serve/provide a very worthwhile form of protection, they are not the cover all cure protection many think they are!!! As you have found out, sometimes they just don't fit in. They are also not the most reliable bit of kit either, often causing more trouble than they are worth.

So, if you have a proven functioning ADS on the supplying circuit you have a safe installation that has proven it's worth for many many years. So get some sleep tonight!! lol!!!

 

[gerard]

Case closed then! Thanks for that.

 

[Inteificio]

Hi possibly a stupid point, but not dealt with inverters before.

If you want an RCD can you not insert one after the inverter? eg an RCD socket.

The current leakage in the inverter might not affect the circuit after...or it might, for some reason can't get my head around it this early in the morning.

 

[clicon357]

Case closed then! Thanks for that.

Whoa there! Lets open the box on this one again. I think I can help:

Go back 10 years and my company were importing kit from abroad fitted with S** (major German company) motors and inverters. The kit we were importing itself had no rcd protection but when we plugged it into the supplies provided on site then we would get tripping problems about 25% of the time (not 25% of the sites but like 1 in 4 times on the same site), this was always where there was 30mA protection. I did some digging and investigated options. Seems that inverters are naturally leaky to earth because of EMF circuitry and that on occasion the leakage was enough to trip the RCD. I contacted the drive/motor maker who's suggestions were 1) fit a drive WITHOUT EMF filters and / or 2) protect with a 100mA RCD. The first, ditching the filtration was unacceptable as it gave us issues with CE marking of the kit. The 100mA RCDs were unaccepable to the sites we were working on. This went on with no real resolution. One S** drive was changed to I think a Jag*** drive that had no fitration and that worked - the CE issue was ignored. Another site was changed to 100mA with non standard 3-phase connections (7-pin) until the client's insurer kicked off. It was because of that site I had to sort it once and for all. I came across the Merlin Gerin range of "Si" units which are specific for sensitive installations. They are NOT as such a time delay type. Had a bit of a time getting them as nobody had heaard of them. Ended up getting two 40A Si units from France via my Telemechanique supplier. Bottom line it worked. Details here:

http://www.engineering.schneider-el...n_against_nuisance_tripping_voltage_surge.pdf

As an interesting follow up to this I then got a letter from S** (they're a huge manufacturer and by comparison I'm Joe Nobody) some time later asking IF I HAD MANAGED TO FIX THE PROBLEM! Understandably I was a bit peeved so called them up. It turned out that they had supplied a customer with something like 200 drives that had gone into kid's fairground style rides - I think the small ride on sort that you stick 50p in and you see at shopping centres. Guess what, a fair proportion were tripping. I suggested the Si RDs from Merlin. I have an excellent relationship with S** seemingly on the back of this in part and use them to this day as my preferred motor/drive supplier.

Hope this helps.

 

[netblindpaul]

clicon,
I know the company Jag***.
I am struggling with the S**?
I know of a large German drives manufacturer, but not just a 3 letter name.
I can think of one possibility, but I did not think they were German, any chance of letting me know to satisfy my curiosity please?
PM if you don't wish the name to be in open forum?
I actually worked for a German drives company at one time, not the one you are describing I think.

 

[Darkwood]

Thanks for link Clicon..... got Sneider catalogues but old ones and dosnt show SI or type B units been looking for a device like this for ages. .......... much appreciated.

 

[srewolf21]

Not sure if I'm barking up the wrong tree but the way I understand how inverters work there is a good chance they may introduce harmonics on to the system which will change the characteristics of the supply frequency and in certain circumstances will increase the neutral current in relations to a RCD obviously a increased neutral current would result in the device tripping.
It may be because harmonics are new to me and I'm seeing potential problems everywhere but with inverters I think this could be a real possibility.

 

[Knobhead]

Gerard, if you are so worried about earthing of portable equipment then maybe you should look at earth proving outlets.

 

[clicon357]

clicon,
I know the company Jag***.
I am struggling with the S**?
I know of a large German drives manufacturer, but not just a 3 letter name.
I can think of one possibility, but I did not think they were German, any chance of letting me know to satisfy my curiosity please?
PM if you don't wish the name to be in open forum?

I actually worked for a German drives company at one time, not the one you are describing I think.

You have a PM sent.

 

[Darkwood]

Not sure if I'm barking up the wrong tree but the way I understand how inverters work there is a good chance they may introduce harmonics on to the system which will change the characteristics of the supply frequency and in certain circumstances will increase the neutral current in relations to a RCD obviously a increased neutral current would result in the device tripping.
It may be because harmonics are new to me and I'm seeing potential problems everywhere but with inverters I think this could be a real possibility.

Check out the link on post#18 it will explain why and what causes nuisense tripping etc, it has a section on VSD's too explaining the nature of their leakage and how these effect standard RCD's causing them to trip or even blind them, its a little too involved to explain on here but has diagrams and graphs etc to aid understanding.

 

[clicon357]

Thanks for link Clicon..... got Sneider catalogues but old ones and dosnt show SI or type B units been looking for a device like this for ages. .......... much appreciated.

No problems. Saved us a lot of headache. A mate worked for the then competition and they were using the same drives and having the same problems we were. Over a beer one night it came up and I mentioned our fix. Turns out they had changed their make of drive totally (to Toshiba I think). It was a bit of a pain as their engineers had to learn new bit of sofware for doing the motor start ups etc. Also they kept with the original motor make which meant entering a lot of parameters manually rather than just telling the drive what motor it was running from an pre-loaded list. Think we got our Si RCDs for about £50 a time. If I think of it I'll dig out the invoice and PM you the exact type we used.