RCD protection for portable equipment with Inverters

[gerard]

Yes, and this is the second time. The first time the Inverters themselves kept tripping because they couldn't handle the motors and they were up-sized. The machine builders acknowledged this and replaced them with higher rated Inverters. Trouble was/is the old Inverters were in a very confined space and this limited the size of the new ones. They should probably have been up-sized by two sizes. I'm not looking forward to telling the boss about this as I have let it go on too long and should have notified the machine suppliers earlier - I didn't know what the problem was then.

 

[Engineer54]

Yes. Yes and Yes. The manufacturer recommends up-sizing the inverter like you say - he said he has seen this problem before and this will solve the problem. Not exactly the cheapest solution though!!

Blimey, ....the manufacturer actually admitted supplying miss-matched/under rated inverters?? I suspect this is a ''duty'' related miss match, rather than anything else!! Have you informed them that you are still having problems with their supplied machine?? I may be, being a little nieve here, but isn't the supplier responsible for supplying a machine that is ''Fit for purpose''?? Exactly, ...how old is this machine, it must be out of warranty if they are expecting the customer to be paying for an upgrade that they should be responsible for??

If you continue to have problems with the RCD side of things, i would personally remove it, and rely on your ADS protection.

 

[gerard]

The machine builder (not the inverter manufacturer) admitted fitting an under rated VFD and replaced it - but they should have gone two sizes up as the new one was still not suitable. It's over a year old - I must dig out the details and maybe I will contact the builder to see what he has to say. If i had had know earlier i would have picked up on this and nipped it at the bud.

 

[clicon357]

The machine builder (not the inverter manufacturer) admitted fitting an under rated VFD and replaced it - but they should have gone two sizes up as the new one was still not suitable. It's over a year old - I must dig out the details and maybe I will contact the builder to see what he has to say. If i had had know earlier i would have picked up on this and nipped it at the bud.

Have you tried the Merlin Gerin SI unit like I suggested yet?

 

[Engineer54]

The machine builder (not the inverter manufacturer) admitted fitting an under rated VFD and replaced it - but they should have gone two sizes up as the new one was still not suitable. It's over a year old - I must dig out the details and maybe I will contact the builder to see what he has to say. If i had had know earlier i would have picked up on this and nipped it at the bud.

When referring to ''Manufacturer'' i am talking about the company that supplied your company with a working (or maybe not ...lol) machine. Not included component parts manufacturers....

 

[clicon357]

I put this whole question to a friend who has a lot of experience with industrial drives in fact he pointed me to the Si way back, his emailed comments are below. Wasn't sure at first what he meant by a Type B Breaker - was confusing it with a Type B MCB but pretty sure now he means a Type B (time delay) RCD.

"Next size up drive may help if,
1. The drive is getting hot, back plain running over 50dec. (bigger drive more cooling)
2. Doing fast stops without brake resistor will overload dc voltage buss.

But installing a bigger drive will not stop RCD tripping.
To improve RCD tripping you can install a Schneider Type Si or Type B Breaker."

 

[clicon357]

MY MISTAKE! I said above I thought a "Type B RCD" was a time delay type - apparently not: "Type B is for DC fault currents no time delay."

Wikipedia link:

Residual-current device - Wikipedia, the free encyclopedia

 

[netblindpaul]

Quite clicon,
Thing is if the d.c bus is going over voltage then we don't know how the drive is dissipating this.
It may have a bleeder system that leaks to earth by design.
This could be what is tripping the rcd.
I am not familiar with the internals of the drive that is being used.
I do know that some of the drives made by the company I worked for would bleed d.c. bus over voltage to earth via a bleeder resistor.
Depending upon the configuration of the drive this could be "seen" by the rcd as earth leakage i.e. imbalance.
If the drive was regenerative (as we called it) any surplus braking energy was converted back to a.c. and fed back into the mains.
We don't as yet know "why" the rcd is tripping on a "scientific" level.
That is what level the leakage is, at what point in the duty cycle it occurs, where the leakage is being generated from etc.
If it is the d.c. bus going over voltage and bleeding off to earth, by design and the RCD is "seeing" this as leakage and tripping, we don't know at what part of the cycle this is.
It could be at the start, it could be that the accel ramp is too steep, and that the conveyor is momentarily over speeding, and then being slowed by the drive, at which point the d.c. bus is going over voltage due to the braking energy.
Please remember some of these drives have very fast reaction times measured in fractions of a ms.
Ours had a base switching frequency of 8kHz.

 

[gerard]

I am going ordering a Merlin SI tomorrow. Meant to do itlast week but replacing a DOL 2 speed motor with an ordinary motor + Inverter on a troublesome machniw these days and am up the walls. Will have it soon

 

[clicon357]

I am going ordering a Merlin SI tomorrow. Meant to do itlast week but replacing a DOL 2 speed motor with an ordinary motor + Inverter on a troublesome machniw these days and am up the walls. Will have it soon

Be careful with the SI unit as the connections go from left to right, N/L1/L2/L3. I bloke I knew (alright it was me) once replaced one in a rush in semi-darkness one evening where the original, ordinary one went L1/L2/L3/N. Things went bang, smoke ensued........

Also if you wire the SI type with no neutral available then the test button doesn't work. You have to strap on a large resistor between neutral and one of the phases - I will dig out the Merlin sheet on it or post up a picture of one of mine in-situ.

 

[William Irons]

I have the same problem today i have two inverters both 3 phase in and 3 phase out they knock off a 300ma rcd as soon as they start so i isolated them from all sources of earthing invluding parallel paths and they work fine but there is a fault voltage between the disconnected earths on the inverters of 14v and the earthing system . I am thinking of making this an earth free zone in a plastic enclosure. Do I need a safe isolating transformer

 

[clicon357]

I have the same problem today i have two inverters both 3 phase in and 3 phase out they knock off a 300ma rcd as soon as they start so i isolated them from all sources of earthing invluding parallel paths and they work fine but there is a fault voltage between the disconnected earths on the inverters of 14v and the earthing system . I am thinking of making this an earth free zone in a plastic enclosure. Do I need a safe isolating transformer

Were they working OK and this is a new thing? What make are the inverters? This 14V, on both? Are you sure they're not just showing natural leakage to earth? Is it a constant or just on start up?

 

[William Irons]

I cant remember the make of the inverters but the voltage is constant when running. when the earths from both inverters are linked and measured between there and the main earth the voltage is 23 v. i am thinking i will set up a earth free equipotential zone inside a plastic adaptable box with all metal work inside at the same potential the cores of the sy cables are still connected to earth as normal but do i need to isolate the supply to the plastic enclosure containing the two isolated from earth inverters by means of a transformer there is no neutral entering the enclosure only the three phases . I am i suppose making an IT system or is this a double insulated system or is this a definately not. I have been reading the regs about this but im unsure any help on this matter would be apprieciated

 

[netblindpaul]

supermajor,
Why have you got an RCD in there in the first place?

 

[William Irons]

These inverters are on a new dairy farm running the vacuum pumps its a TT system they are supplied downstream of a 300ma rcd therefore seperating these completeley from a grounded earth seems like the answer to the problem. It acually refers to this in the instruction book the make of these inverters is emerson.Im really looking for advice if i need a safe isolating transformer to supply the box. sensible replys please

 

[netblindpaul]

OK,
So it's TT.
What is the earthing?
What is the Ra, what is the Zs at the origin of the installation?
What are the Zs's of the circuits?

 

[William Irons]

the earth electrode is 34 ohms and the zs readings are much lower .......parallel paths and structural steel 16 ohms

 

[UKMeterman]

Check which types of RCD are you using, as inverters can have a pulsating and static dc currents in the supply that can upset some RCDs. Ask the manufacture of the inverter for suggestions. Measure the current in the earth if it is excessive you may need to run the inverters off transformers. NB the star point of the secondary will need connecting to the earth terminal of the inverter, this is so the radio intereference filters can work.

 

[Engineer54]

the earth electrode is 34 ohms and the zs readings are much lower .......parallel paths and structural steel 16 ohms

That seems like a high Zs you have there, where bonded building structural steelwork is forming part of your Zs value!!!

 

[netblindpaul]

You have a single earth electrode for a commercial install?