VSD - grounding DC buss before opening cabinet?

I'm designing a system that includes some 40 HP variable speed drives. Customer wants ABB ACS800 series and they have also written in the spec, "A safety-grounding switch shall be included to guarantee personnel safety when VSD system maintenance is to be performed. All switchgear doors shall be interlocked with the safety grounding switch to assure that power has been removed and that all stored energy (e.g. the DC link capacitor banks) has been discharged before having access to the equipment. When closed, the safety-grounding switch shall connect all the buses of the DC link to ground."

Has anybody ever had to do something like this before? I have asked my local ABB distributor and rep if this is possible, but I still don't even know if the capacitors are accessible enough to tap into. After two weeks of pestering, my only response so far is that this is a very dangerous portion of the drive (duh, that's why the customer included this in the spec) and that the rep has to contact some regional engineer at ABB.

I'm not an expert on this by any means, but...

When you ground a capacitor, don't you expect at best a big spark - or at worst an explosion!!

Most current drives have bleed-off circuits that drain the capacitors safely within just a few seconds.

You might ask the customer exactly what they are specifying -- do they have any examples of how they've done this in the past?

Ozee,

I don't disagree. It could be grounded through some appropriate high-wattage resistors to eliminate the big arc/explosion that you refer to.

Communication with the customer is difficult as there are is a signification language barrier. I'm hoping to get as much feedback from ABB and other knowledgable people before discussing the issue with the customer.

My other thought was to monitor the voltage on the caps and physically keep the door locked shut until the voltage is below some safe level.

Most of your VFD/VSD drives have terminals that are connected to the + and - DC bus. This is usually for connecting a remote bus burner. ALL VFD's with a large bus cap bank have bleed off resistors. There is an indicating lamp of some sort built into the drive to show the caps are charged. Typically I have found the Lamp goes out about 75 VDC. Still enough to energy to knock an unsuspecting tech on their buttocks.

For this "supposed safety" circuit to function, you would first need to discharge the cap bank so that you do not cause a rather spectacular pyro-technics display to occur. After the caps are discharged, then you could ground the DC bus to establish the desired "safe" condition they seem to be looking for.

IF you just ground the DC bus when power is initially removed, you will dramatically shorten the life of the caps, and drive.

This sound like it needs to be posted in the "knee jerk saftey thread".

IMO, Who ever wrote this has little or no knowledge of AC drives or was used to DC drives with out cap banks and stuck their digits in where they did not belong.

Sounds like they are trying to comply with NFPA-70E
Arc-Flash protection.

i edited my question

The only arc flash that occurs after powering down a VFD happens when some body tries to rapid discharge the cap bank. Each drive has built in discharge capability and indication of bus power. Once the bus power lamp goes out, usually a couple of light bulbs will finish the discharge process with out arcing/flashing/pyro-technique displays. The discharge resistors are large enough in ohmic value to minimize heating. In some, they are also the balance resistors across serie connected caps. One simple solution would be to time the discharge of the drives in question and then have a timer set to announce when the drives are discharged. Then personnel could open the doors and access the equipment.

With this bus shorting system, I am willing to bet the mfgrs void the waranties. Unless they have their own shorting/grounding system. I have not seen one.

ABB usually marks this with UC+ and UC-. These terminals are as accessible as the line supply (U1,V1,W1) and motor output (U2, V2, W2). This is for connecting braking resistors as I believe the chopper is internal or for common bussing to other drives.

When the power to the drive is disonnected, the DC BUSS will discharge. Most of the ABB literature I've seen says to wait 15 minutes before doing any maintenance. If you put a meter on these terminals you will see the buss discharge within 2 to 3 minutes on the size drive you are dealing with. The larger the drive, the longer the discharge.

Grounding the buss should only be done after it has fully discharged (of course). I don't think it is intended to use high wattage resistors to accomplish this (unless there was some unimaginable reason to "dive" into changing out an AMC board or something on the drive in a rush).

Grounding the buss would also cause a drive fault rather quickly should the power accidently be turned on while the maintenance is still ongoing. You certainly don't want to be in contact with 650 VDC at any time.

Also: If you have several drives common bussed then it won't matter if the AC power is off to some of them. Every drive's buss will be at that dangerous DC level as they are all connected in parallel, so this would be another reason to ground the "link" as it is referred.

One time I saw what happens when the DC- terminal was grounded on an energized drive (The motor ground wire was mis-landed). It wasn't pretty.

Fixing it involved replacing the drive and the breaker that fed it.

That DC- terminal is floating at line voltage potential.

What a bizarre idea... I personally avoid shorting 300-700 volt circuits to ground.

I think the intent is purely preventive. That is, this grounding of the DC BUSS is to be done when power is off and BUSS voltage is zero. When the ground switch is engaged, it protects workers against an accidental power on. It is not meant to discharge the buss or "help" discharge the buss.

IMO anyone who has any business going into the panel to work on the drive should know that VFD's have capacitors in them that need to discharge after the power is shut off.

In high voltage application grounds are put on in case of accidental re energization of a circuit. In a drive cabinet with 40HP VFD's there should be no chance of accidental re energization if one of the supply breakers is locked out. Does the customer want you to have an automatic grounding system for the incoming line as well!!?

Most drives have a warning sticker on them about waiting for the Caps to discharge. You could suggest to the customer putting big labels on the panel saying something like "VFD DC BUS CAPACITORS MUST DISCHARGE AFTER DISCONNECTING MAIN POWER, WAIT 15 MINUTES AFTER SHUTTING OFF MAIN BEFORE OPENING CABINET DOORS."

If this is a new customer I would try and talk to some of their existing suppliers/contractors. It may turn out that the safety policies of the company are ridiculous enough that it isn't worth the effort to do business with them. I had a few contrators from my previous employer tell me that they didn't want to work at our plant anymore because of the dumb safety policies they were forced to adhere to. I couldn't argue with them, a lot of them were dumb.

We had a machine with the same philosophy (it was an induction furnace) when you opened any door of the panel it would dissipate the energy stored on the DC buss via a resistor. This was purely to protect anyone working inside the panel.