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Arcing in MCC

Arcing in MCC

Arcing in MCC

(OP)
I've recently had an MCC tub arcing at the bolts which fasten the tub to the frame.  We found that the tub grounding system was quite poor so we added grounding jumpers to each tub.  We couldn't find anywhere on the tub, front or back, where there had been a phase to ground fault, and the upstream ground fault protection never tripped.

My question is: Could induced voltage in the tub have been enough to cause the arcing.  The arcing was quite severe at one of the two bolts, causing welding to the panel and black burn marks, along with black burn marks at approximately 10 other bolts between sections of the tub, and the tub and the frame and rails.  The tub contains a 600A switch and fuses which feed the MCC.  The max load on the MCC is two 200HP motors @ 600V, simultaneous start.  (Simultaneous start is now prevented)  The motors are on  hydraulic pumps, and the starts are relatively tame.

Thank You,
Gord

RE: Arcing in MCC

gordonl, did you say the the MCC bucket where the burn marks are was the main feed for the MCC?
Typically, these pull-out style buckets have a ground strap which connects first upon pushing in and disconnects last when it is pulled out. Who is the manufacturer?
I do not believe that induction caused your problem. The marks you describe are usually seen after a ground fault when there is not a solid ground connection between components, as you have surmised.

RE: Arcing in MCC

Hi GordonL

DanDel may be right if the burnt marks are as extensive as you mention.
The following thread 238-50453 had some interesting ideas on a possible similar problem.If induction is the problem
give the thread a read.
 

GusD

RE: Arcing in MCC

(OP)
Thanks for the comments, I'll follow the thread.  It's a CH MCC and has a ground strap that shorts to a metal plate attached to the rail, which in turn is attached by one or two screws to the frame.  All surfaces were pre-tapped and painted with no ground washers or paint removal.

A little added description of the burned bolt & system:
- There was arcing and splashing from the bolt over an area of about 1/4" by 1/4" on the tub, with black burn marks radiating out for approx 1".
- The other bolts didn't show arcing, only black burn marks for 1/4 - 1/2".
- The system is solidly grounded, about 500' away with two 3C teck cables with bonding conductor.  (Connected at both ends, I checked)
- The MCC also has a connection to the building ground.

RE: Arcing in MCC

gordonl, did you say the the MCC bucket where the burn marks are was the main feed for the MCC?

RE: Arcing in MCC

You experienced the classical "arcing-groud fault" phenomenon!

Presumably the system neutral is solidly grounded.
can you supply additional info, i.e., voltage?  Did fault occur during start?  Ground-fault protection parameters?

The phenomena is not unusual, and often occurs in switchgear systems having, as a sole ground connection, the drawer wheel/rail arrangement!

Several faults I investigated the fault occured where there was insufficient clearance between the MCC bolts, and the "tub" backup.

RE: Arcing in MCC

Question: Is upstream a ground fault protection device?
Also, an arc protection could be considered.

RE: Arcing in MCC

(OP)
The upstream ground fault device is mechanical 51 relay, recently cleaned, on a ZSCT.  It's a small MCC, I'm not sure arc protection is relevant.

RE: Arcing in MCC

Suggestion: Single pole arc protection circuit breakers are available to 20A.

RE: Arcing in MCC

Sorry, I should have said the "tub's" incoming breaker terminal bolts (not its mounting bolts) and the backplane of the "tub".  The arcing was not obvious without removing the breaker.

Regarding the 51.  Where is it installed?  What are its pickup and delay settings?  Did it operate?

In arcing-ground faults, the resultant rms current is too low to be "seen" by the 51.

RE: Arcing in MCC

(OP)
The ground relay is a CO-8 set at 200A primary pickup, 2TD.  It never picked up.  It is installed at the feeder in the sub approx 500' away.  Bonding connectors running in 3C Teck connected at both ends.  When the incident occured we were starting two 200HP motors.  The motors started fine.  If it was an arcing ground fault it self extinguished.  I suspect the motors may have still been spinning during the start.

I don't have breakers, I'm using disconnect switches and fuses.

MCC is 600V, solidly grounded.  

Would you get arcing ground faults without going phase - phase?

I couldn't find any arcing around switches or fuses, but I was looking through a clear plastic guard.  I would expect to find fairly obvious damage.

Thanks for your attention,
Gord

RE: Arcing in MCC

Suggestion: If the arcing is caused by a harmonic content, especially by the higher harmonics, than they will start heating and damaging insulation in locations where the conductor is closest to the ground through the conductor insulation

RE: Arcing in MCC

Gordonl,
You have a serious problem.  This is the classical case of an "ineffective" ground-return path!

You "tub" bonding fix is a step in the right direction.  Now you must correct the inability of the relaying to detect, and then operate quickly to clear the fault.  What is the "grounding" circuit path, between the MCC structure, and the transformer neutral?

Also, the 51 is a phase relay, and its setting is such that it won't detect the rms value of an arcing-ground fault.  The rms value could be as low as 5-10% of the theoretical phase-to-ground value.

RE: Arcing in MCC

JBartos, what do you mean by harmonics causing ground fault. I would think a ground fault would be caused by a breakdown between phase and ground.

RE: Arcing in MCC


Conductor movement/mechanical stress can be significant during motor starting.  In particular, the rattling of insulated conductors in conduits during inrush is sometimes quite memorable.

Is it possible that the ground conductor in the TECK cable has been incorrectly routed along with the feeder phase conductors through the zero-sequence CT feeding the CO-8?

Is there likelihood of near-simultaneous ø-ø and ø-ground faults, with part of the arc energy not seen by the 51 relay?
  

RE: Arcing in MCC

Busbar, you seem to be making three points. The rattling of conduits during motor starting is new to me and unexpected in my applications, but I am sure it exists in yours. But what are you thinking this has to do with the situation?

In the second, I interpret as possible ways a ground fault not being detected.

Can you clarify your third point of near-simultaneous ø-ø and ø-ground faults as I am not sure what you mean. A phase to phase fault would be more likely to be sustaining, but arcing ground fault may not be.



RE: Arcing in MCC


The point was to offer suggestions to gordonl’s situation that had not already been mentioned.  In an unenergized state, conductors positions and insulation may certainly have shifted, causing a fault that may not be immediately evident afterwards.  2L-G faults, among others, are considered in coordination studies, and have limited zero-sequence current; id est, arc energy may not have been 100% devoted to a ground-return path, in turn limiting 51 operation.

These certainly would not rate as top-of-the-list concerns, but shouldn’t be discounted if other possibilities had been ruled out.  [It’s the best I could do a thousand miles away from the incident.]
  

RE: Arcing in MCC

Suggestion to Modula2 (Electrical) Apr 19, 2003 marked ///\\\
JBartos, what do you mean by harmonics causing ground fault. I would think a ground fault would be caused by a breakdown between phase and ground.
///The higher harmonics are passing through conductor insulation that acts as a capacitive dielectricum. They heat the conductor insulation and may cause the insulation break thus forming arcing and ultimately ground fault.
Notice that the capacitive current Ic leaking through insulation is:
Ic=jx2xPIxfxV
where
PI is 3.14
f is frequency in Hz
V is voltage in volts from conductor to the ground
Therefore, the higher harmonic frequency f is, the higher Ic is, and higher cable insulation is.\\\

\\\

RE: Arcing in MCC

Suggestion: Sometimes, damaged conductor insulation may cause arcing. If conductors are installed, a good care must be taken not to damage their insulation over sharp edges, sharp bends by tools, etc.

RE: Arcing in MCC

(OP)
Shortstub

The grounding return path is quite good.  Approx #3awg. bonding conductor in teck cable solidly connected at both ends.  (Visually confirmed)

In my inspection I had a close look at the areas where the cables pass in and out of the bucket, and found no damage.

I had a CH Appl. Eng. come in and have a look, and his opinion was that the damage wasn't due to a 600V L-G fault due to a lack of damage.  I realize this assumes a good ground return path, but that seems to be the case, other than the connection from the MCC bucket to frame.

I haven't suspected a protection failure because the problem seemed to be transient.  When the event was witnessed, the motors completed their start up, and ran normally.

We have experirenced a severe arcing fault on this sub in the past.  The main incomming to the sub (which feeds this MCC) was busduct which had no ground conductor run in the bus duct enclosure.  The bus duct arced phase-enclosure for a signifigant period of time before going ph-ph and clearing the upstream protection.  (The transformer neutral ground protection failed to operate.)  I haven't visually checked the transformer neutral grounding to the sub ground, I'll do that my next time out.

RE: Arcing in MCC

Gordonl,
Where is the neutral CT installed?

1)  Is it between the transformer phase winding "wye" point and the neutral groundED conductor of a 4-wire system?  
2)  Is it between the neutral groundEd connection in 1) and the groundING conductor to the ground-plane or ground bus-plane?
3)  What is xfmr size?  

RE: Arcing in MCC

You should realize that your busduct enclosure is the only ground conductor required by code, and you should focus your ground checks on the continuity of that enclosure to ground.  That said, where does the other end of your #3 ground terminate at (hopefully, not a ground rod driven next to the MCC)?

Here's some suggestions:

1.  Check resistance of MCC to ground via the busduct.  Don't rely on visual inspections.  Anything over, say, 5 or 10 ohms should be improved.

2.  Check ground-neutral bond at the source panel or transformer.  Resistance/voltage should be very low.

3.  Check for ground currents at the MCC with a gaussmeter.

RE: Arcing in MCC

(OP)
Neutral CT in located in the neutral bushing, so between wye point and ground connection.  (3 wire system)

RE: Arcing in MCC

Hi Gordonl
You said that bonding should be OK now.The burnt bolts and consequent damage has been repaired.
I don't know how long ago you corrected this problem,
perhaps it is  too soon to claim victory, if there are no further signs of burnt bolts.
 As to your question "can induction" cause this problem? For the people that have experienced first hand the effects of induction problems with electrical
equipment,this problem is not that rare.
Electrical Systems are protected for most of the other possible faults,there are no protections for induction caused problems.

GusD

RE: Arcing in MCC

gordonl,
Now we're getting somewhere.  How is the #3 (did you mean #3/0 AWG) grounding cable routed to the CT?  And, the transformer size?  Your data indicates about 4 to 500 kVA.

peebee,
DC resistance won't do the job.  The ground-return path is an AC impedance.

An easy way to measure impedance is to supply the transformer primary with a low voltage source, say 600V, and run phase-to-phase and phase-to-ground current tests at the MCC.  The tests will yield the positive and zero-sequence impedances, Z1 and Zo, respectively.  Then the ratio of zero-sequence to positive sequence will indicate if the arc-fault phenomenon is still present.

RE: Arcing in MCC

Suggestion: The return ground path should be adequate for the projected current that is supposed to safely carry. This means that the proper installation of return ground path connections, especially through the paint and good two locknuts where applicable, should be implemented. The ground conductor should improve the ground path quality noticeably.

RE: Arcing in MCC

Good catch, shortstub.  I tend to use "resistance" and "impedance" somewhat interchangeably, more often than I should.  I would argue that if the resistance of his ground path is OK, then the total impedance is probably OK, too. But you're right that measuring the total impedance would be more accurate.

Either way, measuring resistance or impedance, would be better than an eyeball inspection.

RE: Arcing in MCC

(OP)
Transformer is 2.5MVA.  Wye point ground is about 70' long running to sub ground, as well as connected to ground grid. (Not run in tray with power cable)  The transformer neutral CT is in the bushing of the transformer.

The run to the MCC is two #3awg run in teck cable.  The ZSCT on the MCC feeder has no grounds running through it.

RE: Arcing in MCC

The addition of the internal #3AWG wire proved the inadequacy of the Bus-Duct enclosure as the "ground-return" path.

Now there is another inadequacy.  Based on the size of your transformer, I believe the #3AWG is not large enough to accomodate the potential ground-fault current!  The cable will certainly fuse if you are relying on a 51 relay for protection!

RE: Arcing in MCC

Shortstub, re:  "The addition of the internal #3AWG wire proved the inadequacy of the Bus-Duct enclosure as the "ground-return" path." -- I don't follow you there, please explain.

The #3 does sound undersized, though.

Gordonl:  Where is the other end of the #3 bonded to?  To a switchboard ground bus?


RE: Arcing in MCC

Peebee,
Gordonl's Apr 21 post mentioned a earlier severe arcing fault in the bus-duct of this sub.  This is indicative of an ineffective ground-return path.  Ground-current tends to flow in a path in close-proximity to the phase conductor.  Tarc-fault (presumably degree of damage) of the enclosursub.

Mathematically the Zo to Z1 ratio is very large, resulting in too small an equivalent rms current to be "seen" by the 51relay.   He said that the fault had to become ph-ph before the upstream (presumably the hi-voltage side of the transformer) responded.

Gordonl,
How is the #3 conductor routed to the transformer's neutral bushing?  Via magnetic or non-magnetic conduit?

RE: Arcing in MCC

Suggestion: The grounding conductors tend to be somewhat on the higher side or exaggerated, e.g. 4/0, 500MCM, and 1000MCM in substations.

RE: Arcing in MCC

(OP)
Bit of a misunderstanding, the transformer neutral connection is 500MCM connected to the ground grid in the transformer station, with a riser upto the substation ground bus.  The riser is run seperate from the new power cables which run up to the bus in tray.

The #3awg ground is used from the sub to the MCC, in the teck cable.  To runs in parallel, thus two #3awg in parallel to MCC in question.

RE: Arcing in MCC

Gordonl,

If the #3AWG wire is the groundING-conductor associated with the 3-wire bus-duct supplying the MCC, you still have a problem!  Is the teck cable containing the #3AWG conductor, routed so that it is in close proximity to the bus-duct?

The 500MCM groundING-conductor to the main sub grid, is ineffective in this situation.

RE: Arcing in MCC

(OP)
Let me start from scratch with the current system description.  A 2500kVA transformer feeds a sub approx 70' using power cable in tray.  (Used to be bus duct.)  Grounding from the transformer to the sub is 500MCM run seperate from the power tray.

From the sub to the MCC there is 2-3C-500MCM Teck with #3awg bonding cable.

All of the grounding and bonding conductors are connected to ground busses, columns, and grids.

So total ground return path is:
a)two #3awg ground conductors from MCC to 600V sub for about 250'
b)1 500MCM from sub ground to transformer neutral for about 70'
c)as well as parallel paths through columns, building ground loop, and earth.

RE: Arcing in MCC

So there's no busduct anywhere anymore?

RE: Arcing in MCC

(OP)
no

RE: Arcing in MCC

Gordonl,
You probably measured ground resistance, and are confident there is no problem.  Measure phase-to-ground impedance using an AC current.  I believe you will discover a much higher AC impedance!

But, nuff said from me!

RE: Arcing in MCC

Hello Gordonl

One could speculate forever as to the causes of your problem.History tell us, that similar problems are normally corrected by adequate bonding.If the problem persists after you have taken this first step,then you
may try the many suggested cures.
I would also make sure that my ground relays are properlly calibrated.

Good luck

GusD

RE: Arcing in MCC

Suggestion: There may have also been electromagnetic induction, harmonics, unbalance load, etc. or a combination of those. The monitoring of the ground current in the ground conductor could help to find the root cause.

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