You have a substation converting 13800 KVAC to 480 VAC.

A three phase supply breaker from that substation is set to trip at 800 Amps and is connected to a Motor Control Center Main Breaker also rated 800 Amps by parallel 500 MCM THHN cables per phase.

Four Electricians responding to multiple calls of malfunctioning equipment find both the MCC Main Breaker {MCC-MCB} and the Substation Supply Breaker {SUB-SCB} tripped.

Assume they follow all relevant codes for PPE.

To troubleshoot and then repair the failure of one of the connecting 500 MCM Conductors when and how must they Lockout the SUB-SCB.

Keep in mind you'll be trying to find why the MCC-MCB is tripping and quite possibly need to reset the SUB-SCB and MCC-MCB multiple times.

Code references are appreciated.

Does the plant have control over the 13 KV side or is that utility only?
IF plant has control
what is disconnecting means.
Does plant have ability to ground and bond hi side of transformer?

Is the MCC only load served ie is there only one breaker on secondary or does secondary serve a switchboard with multiple breakers?
Is the transformer secondary to MCC breaker a rackable and removable breaker.
Is MCC breaker rackable?
Are these remote operate breakers and do they have control power fuses in a separate cabinet?
Do I have access to Multi Amp?
Do both breakers have any type of fault monitoring or trip indicators?
Do I have luxury of spare breakers?



Dan Bentler

YES - 13.8 KV Disconnect Switch at input to the substation {note it is one of nine on substations site} We also can disconnect from Utility at Main Switch House if Necessary.



Don't have specifics by Name and Type just know it as a Vacuum Disconnect I think.



YES we do this annually at all nine substations for Annual PM.



There are 12 breakers at 1200A each on the secondary of this substation transformer. If I recall correctly.



Definitely would you install any other type these days at a substation?



No Such Luck it's twin 500 MCM's per phase Line Side and Bus bar Load Side.



No Remote Operation as I would call it. You gotta stand beside them to activate the charging handle and/or reset a trip.


MultiAmp? That term is foreign to me sorrry.


Yes and Yes



Yes you have spares at the Substation but not without butt splices or new wire.
Two Week Lead on a spare for the MCC.

Before going there, how is the coordination configured? Under what configuration can you possibly have both breakers tripped? You specifically mentioned NFPA 70E and there's no way the useless chart can be covering the equipment you are talking about. So you must have done a coordination study first as part of the prep work for an arc flash study. If there are no coordination issues, then it indicates that something strange happened such as a bus bar or cable cutting loose and bridging from one bus to another or some other equally ugly scenario. If you do have a coordination issue, then this gives you more information plus expands the area that you need to look at for problems.

Start meggering everything. Something is shorted either phase-to-phase or phase-to-ground. No reason to energize anything until you have determined what is causing the problem. In fact, re-energizing the circuit breaker is a Code violation and an OSHA violation. You are not allowed to reset circuit breakers and fuses until you identify what caused them to trip and make repairs or adjustments to prevent it from happening again before re-energizing.

So the last line about resetting things multiple times is false and shows a Code violation when it came to doing the troubleshooting.

Now it is quite common for someone to just automatically reset a circuit breaker because motor overloads are so common and usually this problem is taken care of automatically when the overload is tripped out. However, in this case, you are looking at either a loss of coordination issue or a bus or feeder problem. The second common screw up is to go around opening up all kinds of disconnects right away and using this as a means of discovering where the problem lies. Again, really bad idea in this case. First thing to do is to check the downstream side for ground and/or line-to-line faults. Then begin opening disconnects or circuit breakers and rechecking until you isolate the problem. During this time, there is no reason to be reclosing the circuit breaker that tripped. I know that during this troubleshooting exercise that production is going to be breathing down your neck to get everything back up and running but you will get everything re-energized faster by doing it right the first time and not wasting time getting everyone out of the way, taking arc flash risks in a known major fault scenario, and causing further damage to the equipment (and shortening circuit breaker life) by repeatedly reclosing onto a fault. Leaving everything down until the fault is isolated manually will result in getting everything powered back up faster in the long run.

As to repairing a 500 MCM cable, we have a cable which has already tripped the circuit breakers. So you need to be locking them out, even if this means racking them out or some other means of disabling them so that the 500 MCM can't be re-energized while it is being repaired. Newer equipment has provisions for this. Sometimes on older equipment you are stuck with tagging it out which is specifically mentioned only for this problem (no effective lockout point). Finally, before beginning repairs, if there is a possibility of stored energy (capacitors) then you need to attach grounds. Utilities usually do this automatically as an added layer of protection if someone does try to reclose a circuit breaker unexpectedly.

Keep in mind that if you can't de-energize any suspect areas, you may end up stuck asking the utility to drop a pole fuse or other disconnect at their end before you can begin work. I've had to do this more than once with a substation because the main incoming air switches in that substation were the old rotary kind and had burned a switch out, even welding a phase in once. We couldn't touch it without disconnecting power to it which meant we needed utility support.

As always Paul you're correct. We do have a full NFPA coordiantion study and something ugly did happen. Insulation on Phase Conductor B1 failed and shorted to the conduit wall at an LB.



DOOH - Electricians did a full visual inspection of all readily accesible wiring. And finding nothing attempted a power up of the Sub-Station breaker, then brought out the megger. In their defense they did don Level 3 PPE Per the NFPA Posting and maintain approach distance clearance from the powered end during the breaker re-energization.



Full repairs will involve replacing all 6 - 500 MCM conductors this weekend when things are locked out.

OK Bob

First learned trade as nuc submarine electrician and I have experience as a utility electrician at Trojan Nuclear Plant in Or.
I also have 20 years in field as safety and industrial hygiene.
Mulit amp is large circuit breaker tester we used ot test up to 12.5 Kv breakers been a few years 250 or 300 KA fault currents??.
I dearly love rackable breakers.

OK so here is what I know
1. Both the breaker at the sub and input at MCC tripped
2. Both are rated at 800 amp.
3. Cable between two is paralleled 500 MCM.
4. One set of 3 (all three phases is in conduit A
Other set of 3 is in conduit B
5. I have a bad feeder conductor and this is in conduit B (blew hole in conduit)

SCOPE OF WORK
1. Rack out and inspect substation breaker - ship out for testing and trip calibration on multi amp
2. Pull MCC breaker and inspect - ship out for testing and trip calibration on multi amp
3. Pull out cable
4. Clean conduit repair as needed
5. pull in new.
6 Take resistance checks and meggar all record all readings.
7 Install breakers
8. Take another set of readings and meg all.
9. Verify that 500 MCM and conductor type is still good choice

Normally in most plants this would be jobbed out - they do not have the gear to do this pull.
I assume you do have the gear and the competent crew.

SUBSTATION BREAKER
I assume no tie breaker.
Since there may be a fault I will be ultraconservative - weekend job.
You said no control power fuses.
Shut down machinery served by sub
Open all breakers in sub (and at MCCs as needed)
Kill the sub by opening high side disconnect.
All workers lock and tag disconnect operator
Check with appropriate hot stick and gloves deenergized.
Set ground and bonds using appropriate hot stick and gloves.
Check to ensure adequate grounded and continuity of ground path.
Rack and remove breaker feeding MCC - do visual inspection.
Disconnect load conductors in conduit B. Tape and get out of way of all to be energized buss work
Verify conduit B conductors are the bad ones
Disconnect load conductors in conduit A. do resistance and meggar. Tape as above
Check and mark conductors in A for proper phasing at both ends
I would give some though about grounding and bonding all the 500 MCM for conduits A and B
Insstall rubber sheeting to protect live components from contact when further work done inside
Meggar transformer secondary and buss work in switchgear
Replace panels
Either danger tag the door or if you can lock and tag it (supervisor lock and tag may suffice)
Remove grounds and bonds (stick n gloves)
Remove locks and reshut hi side disconnect
Double check MCC is still dead.
Reenergize all equpment and restart machinery.
Ship out breaker

MCC
Verify still dead and A n B condustors still bonded n grounded
Remove input breaker inspect and ship out
Ground n bond conduit A conductors
Setup gear to pull conductor out of B. Makn it easy for me will pull from MCC
Open cabinet back at sub. Rope off area no entry signs etc.
Verify no hot exposed stuff insulating blankest are properly places.
OR do all work at SUB on weekend and deenergized as above.
Attach messenger rope to one of conductors on B
Start pull
Verify all 3 conductors are being pulled from B.
Pull out all 3 (4??)
Repair conduit B as needed - retie messenger
Pull clean swab thru conduit B
Pull in main pull rope
Attach new conductors and pull in.

Brain dead will finish later

Gotta remember...you are quoting the trip point of the breakers at what, 800 A? But what's the withstand voltage? For that matter, what's the available fault current?

Usually these are specified in kA (thousands of amps), well above the 800 A trip point.

The force on a bus bar in Newtons in a fully offset assymetrical peak (short circuit) current is Fmax=L*0.75*(2*I^2/s * 10^-4)
where L is the length, I is the current in Amperes, and s is the spacing in millimeters.

The important point is the I^2 term...so if you went from say 200 A under normal conditions to 20,000 A in a short circuit scenario, the current went up 100 times but the force on the bus bars or cabling went up 10,000 times.

That's the reason that bus bar support structures are so massive and the reason that your cabling gets flung apart and usually "explodes" whenever the cable undergoes even a simple chafing event that leads to a short to ground.

Energy released in this event isn't too pretty either. But hey, you've got flash suits. So what if the equipment blows itself apart and the pieces on an already compromised system come flying out?

Everybody please contimue to contribute I am learning a lot.

Just to update - Repairs have been jobbed out.

Replacing all 6 current carrying MCM 500 wires and associated grounds.

Breakers Check and test good.

Visual Inspection of Buss Bars and anciallary hardware finds nothing.

IR Camera will monitor after equipment is in use again for a day ot two.

1. The fault was between the SUB and the MCC breakers - uhhh was it?
2. Fault current never passed thru MCC breaker granted there could have been fault current in the good set of conductors but that should have "stopped" at MCC input terminals / bus work.

In theory only the SUB breaker should have tripped on overcurrent.

What I am curious about is
Did the MCC breaker really trip on overcurrent
OR did it trip because of loss of voltage on one leg
OR did it trip because of loss of control power
Is there any way to determine true cause of MCC breaker tripping?

Did you have the breakers trip set points calibrated?

Dan Bentler

Yes - Conduit #2 Phase Conductor "B" - Insulation Failure near an LB



Phase Current passing through ground most likely tripped the MCC Breaker



I would suspect a couple of possibles. Loss of Phase Trip and Ground Current Trip

OK, so you suspect line-ground fault. I concur based on your description. Based on the force involved it also sounds like you've got a solid-grounded delta-wye system. For future reference you may want to consider a high resistance ground. I think NEC now sort of pushes you in that direction for anything over 1000 A anyways which it sounds like you are either at or close to. They are not really that complicated to deal with and have the advantage that line-ground faults do not exceed 25-50 A. The downside is that you have to add an extra protective relay or two, and a resistor. But the cost isn't that much compared to what you typically see with MCC/MVC gear overall.



Loss of phase with a motor circuit (I'm assuming motor circuit here...correct me if I'm wrong) implies that the motor will then start returning energy through the missing phase leg, "single phasing". Current in this leg temporarily increases to roughly twice normal (actually 1.73 times) until the overload trips it out. Instantaneous trips usually need not apply in this case.



Whoa...OK, that's also important to know. It is definitely POSSIBLE that the breakers were off but doubtful. If you bought new, I hope you bought electronic trip units because they are much more reliable and generally speaking, calibration is simply not a factor. The various heating/temperature problems go away, too. Based on the size you bought, I'd default to vaccuum circuit breakers too because they will be smaller and immune to those things that shouldn't be inside the compartment (dust). Circuit breaker life is longer too...usually the limiting factor on life shifts from the contact tips to the mechanical components.

There is still clearly a coordination problem here so you really should be looking really closely at your coordination curves and getting this straightened out. If this is after the fact and you find you have a very difficult coordination problem and can't live with it (sounds like this is one of them), you can contact Utility Relay Company (they have a web site) and buy a rebuild kit for about $2500-$3500 for low voltage draw out circuit breakers. This rebuild kit will convert existing draw-out circuit breakers from mechanical to electronic trip units and also includes all the parts to rebuild/overhaul old units. If it's a molded case type, then your only choice will be replacement. Check out the electronic circuit breakers, often with current limiting functions, available from all the major manufacturers. They are surprisingly inexpensive, often cheaper than the old standard thermal/magnetic types at the sizes you are looking at. If it's a medium voltage unit with a separate protective relay, then just buy an upgraded relay. Often they will fit right in the exact same slot as the old relay. These newer relays add LOTS of new settings and you can trivially tune your coordination curves to avoid just about any kind of coordination problem with the possible exception of the notorious problems with mixed fuses and circuit breakers (hard to match a circuit breaker to a fuse curve and vice versa).

An additional advantage is that you can probably drop your arc flash ratings down to near 1. If you really want to do something about that in particular, check out VAMP or ABB. Both sell after-market fiber optic trip units that can be retrofitted onto existing switch gear to protect your high power buses. The result is often that you can take even a hazard class 4 unit down to 0 or 1. They are relatively inexpensive (<$10K) because it consists of just an unclad fiber that you fish through all the compartments, a sensing relay, a second low burden relay that you connect to one of your CT's on the front end, and a trigger relay to trip the main CB when an arc flash event occurs, all in under 10 ms. The limiting factor on speed turns into the speed that your primary CB can open (2 to 5 cycles), not the speed of the "instantaneous" trip units.

Since you've only had roughly 2 months of run life, my biggest suspicion is installation error on cable pulling. With 500 MCM cable, if you don't make sure to put in the proper number of pulling boxes, debur every single conduit, cut with the proper threading head (NSPT, not the more common NPT), put plastic bushings on the exposed threaded ends, use a proper pulling eye, and liberally use lots and lots of cable pulling compound, your chances of nicking or stretching/damaging a cable go way up. There's a right way to pull 500 MCM conductors which can be done without causing mechanical damage. Done right, you will never have a failure like what you described. I usually don't spend a lot of time hanging around the site when cable pulling is being done because there's really not much to see. But for the large conductors and for proper terminations on 5kV and higher cabling, I don't make a big show of it but this is one time to spend ALL your time on the site monitoring the activities. 5kV and higher cable is supposed to be insulation tested (hi pot) specifically to detect any problems such as this. I'd suggest you consider hi pot testing it now while it's down in case you find another one. I'm a very anti-hi pot guy except on new installations and this is probably close enough to be considered new. If the cable wasn't so new then I'd recommend meggering it which at least isn't a potentially destructive test. I'd let the installation crew know just how disappointed I am too but since there's no way at this point to detect where/when the screw up occurred and you didn't test the cabling, it is impossible to go after them contractually.

Note that I only recommend hi pot testing on NEW cable when installed. If you attempt to hi pot test once cable has been in service more than likely it will test bad. AND worse yet, hi pot testing on used equipment actually CAUSES insulation damage, becoming a self-fulfilling prophecy. Also, hi pot testing on transformer bushings tends to be very vague on results and often "fails" on medium voltage transformer bushings, so you sort of have to "interpret" the readings a bit. The hi pot testing literature is rife with descriptions of this.

This is a little off topic, but what determines what arc flash rating a panel or switchgear is? Is there some kind of formula that you can use to determine this or is it just a set of arbitrary rules that determine it?

Duff - It seems like "black magic" for a poor maintenance guy when your Plant Electrical Engineer first whips out the One Line Electrical Drawing for your Plant with all the Arc Flash Ratings notated on it. But it turns out there is a very well reasoned process behind Arc Flash Rating. It deals with Available Energy to cause an Arc Flash and Duration of it's availability {think I squared T or instantaneous current over a time}. Paul can probably give use a ton of information more if you want to start a post on that topic be my guest.

It's actually quite easy to do for your typical electrical engineer. My uncle is a nuclear plant operator. He once told me that they have lots of electrical engineers running around nuke plants. He said that none of them can even fix a good pot of coffee but they sure can integrate those equations. I promise you though that there are no integrals to solve, only algebra.

The key is indeed the amount of energy you are exposed to. It depends on the distance you are away from the panel (there is a standard for this), the distance between the phases, the voltage, the fault current, and the time involved before the fuse or circuit breaker opens to clear the fault. Obviously depending on the fuse/circuit breaker curves, you may have to run the calculation multiple times to find the worse case scenario. There is also clearly a difference between standing in front of an open panel in a cubicle (like a typical MCC) and in an open air situation like an overhead substation because the cubicle tends to direct the blast towards the victim. Arc-in-a-box however hasn't actually been tested for all scenarios so you can't get accurate results in all cases. In addition, you are required to run two scenarios; using the full bolted fault 3-phase current as well as 85% of that value. The 85% factor comes into play because arcs are not perfect conductors. It usually varies between about 70% and 90% of the bolted fault case, and the variation mostly has to do with the voltage. So 85% is a good compromise.

There are two procedures for doing the calculation. The original procedure is in IEEE 1584. The other one is in NFPA 70E. You can view NFPA 70E online for free. Head on over to www.nfpa.org. Select "Codes and Standards". Select "List of NFPA Codes and Standards". Scroll down to 70E. Click "View the 2009 edition"...then just follow the messages until you can see it on screen. Clicking on the "table of contents" thing lets you click on sections to browse around quickly. You can't "print" but you can screen capture if you need to. You can also view NFPA 70 (NEC Code) this way in a pinch if you don't have a copy nearby.

You will need to be able to use a coordination curve to figure out the trip time. Head on over to www.bussmann.com and download their SPD handbook. It does try to denigrate the competition (circuit breakers) but it explains coordination curves in basic layman's terms. These curves just show time vs. current so that you can figure out at a given current how long it takes for a fuse or circuit breaker to trip. One additional piece of information you will need to calculate is the impedance of the circuit including the transformer and wiring because you will calculate a 3-phase bolted fault by assuming a short circuit and just calculating the current by Ohm's law (V=IR). I believe the Bussmann book covers calculating impedances for short circuit fault calculations, too.

There is a section in NFPA 70E which gives you tables which are supposed to work if you don't know anything...trouble is that these tables are self-contradictory and give very incorrect and misleading information. They often give you grossly overrated and often underrated numbers. My opinion on the tables is to use to ignore them. The equations aren't fun but not impossible to use either. Every review committee threatens to take it out because there is universal dislike/distrust but since nobody has a reasonable alternative, the tables have not been removed.

What you end up with is the number of calories of energy per square centimeter of exposed electrician. A calorie is the amount of energy needed to heat 1 cubic centimeter (one milliliter) of water 1 degree Celcius. It turns out that the amount of heat needed to cause a second degree burn is 1.2 cal/cm^2. So NFPA set this as the target in the NFPA Code...we will accept a worst case scenario of 1st degree burns or less.

You can do this by hand for one or two locations. You can even use a spreadsheet to do maybe a dozen or so. But for most plants, the amount of labor involved in grinding through the math is incredibly time consuming. There are computer programs that do it automatically. SKM is the most popular and has the most number of parts preprogrammed into it's libraries but it is the most expensive. ETap and Easypower are two competitors that have much more reasonable price tags. These days, it is becoming almost a requirement that every plant has a copy of one of these programs and makes regular updates to the plant single line diagram in the software.

Excellent comments Paul - Not dissappounted as usual.

I use the NECPLUS.Org a lot but then my boss pays teh $9/month for full access and search capabilities.

Yes, thank you Paul. The amount of knowledge you have is overwhelming. I will have to look over this information more in depth when I get time. I'm interested in learning more about this since our company has started doing studies on it. I have too many things I'm trying to learn about and not enough time! :-P