Arc Flash labels on transformers and line side of secondary overcurrent

Part of my distribution system shows a 5KV fuse protecting a 500KVA, 3ph 4160V-480V. It's secondary is connected to a switchboard 480V main breaker. In order to have decent coordination, the arc flash level on the secondary of the xfmr is very high. This also translates to a very high level on the line side of the breaker. Two questions:

- Is it normal to have high arc flash on the secondary of a transformer of this size and is it required to place a label on this equipment? I'm thinking in reality - why would anyone need to work on the secondary of a transformer while it is energized?
- How do I tag the switchboard? Is it based on the main bkr line side arc flash value or load side/ bus value?

For a switchboard it would depend on the compartment design. You have to consider the line side of the main when determining AF values. Some switchboards have open communication between the main breaker and the branch breakers, if that is your case, then the branch breaker area is the same as the main. If there is a seperate metal compartment, and only a small open area around insulated bus, then I would say you could use the load side of the main to calc the branch breaker side.

With that said, it sounds like you have some coordination capability. You should check and try to set the breakers as low as possible but still give coordination. I mean you don't need a big gap between the deadband of the branch and the main, as long as they don't touch.

Thanks Haze. Any thoughts on the transformer question?

That is a normal problem, pretty rare that anyone would need to access that part but the same label would apply to your main breaker. In my experience if there is a doubt it is better to label it, CYA.

You should label both sides of the transformer compartments. The secondary side of a transformer is often accessed for oil analysis sampling. It is often not possible to shut the transformer off to sample and companies like SD Meyers will sample live. While not routine, there can be troubleshooting that needs to be performed at the secondary, like setting tap voltage, or verifying phase rotation.

One thing to look for to lower Ia is to use fuses with a fast time response curve and don't set them too high. Unless I know I have some very big motors starting with a lot of in rush current, I will routinely protect my building transformers at around 130% to 150%. If all you have is lights and a bunch of small motors then you can go with a fast fuse curve and 130-150%. If you have some or one large motor, then you model the transformer at some predetermined load, ie-70% and then add in the in rush current on the motor. Convert this to 4160V and then compare to the primary fuse. Make sure you have about 20% safety margin minimum. If you are feeding with a medium voltage breaker you can pretty much tune the trip curve to anything you want.

The primary side of the transformer will probably be a lower IE than the secondary. If each side is in its own compartment with a door that latchs you can use different values on the IE label. If they have a common door, or as in most padmounts the primary side can not be opened unless the secondary side is opened, then put both sides the same as the secondary (or whichever side is highest). IE labelling does NOT have to match the actual calculation at that point. It just has to be equal or higher than the calculation. The purpose of the IE number is to determine the proper PPE. Think in terms of having the electrician be able to know what PPE is needed, and don't get caught up with the need to post the scientific number calculated.

A point of caution. When I switch on an oil filled transformer that has been deenergized, no one is permitted within a 60ft circle. If an oil transformer were to short and exploded, it would be quite a fireball, and energizing after repair is the most dangerous time. If you have loop fed and you have to have an electrician with a shotgun in the primary cabinet changing feeder switches, then he is in HRC4. At that point I am not concerned about the IE I am concerned about a deflagration that sprays him with burning oil. I typically handle this in training, and also require a Live Work Permit for all medium voltage work including switching.

Haze, I'm using a CL fuse to protect a 4160V xfmr. The fuse is above the inrush point and below the damage curves for the xfmr. Is it safe to say that with this technique, the transformer, under short circuit conditions, will not explode and spray burning oil?

My initial concern is the very high arc flash. It is above HRC4.

Also during use of chart recorder etc.

I think we need to be careful and continue to use our minds in all aspects of our work. To some degree Arc Flash regulations are boiler plate rules in an attempt to prevent the inexperience and untrained from being hurt by their own action. Yes it also protects the trained and knowledgeable when the equipment itself is deficient or faulty, but for the most part human error causes the majority of AF accidents. I say this tongue in cheek because this is just my opinion and I have no data to back it up.

I'm sure someone will correct me where I go astray, but I remember the first Arc Flash guidelines being limited to the energizing of oil filled equipment. I've never personally had a transformer explode but I have seen videos of some that have and it was a real fireball. I have had transformers damaged mechanically and seen oil spray 20 feet into the air when a radiator tube was punctured. So I see the potential of what could happen by association.

I am particulary cautious when energizing any oil filled equipment, although de-energizing it or working on it while energized does not concern me nearly as much. Something that is working normally will probably continue to work normally for the short time I am near it. But whenever work if performed on de-energized equipment I am always cautious when energizing. Seems that most human mistakes are made by people, and a cardinal rule of maintenance is that if it worked previously but is now broken, go back to the last repair made. So when energizing, I keep clear a 60 ft radius around the transformer, and if an electrician is needed within that circle because he has to throw a loop switch at the transformer, then he is in HRC4.

Now just to make my philosophy clear, if the secondary side of the transformer came out at 7.9 cals, I'd only require the electrician to be in HRC2 for tasks like voltage testings, installing a chart recorder, or taking an oil sample. The normal probability of the transformer continuing to be normal exists while these tasks are being performed. Just having the doors open, or just being near the transformer, is not the risk. Think about all the 6 year old kids sitting on top of the padmount transformer at the library. Is there really a high probability that the transformer will just spontaneously explode?

If a transformer is loop fed, that means I have to 'make' two circuit before I 'break' from one. Did the person before know how to properly 'phase' a transformer? I for one will never trust another persons phasing until I prove it myself. Could the electrician have messed up and installed a ground strap to X1 instead of X0? Mistakes happen, especially during construction or repair.

Now what about primary protection and the damage curve? Under proper conditions protection that protects below the transformer damage curve should protect the transformer from any physical damage. Without the physical damage, how would you generate the arc that ignites the oil? Well, in truth oil filled gear is very rugged, safe, and almost never explodes. In my heart I must be a gambler because I am constantly assessing the odds. I think nothing of changing a 120V ballast in a light fixture hot, but shotgun in hand throwing a loop switch on an oil transformer finds me in my HRC4. To me its a question of the probability of being hurt, and the severity of that injury. Many will argue, and rightfully so, that me changing the ballast hot will have a high probability of me being shocked or hurt, and I agree they are right -BUT- how badly hurt will I really be? Disfigured for life with amputated limbs, or, shaking off a bee sting. The probability that the oil filled transformer will explode that moment in time when I throw that switch, and that the overcurrent protection, or something internal to the transformer will fail even though its within the fault curve - is extremely low. The consequence that it does happen, as rare as it may be, is that I am disfigured for life and have amputated limbs. As a kid I traded punches in the arm with friends until someone cried 'uncle'. I can deal with the pain of the former but not the later.

Yes, fuses are safer in my opinion than breakers, and breakers almost always work as designed, and the manufacturers publishes a fault damage curve I can work to - but - I am not willing to trust my life to that gear, or that equipment, or to that factory worker who soldered the cable to the bus - in spite of his wife informing him the night before that she filed for a divorce.

We all develope a philosophy of work as we grow in our careers, some will surely disagree, but this is mine.

Why would anyone expect HRC4 to provide protection from burning oil? Tested to not ignite and continue to burn in the presence of an arc when clean, I would assume that when coated with burning oil the flames will persist until the fuel runs out or are otherwise extinguished.

HRC4 is NOT flame proof. I never said it was. But you miss the point. The reasonable person does not stand in the middle of flaming oil shouting to the world that 'it doesn't hurt!'. I am much better off running with my HRC4 in flames, and getting out of the flames, and then dropping and rolling, and being the range of help from others than I would be in my HRC2.

I don't know of any garment specific options for working on oil filled gear other than those I outlined. You can buy suits up to 140 cals, and even aluminized reflected suits. You can get fire shields and have a fire truck standing by. Its all a matter of how far you want to take something. For me, reasonable risks require reasonable measures. You can not prevent all injury for all humans everywhere. If the work risk is no greater than the risk the person took driving his car to work, then I think it acceptable.

If its dangerous, limit the exposure to the least number of people, and give those nearest the danger the best protection you have. I feel very comfortable and confident that my HRC4 will get me out of the incident alive and in reasonable health.

There are others on here that have more utility switchgear experience, maybe they can chime in.

And the basis for this statement is? I am still having difficulty applying any correlation between the way the garments in question are tested to the situation described. Seems to me that a garment that more readily soaks up oil might result in a more serious injury than a lower HRC rated garment that does not.

HRC4 has a full face hood and is double layer. The outer layer is often the same as the lower HRC fabrics. So both would absorb depending on material but one is thin and the other thick. Oil is only going to burn on the surface, not in the middle of the cloth as it wouldn't have oxygen. So you stand in your HRC2 coverall and faceshield, and I in my HRC4. Neither has been tested for the specific risk, its a judgement call.

The damage curve is for through faults (on secondary of the transformer), not for faults inside the transformer.

WHump,
My take on this subject.

I find in most if not all cases the secondary of any such power distribution has an extreme danger level between the secondary and the switchgear line side breaker or fuse.

This is very extreme hazard if the transformer and upstream protective device are separated by a good distance >20 from each other using busway.

About 10 years ago I had 480v- 4000A busway exterior of the building short out arc and burn for several minutes >10 before the 12.47kv fuse cleared. The fuse finally blew right before electrical personnel arrived to pull the switch blades. It was at night and the engineering manager was called. When the EM left his house 3 miles away he could see the flashing and sparking as he got in his car.

This incident was created by water seepage into the busways joint connections on the exterior of the building before entering the switchgear room. It required the replacement of some busway so not a big deal and no one was in the area. But it goes to show that if it were someone creating the arc it could be bad.

This generally is a position in which you would say no energized work allowed.
I understand the oil test situation and that a tough call other then to suit up.
We are looking at alternatives.

With IR viewing I would recommend special windows installed during a plant outage at specific points on the transformer cover or switchgear housing for thermal inspection without opening any covers.

Look at the coordination curve and create a short circuit on the LV side and see what the time is for the fuse to clear on the HV side. In most cases it’s going to be a good amount of time no matter what type of HV fuse used. It’s better to have several small transformer power distribution systems <2000KVA then one big one when you want to reduce the arc flash hazards.

WHump,

I ran into a guy a few days ago that was pulling oil from a 2500KVA system when something happened and a SC occured on the LV side. He was wearing the higer cal suit, he said it saved his life. I told him to join the forum and tell his story.

Just thought of that.