PPE for Arc Flash Scenario 5

[nightfox1925]

We had a 480VAC MCC with a calculated bolted fault at the bus at 11kA sym.,and the calculated Arc Flash is around 40cal/cm^2. Our contractor is suppose to plug in a new motor starter to this existing MCC (which is enegized) but because he does not have his PPE rated for the Arc Flash, the work was stopped.

My question is, if the MCC main and vertical bus is enclosed in a separate compartment within the MCC which does not expose our contractor to live parts, is he supposed to wear a PPE rated for >40cal/cm^2.

What about if the work only calls for example opening of a de-energized MCCs spare starter for visual inspection, are we suppose to be wearing >40cal/cm^2 PPE rating?

I am still in the process of familiarizing myself with the Arc Flash standard.

 

[dpc]

Plugging a starter into an energized bus is inherently risky, even though it is done routinely. If a fault occurs while the starter is being plugged in, it can be blown out and serious arc-flash injuries can occur. PPE is definitely required for this operation. If you read the instructions of the MCC manufacturer, there is a good chance they advise to never plug into an energized bus.

Opening a door for inspection is probably subject to interpretation. The starter is not de-energized if it is stabbed in because the line side of the disconnect will be energized. As consultants, our advice is to wear PPE anytime you open the door. Simply opening an enclosure door has initiated many faults.

 

[Thedroid]

People pull buckets out of live mcc's all the time with no problems. I work in a dusty environment and although we vacuum our substations on a weekly basis there is no doubt that the busses and lugs have dust built up on them. Our electrical department has over ten years with no types of injuries, but I don't want to be the one that breaks that record. I don't like to pull buckets if the mcc is live, but if it is an emergency i will wear at a minimum a salisbury arc hood, jacket and gloves. I don't want to hear the sound that only dead men hear.

 

[Zogzog]

"People pull buckets out of live mcc's all the time with no problems"

I have heard this same statement from dozens of accident investigations, it is false, a true statement is "People pull buckets out of live mcc's MOST OF the time with no problems", in fact it is done thousands of times a day in workplaces across the US and on average ONLY 4-5 people a day are submitted into burn units from the arc flash that occurs, and yes, some of them live to go through several years of rehab to obtain some type of life after thier rehab.

Your plant is way off base and is a stastic waiting to happen.

 

[Zogzog]

Nightfox, find a new contractor, if they dont have the necessary PPE for this type of work I highly doubt they are trained and qualified for this type of work. DPC's comments were right on, but I must add that the level of protection may only be reduced if you have arc rated switchgear.

There is no PPE that can protect you from >40 Cal/cm2 ,there is stuff on the market rated for up to 100 cal/cm2 but it only protects you from the heat, not the pressures from the blast, lets you have an open casket instead of a closed casket. 70E handbook discusses studies done on these pressures, any equipment with >40cal/cm2 should be labeled dangerous and cannot be worked on while energized weraring any PPE.

You need to look at arc flash mitigation in these areas, there are dozens of soluions to this problem based on the types of solutions.

Decrease fault current (Current limiting devices)
Decrease clearing time (Ex. arc flash reduction switches)
Increase working distances (remote opertors and racking,

http://www.remoterackingsolutions.com

)

 

[ruggedscot]

on average ONLY 4-5 people a day are submitted into burn units from the arc flash....

Gee thats a calculated risk and well its not worth getting upset or worrying about, Yep. Ill just say that to the Joe recovering in the intensive care unit, you were just plain unlucky.

This is something that has to be knocked on the head once and for all - working should not be dangerous - the drive to anhd from work should be the most dangerous part of work. If there is a risk then it has to be curtailed and managed. No one should be taking this sort of risk in this day and age, Proper gear and proper training. Risk assesment and compliance with the requirements of the task.

Its all very well saying that the risk is low but the risk is there and the effects are horrific, severity and impact to life and those that this person cares about and those that depend on him. Ive seen accidents and witnessed that aftermath of a pretty severe arc fault- it happens. Ive been sparking since 1988 and well if Ive seen such a fault happen then it is an occurnace and you need to take the appropriate actions. If I caught someone doing such work without the right PPE then I would be asking them to stop the job and Id be putting them of site. Its not worth it and Im not going to see anyone hurt for the sake of not being adequately protected.

 

[Electic]

As a bucket is inserted into an energized MCC, there is a period of time when the vertical bus will be exposed, though technically not by line of site, to the worker. Industry convention has deemed that as 'exposure'. Also, the conductors going to the top of a typical bucket circuit breaker will energized and exposed, though, that is unlikely to cause a disturbance in the electrical field.

40 calories per sq cm is the limit for NFPA risk category 4. If that truly is the calculated exposure, the work may be completed with the prescribed PPE. If the calculated exposure is slightly above 40 calories per sq cm, that is deemed unworkable by NFPA 70E, and would require a shutdown to pull the MCC bucket.

If the facility can afford to take power outages, it is preferred to do this work on a deenergized bus.

If this is a continuous process plant where outages also constitute a risk, you might wish to retrofit a detached secondary main breaker with instantaneous tripping that in most cases can be adjusted to achieve risk category 1. I would expect that a 480V installation with 11kA fault current to have workable calculated arc flash exposure if the breaker has a well adjusted instantaneous trip unit. There are also other methods to reducing the calculated arc flash exposure, such as temporarily reducing primary breaker instantaneous pickup by way of a 'hot work' switch.

Many facilities insert and withdraw buckets from energized MCC's. Much of this equipment is designed for buckets to be inserted and withdrawn while energized. With proper planning and preparation it may be done within the guidelines of NFPA 70E, though that does not remove all risk.

 

[Thedroid]

Who's plant is a statistic waiting to happen? I didn't say at my plant we pull buckets live all the time. I'm 100% against the practice. I made a mistake by starting my last reply, "all of the time", but I assumed everyone knew what I meant. It happens. If a bucket needs to be pulled I always try to shut off the power to the mcc. Sometimes it's not possible, not just at my plant, but all over the place. If that's the case, I feel way better with an arc flash hood, an arc rated jacket, and linemans gloves.

Andrew M.
Cement Plant Electrician and Instrumentation

 

[Zogzog]

rugged scot, I was being sarcastic when I said ONLY 4-5 people. I have made too many trips to a burn unit, never want to step foot into one again.

 

[Zogzog]

I thought you meant your people pulled MCC's live all the time.

Even so, your PPE you list as wearing is not adequate. You are missing several key parts.

 

[ScottyUK]

Zogzog,

In the UK we pretty much do pull LV starters 'live' all the time and manually rack LV breakers onto some very high energy boards. Most of our MV breakers are truck mounted and I can't see how the automatic racking mechanism would adapt to that style of breaker for all it is an interesting idea.

I'm not defending the practice at all - just saying what happens when there isn't legislation to prevent it. Our company and a lot of our competitors have voluntarily undertaken, or are undertaking, arc energy studies and putting the workers in PPE even though there is no direct legislation to require it. I'd like to thank this forum and especially the US members for raising my awareness of the hazard a few years ago, from which I set wheels in motion to protect our workforce.

I do have some concerns with our 11kV system which potentially has two 48MVA transformers plus a 45MW generator on the bar. We switch remotely but racking is manual. There's not much we can do on the protection settings to bring the energy level down and the incident energy is so far off the scale that even a 100 cal suit wouldn't be adequate. What happens in the US where such problems exist? Killing the bar is an operational impossibility - we'd shut down 1GW of generation by doing so.


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If we learn from our mistakes I'm getting a great education!

 

[Zogzog]

All this arc flash protection is just catching on here in the states even though the laws have been in effect since 1979, although some major changes were made in 2000 that got the ball rolling. Funny thing is the utilities dont have to follow the 70E and the utilities are better about PPE and Electrical Safe Work Practices than the other industries that ARE required to comply, especially the EC's, they are the worst, they all think they follow 70E and are qualified when in fact they have never had the proper training.

We have already made about a dozen custom adapters for different racking solutions. What type of breakers do you have? There is the traditional screw type racking system model and an extraction model for types you have to pull out like the earlier DHP's.

Scott

http://www.cbsnuclear.com

 

[Thedroid]

An electrician I work with just came from another plant where you had to put on a full suit, cotton underwear and all, to open the door on any enclosure. How do you take any kind of readings with those things on?. I'm new to this trade, and I'm trying to pick up all the right habits instead of the wrong one's, but I can't see being able to work safely with a full suit on 24/7. I imagine your more likely to cause an arc flash trying to work on something live with a big old suit on than not. Definately not condoning the practice, just a thought.

Andrew
Cement Plant Electrician and Instrumentation Technician

 

[dpc]

TheDroid:

Get used to it. It's a done deal and we're not turning back. That's about as likely as seeing new cars without air bags.

If a proper arc-flash hazard analysis is done, it will generally be found that most 480 V equipment downstream of a molded case breaker or current-limiting fuse will have arc energy levels of less than 8 cal/cm2. This requires a face shield and arc-resistant clothing. It is not that difficult to work in this PPE. For higher energies, heavier PPE is required, but that's just the way it going to be.

In my career I've listened to guys say they couldn't work with a safety harness on, couldn't work with a hard hat on, couldn't work with safety glasses, couldn't work with ear protection, etc. But they did.

If you have to work in a 40 cal/cm2 hood, bring lots of lighting with you, because it will be dark in there.

Cheers,

Dave

 

[LionelHutz]

Andrew

Your electrician friend likely came from a plant where they did not really understand the hazards. If they did a proper study of their power system they would have found the protection requirements for the equipment in the plant and assigned the proper PPE for each piece of equipment. There's likely lots of equipment that didn't need that level of PPE to work on. There might also have been some equipment that could not be worked on even with the 40 cal/cm^2 PPE.

 

[ScottyUK]

Zogzog,

It's Whipp & Bourne DV40 vacuum gear. Breakers are 2500A / 11kV and 3150A / 11kV on the newer section of board.

I agree with you about utilities and generating sites taking electrical safety more seriously than a lot of construction outfits. As an industry I think we do a pretty good job of looking after our workforce, but we can always try to improve and to learn from other people and other countries.

There are some parts of the plant where I refuse outright to undertake live testing. An incident energy level of 170cal/cm² won't take any prisoners if it goes up.



----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[Thedroid]

I'm going to look into the energy levels of some of our equipment. We have numerous substations each containing multiple mcc's. Most of the substation transformers are dry type 4160/480 1000 kvA. We also have numerous medium voltage motors from 400hp up to 1500hp. All of the transformers are protected by GE Magne-blast breakers, and the mcc's are fed through a secondary AK breaker and each mcc has it's own AK breaker. The AK's all have AC Trip units installed. No one has ever done an arc flash study on our equipment, so I'm going to try to plug in some numbers and see what the potential energies involved are. All of this equipment was commissioned in 1959. The plant is MSHA governed, so PPE is taken very seriously, but arc flash gear is not specified to my knowledge. Definately something I want to look into more closely.

Andrew
Cement Plant Electrician and Instrumentation Technician

 

[Zogzog]

"The plant is MSHA governed, so PPE is taken very seriously, but arc flash gear is not specified to my knowledge."

Not too seriously apparantly, from the sounds of it your safety department is misinformed on the requirements or just dosent have the power at your plant to make the right policies (Internal money vs safety issues). I have seen this at hundreds of plants, they all say safety is #1 but many do not walk the walk.

Odd that the Magneblast breakers protect the transformers, usually in a system like you described they protect the MV cables and there is a fused load break switch protecting the transformers. What type of relaying is being used on the magneblasts? Differential? that would help.

You said your AK's have AC trip units, you mean AC PRO? If so are they QT ready? Is a QT system installed?

What are you "plugging numbers" into? An incorrect hazard analysis is more dangerous than no hazard analysis. Be careful, be conservative, be sure you know what you are doing.

 

[burnt2x]

Thedroid,
Luck is not something we can rely upon. In a plant where I have been involved with last year, same activity was done. We got the whole mineral processing plant blacked-out when a contractor plugged an new feeder breaker with bus energized. They have the necessary arcflash PPE but still got the plant idle for 4-5 hours repairing/ cleaning the MCC panel. Planning ahead and working with dead bus is still the best option. Otherwise, you can just hope nothing untoward happens.

 

[Thedroid]

Your right the magne-blasts do protect the cables, and I'm not sure what protects the transformers. The units on the AK breakers are AC PRO's and I'm not sure if thay are QT rated. I don't know what QT means. I'm new to the industry and also new to the plant I'm working in. I've been working with electrical, mostly motor control and machine troubleshooting for the last few years infrequently, and exclusively for the last few months. I'm here to gain as much knowledge as possible and to grow in this profession. I wanted to do my own basic arc flash calculations using the one line diagram for our electrical system. I've seen several formulas for determing how much energy can be supplied to a worst case scenario. More for practice and my personal knowledge.

Andrew
Cement Plant Electrician and Instrumentation Technician