Mitigation of Arc Flash Hazard on 3000kVA txfmr feeding 480V swgr 2

[bdn2004]

We have a 3000 kVA transformer that is being fed from a 15kV delta primary, 480V grounded wye 6-ph secondary. The transformer feeds a 480V switchgear via 5' of copper non segregated bus duct. It lands on a single 4000A fused switch that feeds a large electric furnace. The arc flash sticker on the gear shows a calculated energy level of 52cal/cm*2 and a required PPE of 5.

If we were to try to mitigate this hazard what would be the best way to do it? Is changing out the switchgear to an arc flash type and installing a circuit breaker with series current limiting fuses an option? Or is just changing out the 15kV fuses to faster ones also an option? Or with this much energy are we just wasting out time worrying about it?

 

[rbulsara]

There are all kinds of "options" available, but basic question is why?

My preference is to find ways to refuse to work live on the gear. Options could vary from just adjusting INST settings to wholesale replacement of the gear with whatever the latest is. OR even a redundant system installation with bypass circuits that allows shutting down a gear without affecting the critical loads.

Justification depends on why would you want to do it, what it serves and if the cost is worth it.



Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

I love the "just work on it deenergized" argument. It's not deenergized until the grounds applied and before the grounds can be applied it is necessary to test for voltage. The testing for voltage and the application of the grounds are hot work, not subject to "just work on it deenergized".

 

[bdn2004]

The reason "why" is because we are changing out the furnace and I noticed this 480V switchgear is as old as the load it feeds. It's certainly not arc resistant. Safety is a big selling point when it comes to getting monies authorized for projects. So now would be the time to make the case to change out this switchgear. But it has to be a valid reason: like we can reduce the energy levels 50%.

Again, my question is...installing a circuit breaker closest the load in series with current limiting fuses a good option?

 

[rcw retired EE]

The circuit breaker will still have the same arc flash level at its line side unless other precautions are taken.

Look at adding a current transformer and an optical arc detection relay like SEL, ABB and others offer. If it trips the upstream breaker, the arc flash level can be reduced becaue of the fast response time of the relay.

 

[rbulsara]

Anything that opens faster on a fault is good. How good? That is always questionable. If you consider getting a 2nd degree burn at minimum, and you can achieve that level, it is good enough for you, I guess. May not be for some.



Rafiq Bulsara

http://www.srengineersct.com

 

[Zogzog]

"We have a 3000 kVA transformer that is being fed from a 15kV delta primary, 480V grounded wye 6-ph secondary. The transformer feeds a 480V switchgear via 5' of copper non segregated bus duct. It lands on a single 4000A fused switch that feeds a large electric furnace. The arc flash sticker on the gear shows a calculated energy level of 52cal/cm*2 and a required PPE of 5."

OK, first problem, there is not and never has been a HRC 5, adding a HRC 5 was discussed about 8 years ago but never made it into the standard.

"If we were to try to mitigate this hazard what would be the best way to do it? Is changing out the switchgear to an arc flash type and installing a circuit breaker with series current limiting fuses an option? Or is just changing out the 15kV fuses to faster ones also an option? Or with this much energy are we just wasting out time worrying about it? "

Best way, mitigate the hazard. Your problem is not your 480V gear, it is the 15kV fuses, and the way they see a fault on the secondary of the transformer. Long clearing times. First thing to check is the clearing time used for the analysis, you can use a maximum of 2 seconds per IEEE 1584 but from the info you provided it sounds like that was already done.

Next thing to look at is the 15kV fuses, I have had success with some different fuses with special curves for a solution here, don't worry about them being compatible with your 15kV switch, that can always be changed, we do it all the time.

Another solution is to replace your 15kV fuses with a mini VCB with seperate current sensors on the 480V side of your transformer, this VCB is placed where your 15kV fuses go in your switch via a retrofill modification. Lots of options there.

Now if the operation of this 480V fused switch is your primary concern there are wireless remote operators available so you can operate the switch outside the arc flash boundary. But as David Beach mentioned if you plan on entering this cell and working on the switchgear you would still have an arc flash issue to deal with until grounds are installed and it is verified de-energized. But if just switch or breaker operation is your concern, the remote operator may be the easiest solution, and requires no modifications to your equipment.

 

[rbulsara]

You can always perform an arc flash analysis and calculate the incident energy. Even do a few "what if" scenarios. If you do not have the means, have a professional do it for you. Yes, that would cost some money too, but there is no other way to accurately estimate impact of the proposed changes. (Based on the approved standards today).

Rafiq Bulsara

http://www.srengineersct.com

 

[bdn2004]

Zogzog,

In your opinion is this energy level so high as to make an effort to mitigate it justified? is this a safety hazard? Can you explain what is HRC 5? Is that the same as PPE level 5?

 

[Zogzog]

There is no HRC 5, or what you are calling PPE level 5. Whoever did your arc flash study was misinformed, but I have seen the same mistake made, once in every plant of a very large automotive company.

What the level 5 is refering to is from a note in an old 70E handbook, 2000 edition if I recall, the leve 5 is telling your the Ei is >40cal/cm2. Today most engineering companies will label that as "Dangerous" refering to another FPN saying that the pressure waves for an arc flash with Ei >40cal/cm2 may not be survivable no matter what PPE you are wearing. Nothing that really is enforceable about not working at these levels but the jury is still out on these levels. 95% of companies I have worked with decide to not allow any work >40cal/cm2.

So yes, mitigation is justified in this case. The majority of our remote racking and switching customers are using those devices for the same situation you have, which is fairly common.

Here is a link to the mitigation presentaion I gave at a recent EPRI conference that discusses options for your exact issue and solutions.

 

[dpc]

I basically agree with Zogzog. Without more knowledge of the primary system, we can't fully assess all options.

The reduce the arc-flash energy on the 480 V side (at the main fused switch), the clearing time needs to be reduced. A primary breaker with current sensing on the 480 V side can do this.

HRC = Hazard/Risk Category in NFPA 70E. The thing is, there is no Category #5 in NFPA 70E. Per NFPA 70E, the maximum arc-flash energy that can be addressed safely with PPE is 40 cal/cm2. This is Hazard/Risk Category #4. PPE with arc ratings greater than 40 cal/cm2 are available, but that does not change the limitations in NFPA 70E. However, NFPA 70E is not legally a mandatory standard. BUT - OSHA uses it like God handed it down when there is an accident. So if you are not in compliance with NFPA 70E and someone is injured in an electrical accident, you'll need a really good story to avoid fines, lawsuits, etc.

It is most definitely a safety hazard and if someone is injured while working on this gear energized, it could be very costly.

dpc

 

[bdn2004]

Zogzog,

I think you forgot to attach the link to your presentation. I'm looking forward to seeing it.

And thanks again for the informative and helpful posts.

 

[Zogzog]

Oops, sorry

http://www.cbsnuclear.com/free-information.htm

 

[ScottyUK]

Do you regularly apply safety grounds to LV equipment in the US? In the UK earthing is only a requirement for HV equipment, i.e. >1000V AC or 1500V DC by the present definitions.


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

 

[Maak]

Current limiting fuse is a good option only if you can guarantee that the current will be high. In low voltage systems the arcing current is often less than 50% of the bolted fault current.
"In the case of current-limiting fuses, the maximum arc energy may occur at current levels below the maximum interrupting rating." (IEC 62271-200)

I would recommend protection systems based on detection of light and overcurrent (dual sensing).