system versus arc flash protection 1

[stevenal]

My interest is from the utility side. The NESC proposals for 2007 include arc flash protection requirements. Similar rules are being considered by OSHA. The rules themselves are pretty simple, assigning caloric clothing values to incident energy levels or to tabulated fault currents and clearing times. No more guidance is offered, leaving us with IEEE 1584 for details. Regarding clearing time, by my reading, 1584 says to look at the first upstream device from the possible arc flash location. Contrasting this procedure with system protection leaves me somewhat puzzled. We try to set primary and backup protection both to protect the system. Both secondary and primary side protection on a transformer, for example, lie beneath the transformer damage curve. Likewise for line protection. Whether backup is local and triggered by breaker failure or is remote and coordinated, backup protection clears a fault before the line burns down. So I'm left with the uncomfortable proposition that the equipment is more important than the safety of the workers. Sorta goes against the engineering code of ethics. Any comments on this observation? Thanks.

 

[davidbeach]

Traditional protection is system equipment protection. Personnel protection has been long ignored, other than 5mA ground fault circuit interrupters, and has only recently begun to be considered. Things like hot work tagging that turns on instantaneous trip of a line if it isn't normally on and turns off reclosing is one effort, but sacrifices coordination.

To provide coordinated protection for both personnel and equipment it is necessary to use zoned protection that can trip instantaneously for all faults in the zone. Differential protection is the clear winner here but other schemes can achieve the same results. Blocking schemes that allow a breaker to be tripped nearly instantaneously if it does not receive a blocking signal from a downstream relay could be the next best; these can be implemented with relay times in the 3-4 cycle range. Distance protection, alone, would not provide that level of protection near the ends of lines but could for the middle half to two-thirds. In an industrial setting, distance could be used on radial transformer feeders to instantaneously trip for any faults on the feeder or primary of the transformer, but would not provide instantaneous protection for beyond some portion of the secondary. That distance element on the feeder circuit plus a transformer differential, plus a blocking or zone interlocking scheme on the secondary could provide instantaneous trip for all faults between the source breaker and the load switchgear. I am analyzing another scheme for the primary circuit that wouldn't require the distance element but would provide the same degree of protection using functions available in relays much more economical than those with distance elements. If my testing works out, look for a paper at WPRC in October.

I'm not sure that the typical utility distribution circuit lends itself to protection intended to minimize the danger to personnel and provide selectivity. Not that it couldn't be done, but it would be costly to do so. Reclosers with instantaneous trips and the communications infrastructure to send blocking signals to the relay at the substation to block its operation. There may a point to a system that sacrifices selectivity during the time personnel are known to be working on the line and maintains selectivity without personnel protection the rest of the time.

 

[dpc]

The two goals of coordination and protection are often directly conflicting, as I'm sure you know. Compromise is generally required.

IEEE 1584 has an industrial/commerical facility background. I also think their procedure attempts to address determination of arc-flash hazards for an existing facility designed to meet existing codes and standards. If one were starting from scratch on a new facility or new equipment, there would be a lot of things done differently.

Also, IEEE 1584 is clearly just a method for calculation of arc-flash hazards. It was not intended to address safety, protection or installation requirements. That is covered, to the extent it is covered at all, by the NEC and NFPA-70E in the non-utility world.

But you're right in the sense that the IEEE 1584 procedure assumes that the first upstream device is going to operate and is going to operate within its specified tolerance. No allowance is made for breaker failure, relay failure, etc.

 

[stevenal]

David,

The point I was trying to make is this: System protection >> Assume primary protection fails and set both primary and backup protection to protect the system.

Arc flash protection>> Assume primary protection works every time and set it to protect personnel.

I'm not really trying to find nuts and bolts answers to improve over 1584 at this point. Just trying to get my head around the philosophical differences. Thanks.

 

[JBD]

The rules for protecting utility line workers (those covered by the NESC) are different than those for general workers (those covered by NFPA70E). This is partly because the utility workers work on energized circuits in a different manner than the standard worker.

Protecting the worker will always also protect the equipment/system. However, utility systems are designed to "not trip off line" on minor faults instead hoping the fault will clear itself. Maybe some thought needs to be given to more zone control, so tripping on minor faults can be done without affecting large portions of the grid.

 

[stevenal]

I don't have a copy of 70E, but my understanding is NESC and OSHA are both incorporating more of it into upcoming standards. Natural fibers alone won't cut it any more for utility line workers.

A few other thoughts on the subject: Breakers, relays, associated instrument transformers and DC power systems, reclosers, fuses, etc. are now becoming PPE of sorts. Are the manufacturers down with that? Are existing standards adequate for these items in this role? Is it appropriate to expect a 40 year old breaker and relay that were not designed for the purpose to meet these new expectations? How does this affect your maintenance programs? Will the questions ever stop?

 

[mc5w]

Some of the arc flash concern is that equipment is getting old leading to more instances of circuit breaker ruptures and so forth. Besides old age there has been some increase in short circuit current such that the impedance of the primary side of a transformer can no longer be considered to help with lowering short circuit current.

You can expect 50 year old and 60 year old equipment to blow up sometimes. Ford had an old 4,800 volt metal clad cable blow up and kill somebody and as a result the NEC people lowered the limit for unshielded conductors to 2,400 volts. That is NOT the solution. The solution is to replace old wiring. In some cases circuit breakers and fuses with higher short circuit rating need to be installed and even entire switchboards upgraded to not just new part but better parts for more short circuit withstand.

 

[stevenal]

mc5w,

I was not referring to any increase in fault current, and I don't believe the NESC '07 code writers are expecting us to rewire the entire transmission and distribution system to meet the new code.

Your mention about breakers blowing up is more to my point, but assume no one is injured directly by the explosion. If the explosion should occur during an arc flash exposure downstream situation, the arc duration will be longer than the IEEE 1584 alloted time. Protective clothing may be insufficient for the longer clearing time of the backup protection. Loss to the system is repairable. Loss to the worker may not be.

 

[dpc]

But even insufficient PPE is going to be much better than what they are likely wearing right now.

No matter what level of protection or precautions are used, it is always possible to postulate a scenario where this protection might not be adequate. People are still killed in automobiles equipped with air bags and seat belts, but no one seems to be advocating reducing speed limits back to 55 mph which would, unquestionably, save many more lives every year than anything we are talking about here. So whether we like it or not, the cost of additional safety measures is always a factor.

I agree that there is inconsistency when comparing arc-flash protection recommendations with some types of equipment protection in some situations. But some protection is always better than no protection. And I think we will see a lot of changes in arc-flash PPE and operating procedures as more testing is done and there is more consensus in the industry.

Besides, the NEC was originated by insurance companies who were tired of paying claims on building that burned down. It's never been focussed on safety of electrical workers.

 

[jghrist]

I'm not sure how the NESC will implement arc flash protection standards. Would it apply to hot line work? I would imagine that the arc flash energy available on the typical distribution pole would require all linemen to suit up in "spacesuits". Expect a bit of decreased productivity.

 

[stevenal]

Add new Rule 410A3 and renumber existing Rules 410A3 and 4 to 410A4 and 5:
Effective as of January 1, 2009, the employer shall ensure that an assessment is performed to determine
potential exposure to an electric arc for employees who work on or near energized parts or
equipment. If the assessment determines a potential employee exposure greater than 2 cal/cm2 exists
[1], the employer shall require employees to wear clothing or a clothing system that has an effective
arc rating at least equal to the anticipated level of arc energy.
When exposed to an electric arc or flame, clothing made from the following materials shall not be
worn: acetate, nylon, polyester, or rayon.
The effective arc rating of clothing or a clothing system to be worn at voltages 1000 V and above
shall be determined using Tables 410-1 and 410-2 or performing an arc hazard analysis. For secondary
systems below 1000 V on non-network systems, clothing or a clothing system with a minimum
effective arc rating of 4 cal/cm2 shall be required to limit the likelihood of ignition.
When an arc hazard analysis is performed, it shall include a calculation of the estimated arc energy
based on the available fault current, the duration of the arc (cycles), and the distance from the arc to
the employee.
NOTE 1: A clothing system (multiple layers) that includes an outer layer of flame resistant material
and an inner layer of non-flame resistant material has been shown to block more heat than a single
layer. The effect of the combination of these multiple layers can be referred to as the “effective arc
rating.”
NOTE 2: It is recognized that arc energy levels can be excessive with secondary systems. Applicable
work rules required by this part and engineering controls should be utilized.
Revise Rule 420I1 as follows:
1. Employees shall wear clothing suitable for the assigned task and the work environment. See Rule 410A2
Add new Rule 420I2 as follows, and renumber existing 420I2 to 420I3:
2. When employees will be exposed to an electric arc, clothing or a clothing system shall be worn in accordance
with Rule 420A3.


Tabulated values are also included.

 

[dpc]

Actually, for hot-line work with hotsticks, the calculated arc-flash values are not as bad as I expected, because of the increased working distance. (assuming hot-line tagging is used, with high-speed (inst) tripping).

 

[stevenal]

The tabulated values assume 15" separation, arc to worker for system voltages to 46kV.
Higher voltage separations are the minimum approach distances less twice the assumed arc length. The assumed arc lengths are based on the line to ground voltage at 10kV per inch.
Doesn't look like they are using stick length to assure separation. Order up those moon suits.

 

[stevenal]

The committee passed the proposal. I tend to agree with two of the dissenters:

McKinney: The major reason for this CP was OSHA’s desire to have an industrial consensus on this issue to
help them with future rule making. Currently, the FR clothing rules for the industry are only found in NFPA
70E, which specifically excludes utilities. We believe there is insufficient data from OSHA to justify requiring
FR clothing for the industry at this time. There are a small amount of documented accidents in which
burns are received. However, in almost all of these cases, the person burned has violated an existing NESC
rule. If we are going to spend extra time and money to reduce these injuries, we should consider increasing
enforcement of existing rules rather than just creating another rule. Sometimes it is better to just make sure
someone wears their belt instead of requiring suspenders in case they forget to wear their belt.

Woodings: Effectiveness, acceptance, and ability to perform required work are my reasons for rejecting CP
2815. As for the effectiveness of wearing FR-rated material in a thermal situation, yes, it is beneficial, but if
employees would all wear long sleeve cotton shirts and a plain white cotton T-shirt underneath, the
acceptance level on both employee and employers would be far greater than it currently is. A contributing
factor in reviewing many incidents is the need for more and better training, as many individuals do not realize
the danger of wearing synthetic fabrics nor the potential danger of a thermal situation. Another factor in the
acceptance of this requirement is the problem of working in a hot environment with an FR-rated shirt or
jacket, as they can be too hot for safe working conditions.

 

[dpc]

A 15" working distance will give a high arc energy unless you're clearing the fault with essentially no delay.

That seems like an extremely short distance to assume for working distance.

Per NFPA-70E the working distance is based on distance to the worker's torso or face. This would be greater than the minimum approach distance, I would think.

If arc energy exceeds 40 cal/cm2, NFPA-70E says (essentially) that there is no PPE that can provide adequate protection and that any work must be done de-energized.