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 Post subject: activating de-energized disconnect
PostPosted: Sat Mar 31, 2012 3:48 pm 

Joined: Sat Mar 31, 2012 3:36 pm
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hello
we have disconnect switches (for LOTO purpose) that our operators use in order to safely cleanout a de-lumper.
before arcflash they were allowed to place these switches in the off position and lock them out.now they need to wear cat0 clothing.because there is no voltage on these switches when they place them in the off position, do they still need to wear cat0 non melting clothes?
joe


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PostPosted: Sun Apr 01, 2012 4:52 am 
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If there is no voltage, you've locked it out upstream of the disconnect in question, and you can prove that this is the case through the means of a device that proves the absence of voltage AND can be tested both before and after, then the equipment would be considered in a safe working condition and they can do whatever they want with it however they want to do it. There is no PPE requirement for doing anything with equipment that is in a safe working condition.

However, if you are referring to the conventional situation such as opening and closing a fused disconnect in a 600 V class MCC where the bus upstream is energized, that's an entirely different matter. You are assuming that there is no current but there is certainly voltage present.

Z462 is written such that the bare minimum required clothing for ANY electrical work is H/RC 0. Even with 120 VAC equipment, we've all either seen or experienced first hand for instance a light switch that arced over and exploded on us. This is a voltage and current that is well below the point where a self-sustaining arc can happen. But if the clothing being worn at the time is highly flammable, there is a risk that it could catch on fire and cause a burn. Thus the reason for an H/RC 0 minimum. This is sufficient defense against even relatively harmless, low level arcs of any kind.

Remember though...Z462 is a guidance document. It is not regulatory unlike say CEC. You are free to consider the likelihood of any failure at all of the disconnect switch and consider the consequence if it were to happen. I would encourage you to find a copy of IEEE Std. 493, or "Electric Power Distribution System Reliability" by Richard E. Brown. From the latter, failure rates for disconnect switches are from 0.002 to 0.01 failures/year. Failure rates for molded case circuit breakers are 0.003 to 0.0176 but this seems about 1-2 orders of magnitude higher compared to the referenced datat (IEEE Std. 493). Draw out air insulated circuit breakers are listed as 0.001 to 0.030 failures/year. "Failure Rate Modeling Using Equipment Inspection Data" also by Brown gives some insight into the methodology on the ranges. If you consider all failures random and routinely inspect (and pick up about 90% of failures during inspection), you can go from the ultimate failure rate of the equipment down to something more reasonable. He gives air insulated switchgear (drawout) circuit breakers in that paper as having a range from 0.0005 to 0.060 failures/year. Disconnect switches are 0.002 to 0.320 failures/year. In the case of circuit breakers a certain number of failures (roughly 90%) are not going to pose a hazard to the operator, and only a fraction of them (about a third) are going to pose a hazard during use (as a disconnect). In the case of switches however failures almost never happen during use...most failures occur during opening.

As a guideline, many companies set their level of risk tolerance around 0.00001 to 0.000001 (1 in 100,000 to 1 in 1 million). Even if you are doing excellent maintenance and using the equipment in a clean environment so that you can claim somewhere around 0.0005 to 0.002 failure rates, that's nowhere near where you can consider this a risk free scenario. Given enough time and enough switching operations, eventually a switch is going to fail on you.


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PostPosted: Sun Apr 01, 2012 1:06 pm 

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thanks for the very informative reply.
the disconnect is non fused and there is NO voltage at the time of switching because the fuses and contactor are in a control panel.when the contactor is DE-energized there is no voltage.


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PostPosted: Sun Apr 01, 2012 6:48 pm 
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There is voltage on the upstream side of the contactor, unless something else is disconnected upstream of it.


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PostPosted: Mon Apr 02, 2012 8:42 am 
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Paul... thanks for a very informative post. I do however, question the statement where we have all seen 120v light switches explode and that HRC #0 is required. I have heard a slight pop when a switch failed and, technically, every time we operate a light switch, there is an arc. Your statement suggesting that any operation of a 120V light switch requires HRC #0 PPE is a little far-fetched IMHO.

While I'm not familiar with Z462, the NFPA 70E 2012 states "Under normal operating conditions, enclosed electrical equipment that has been properly installed and maintained is not likely to pose an arc flash hazard". It also states that any body part that crosses the arc flash boundary must be protected. With that in mind, many CAT #0 AFB are just a few inches.


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PostPosted: Wed Apr 04, 2012 9:05 pm 
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While I'm not familiar with Z462, the NFPA 70E 2012 states "Under normal operating conditions, enclosed electrical equipment that has been properly installed and maintained is not likely to pose an arc flash hazard". It also states that any body part that crosses the arc flash boundary must be protected. With that in mind, many CAT #0 AFB are just a few inches.

This forum section is for CSA Z462, not 70E.

NFPA 70E and Z462 do not have a "category -1". I agree with you about the "normal operation" argument but the tables in 70E for low voltage equipment do not say "no". They give a specific numerical arc flash PPE requirement. I would agree that the hazard is minimal but wearing nylon and/or propylene and performing tasks where an ignition source might be present, however small, poses a significant risk if the clothing melts and continues to cause damage well after the incident is over. Hence the reason to consider whether or not any PPE at all should be required.


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PostPosted: Mon Apr 09, 2012 7:22 am 
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Only Qualified Electrical Workers that will potentially be exposed to an arc flash hazard require arc rated clothing.

Under normal conditions energized electrical equipment that is approved and installed to the CEC or NEC doesn't pose a hazard to anyone and no special PPE is required.

Qualified Opeations Workers that have been training and are authorized can complete low votlage isolations under normal conditions with no special PPE.

We need to make sure that we properly interpret CSA Z462 or we will shut down industry which is not the intent. We need those Qualified Electrical Workers that are exposed to shock and arc flash to de-energize first, apply other preventive control measures and PPE last. See CSA Z1000 for the hiearchy of preventive and protective control measures.

Regards;
Terry Becker, P.Eng.
ESPS Electrical Safety Program Solutions INC.
See my LinkedIn Profile


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PostPosted: Tue Apr 10, 2012 6:08 pm 
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Under normal conditions energized electrical equipment that is approved and installed to the CEC or NEC doesn't pose a hazard to anyone and no special PPE is required.


Terry, though I don't disagree with your attitude about going crazy about PPE for those who are not exposed, you cannot possibly mean this. Medium voltage switchgear is definitely installed to meet CEC and NEC requirements and yet even Z462 and 70E require H/RC 2 clothing for switching operations on medium voltage gear, something that can be done by personnel who are not going to open the doors and work on the equipment. And that's not even considering certain relatively hazardous operations of equipment that has been installed as per CEC/NEC regulations and maintained as per NFPA 70B or NETA-MTS. Much as it drives everyone crazy, draw out switch gear is a good example of equipment that though it is certainly designed and hopefully maintained to those standards, has a pretty high propensity (roughly 80% of failures are in the draw out mechanism as per CIGRE reports) to fail and most of those failures result in arc flashes. I have equipment here in the States where the only way to lock out a breaker is by drawing it out and putting a lock through the rail. It meets even current CEC/NEC standards though we probably won't buy that style ever again.

Unfortunately, CSA Z462 and 70E never, ever quite come right out and articulate what you just said. They allude to it vaguely in the definitions section, and again in the hazard tables. But there is no list of "PPE not required" anywhere in the whole document. Life would be a lot easier for those who have done the engineering studies as per IEEE 1584 if the 70E Technical Committee would just come right out and document this. But nowhere in those documents do they ever state exactly what you just said.

Care to come to Chicago in May with Neil Wight and myself so we can beat up the "other guys" who are trying to articulate the opposite argument?


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PostPosted: Mon Apr 30, 2012 8:33 am 
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Paul:

We need to make sure we interpret when an arcing fault and arc flash can really happen. Use risk assessment with probability and then decide when arc rated clothing is required depending on worker role.

A Qualified Electrical Worker requires arc rated clothing. Other "Task Qualified Workers" may need arc rated clothing as well for example an AC & Refrigeration Mechanic.

"Operators" of energized electrical equipment do not need arc rated clothing. They should not be working on energized electrical equipment. We need to define their limits in a Role description in an Electrical Safety Program. We do allow them under "normal" conditions to perform low voltage isolations for work on mechanical equipment. They should be instructed on a simple isolation procedure to follow and a bump test of the motor that is driving the mechanical equipment. This is an industry "Standard" practice.

With the doors closed, LV load turned off first, no arc rated clothing is required for LV circuit breaker or disconnect switch operation.

Regards;
Terry


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