Arc Fault Calculations 1

[ruggedscot]

I see that out american cousins are heavily into arc fault calculations. Is this something that will come to the UK and start to feature alot in our own electrical systems ?

If so then where should I be looking at to try and get a head start and look to start prepreation of a plan of attack to cover what we will be required to cover.

Installation comprises of a few generators of about 1500kVA, 2 transformers 2.5MW some UPS gear and an intensive power delivery distribution system.

Rugged

 

[ScottyUK]

rugged,

We're drawing to the end of a study for our site. It brought out some interesting results - one or two things where the risk is much lower than we instinctively thought, and one or two where things are significantly worse. The worst results by far were certain boards fed from 2.5MVA 11 / 0.415kV transformers because in our installation these have a short LV cable group to the switchboard resulting in very high fault level and because the protection has a definite-minimum-time element which causes a high energy let-through.

I think it will probably become the norm over here, but at present North America is quite some distance ahead of Europe. We dealt with a company called Teesside Automation Services (TAS) who were a genuinely nice outfit to deal with and were not outrageously expensive. Website is

http://www.tas.co.uk

and the guy to talk to is John Easton. The study was done using SKM Powertools to IEEE 1584; the model they built will be useful in future for other tasks such as relay coordination and motor starting as the plant evolves. The NFPA 70E document is worth getting hold of too - there are some UK suppliers who can get it quite quickly.

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Sometimes I only open my mouth to swap feet...

 

[ruggedscot]

Thanks for your information on this one - As I said this is one that I think is going to become very important here in the UK and Im hoping to prempt this and ensure that we are ready...

Rugged

 

[Modula2]

Doesn't the IEC already have some documents on safety, approach distance, etc., that could form the basis for an IEC standard?

 

[ScottyUK]

There are a few IEC documents, but I couldn't find anything which you could reasonably describe as a coherent standard. Maybe I missed it - have you an IEC standard number?

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Sometimes I only open my mouth to swap feet...

 

[SteveHuck]

The truth about arc flash in the states is, there is no standard. Ask 5 engineers to calculate and you get 5 different answers. It has gotten to be a bunch of hocus pocus. Chrysler has new standards where 24volt control wiring is arc suit work because nobody can agree on what is needed. I agree that safety needs to be a priority but it also needs to be workable.

 

[ruggedscot]

As usual the states seem to be pretty quick with a way to generate money - looking into an arc fault calculation requires publications and training and all sorts so as you can imagine money exchanges hands. But as per America we seem to follow suit. Getting in to this beofre it all kicks off would be an advantage and well it would be a worthwhile exercise. I think that 24v control needs to be checked but to suit up and take precautions may be a little over the score. But working on a 2.5MVA 11kv to .400kv transformer feed requires some form of protection to be employed. Now Im not even talking about working but actual switching and operational proceedures on the system. Better to have the work done and the safety in place before we have an incident, and have the ability to be able to say yhat we knew of the risks and did the risk assesments and implimented the PPE and Proceedural control before anything went wrong.

Rugged

 

[wbd]

I think that the 24 Vdc in an arc suit is a bunch of BS. If one reads the current consensus standard (IEEE 1584) that is the basis for arc flash standards, it does not apply to dc and states so. The Chrysler engineers have their collective head up their ass.
That said it is a worthy standard to look into and analyze your system for. There is standard calculation methods based on IEEE 1584 and if the results of the calculations and the PPE from them save a life, what price would you put on that?

 

[Zogzog]

Granted there are companies out there doing these studies that have no idea what they are doing but there are also some that do. Chrysler (Like Ford) decided to do the studies in house, and now they are running into many problems.

For you UK guys, we have a web based training program that can fill you in on the requirements. Let me know if you are interested. ScottyUK, your results are typical, the key to the high hazard areas is understanding how to mitigate those hazards.

 

[ScottyUK]

Hi Zogzog,

I have some concern that the additional stress of having to work in Cat 4 PPE could have a negative effect on a worker's concentration and might actually contribute to an incident. Do you know if there are any recorded instances where the protective clothing might have been a contributing factor to an incident?

To deal with equipment which has very high arc energy, we're looking at sacrificing protection coordination in order to reduce the available energy, especially reducing the definite-time characteristics which seem to be major contributors to high energy let-through. Where that isn't possible, we'll have to switch it off and work on it dead and isolated.

What does the training program involve?


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Sometimes I only open my mouth to swap feet...

 

[wbd]

I have heard that working in a Cat #4 arc flash suit with the hood, is limited to 3 minutes. This is due to the fact that the oxygen level decreases to 19.5%. For those of you that have had confined space training this is a threshold level to exit the confined space area.

 

[dpc]

NFPA 70E, which is the primary arc-flash safety standard in the US, does provide some leeway in determining PPE based on specific task-based requirements. The heavy flash suits are available with forced-air cooling.

It's a point of discussion and ongoing debate. But I think most of the people active in the electrical safety field think this is usually just another excuse for not doing the right thing.

As far as wearing PPE for 24V dc work, that seems like serious overkill to me. NFPA 70E doesn't worry much about anything less than 50 V. IEEE 1584 says arc-flash calcs are not needed below 230 V, except for very large systems.

 

[wbd]

dpc,
The point I was making was that in buying an Cat 4 suit is to make sure it is right for the job. If only being used for racking in/out breakers where it can be done is a short period of time, oxygen depletion is not an issue.

 

[davidbeach]

Where that isn't possible, we'll have to switch it off and work on it dead and isolated.

Scotty, How do you get to dead and isolated? I keep hearing people say that they'll just have to work on certain gear dead if the arc energy is too high; but what is dead? My understanding, as someone who doesn't actually do the physical work, is that dead doesn't really exist, that it needs be treated as energized unless visibly grounded. But before applying grounds there needs to be a check of voltage to verify that the circuit element is not connected to a source. Isn't that verification and the work of applying the grounds hot work?

 

[dpc]

NFPA 70E refers to it as "electrically-safe work condition". There is not an absolute requirement for applying temporary safety grounds in 70E - that is situation-specific, I believe.

(The safety hazards associated with improperly applied or installed temporary "safety" grounding is a separate issue.)

However a visible disconnecting means and a voltage test is required to verify that the system is de-energized. Appropriate PPE is required for the voltage test and application of grounds (if required). Once the grounds are in place and the voltage test completed, the PPE can come off.

 

[davidbeach]

dpc, but if the gear is question is "Dangerous", i.e. not PPE available, how is the voltage test and application of grounds to be performed?

 

[ruggedscot]

I think that I need to be a little clearer here....

While switching or operating ACB's I feel that there is a significant risk being presented and I want to take adequate steps to try and reduce these risks and manage them in a proactive manner.

While racking out an ACB if there is a failure within the transport mechanisim then there is a real risk that there could be an open short circuit in the vicinity of the engineer and this is what I want to try and manage.

These units being connected to around 2.5MW rated transformers by short busduct runs - so the availiable let through is pretty significant.

I have seen the effect of a catastrophic failure and the damage caused was pretty significant - if someone had been close by then there could have been a pretty significant injury.

Rugged

 

[ScottyUK]

Hi David,

There is no requirement under British legislation to earth LV equipment prior to working on it. A isolation only needs to introduce an air gap of a minimum size and which can be made secure by application of a lock or similar. Once that is done the work is classed as 'dead working' even without an earth, which is different practice to the HV rules which require a connection to earth. That said, if I was working on a high energy LV board which had been isolated by taking out its feeder transformer I would insist that the HV breaker was locked out and preferably the HV circuit earth locked in also.

Proving dead is 'hot work' and arguably should be carried out using precautions for 'hot work' at that energy level. I'm interested on opinions whether this task would require full arc flash PPE or not.

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Sometimes I only open my mouth to swap feet...

 

[stevenal]

Table 130.7(C)(9)(A) of NFPA 70E includes voltage testing and application of safety grounds. Hazard categories go from 1 through 4 depending on voltage level.

 

[rbulsara]

Yes, proving 'dead bus' is hot work, according to NFPA 70E and its interetation by "experts" in a good seminar I attended for arc flash analysis. Proper PPE will be required while measuring voltage (or lack of) on bus which is supposed to be dead.