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Topic Title: A warning to us all
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Created On: 07 August 2012 06:49 PM
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 08 August 2012 10:38 PM
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dlane

Posts: 690
Joined: 28 September 2007

Good to hear that your friend is doing well, I hope he continues to improve at a good rate.

Originally posted by: OldSparky

The cutout fuses didn't blow, they were still intact....

think safe chaps


This is the usual problem for arcing faults and is one of the root causes for the amount of heat generated by them.

At LV the arcing fault current is limited by the resistance of the ionised air it is travelling through and may be down to 30 to 40% of the short circuit fault current. Due to this protective devices often take longer to trip or may not trip at all.

Originally posted by: Fm

Strange that we wear arc protective gear when switching more than 60 amps, never did figure out where that figure came from.

No more complaining about the fire resistant overalls on hot days.....


It will depend upon the characteristics of the installation as to the heat produced from an arcing fault. The 60 amps may be due to the sizing of the protective device feeding it.

In my experience, at 200 amp BS88 fuses the arc energy produced is between 1 to 2 cal/cm^2 and general flame resistant PPE, good pair of gloves and an arc resistant phase shield will suffice.

When the fusing starts to get above 200 amps and up to the 700 amp level the arc energy can rise rapidly anywhere in the region of 8 to 20 cal/cm^2 and higher levels of protection are needed.

As a gauge to those figures it takes 1.2 cal/cm^2 of heat energy 0.1 seconds to cause a second degree burn on bare skin.

Flame resistant clothing is not the same as arc resistant. Under arcing faults flame resistant clothing can break open. Arc resistant clothing utilises different fabric weaves and construction methods to prevent them being torn open by the arc energy.

At the higher energy levels a proper arc resistant faceshield is a must as standard faceshield for grinding etc will melt due to the heat produced. We also tend to spec Nomex gloves or rubber gloves with leather gauntlets over the top if there are no issues with the dexterity required. You can also increase protection by wearing multiple layers of clothing and provide better comfort.

It is also important than non synthetic clothing is worn underneath flame / arc resistant overalls as synthetic clothing can melt and burn underneath overalls and cause injuries even though the overalls may remain intact. Any coat / rainwear will also need to be flame resistant as standard coats will burn over the top of the overalls and again will cause injuries.

Kind regards

Donald Lane
 08 August 2012 11:02 PM
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OldSparky

Posts: 592
Joined: 28 June 2011

so then what sort of current would be expected to flow with a dead short (say) and a 100amp fuse stay intact... with such a flash/ arc to be produced

i know it could be a complete guess..
 09 August 2012 12:55 AM
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alancapon

Posts: 5842
Joined: 27 December 2005

Originally posted by: dlane
. . . It is also important than non synthetic clothing is worn underneath flame / arc resistant overalls as synthetic clothing can melt and burn underneath overalls and cause injuries even though the overalls may remain intact. . .

I agree. We have overalls rated at 12cal/cm2 and are also provided with 100% cotton shirts & trousers etc to wear under them.

Regards,

Alan.
 09 August 2012 06:46 AM
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ebee

Posts: 5814
Joined: 02 December 2004

Old Sparky,
I`m sure we all hope your mate makes a speedy recovery.
It does bring it home that it happened to your mate (as being with someone we "sort of" know) rather than being told by some person we do not know.

As careful as we all try to be we can all have lapses/take short cuts and I`m sure that most of us have only had pure luck on our side at times, however smart we think we are.

Take me, when I talk to Electricians my own age and tell them I`ve never had a shock at working age (except of course the good old catch the capacitor ruse!) they tend to look at me in disbelief . Well as a 12 year old in hospital getting a belt from plugging in a TV with unsleeved pins and getting a shock that hurt and frightened me to bloody death, I always make sure I wont get a belt. Even so there have been times when I`ve been lucky (I did once thwack thru my side cutters on a piece of dead T & e that wasn`t to name just one instance).

Rock,
You seem to have a bit of animosity to us - the great unwashed. I did read thru your explanation a couple of times and have not quite grasped your point here! I have put that down to my misunderstanding of what you said rather than any error on your part and I`m sure I`ll "see the light" soon but please have a bit of patience with us though. We are not all brain surgeons

I`m one of those who is never happy to be told "when we do X the Y happens" I gotta see why Y happens - Say we put a whole cabbage in a box and out comes shredded cabbage I gotta see that there are blades inside the box causing this to happen. I did the same with the "Monty Hall Conundrum" as I mentioned before, that one hit me in a blinding flash two weeks after I`d given up on it and now it seems so simple I could kick myself.

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 09 August 2012 08:36 PM
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dlane

Posts: 690
Joined: 28 September 2007

Originally posted by: OldSparky

so then what sort of current would be expected to flow with a dead short (say) and a 100amp fuse stay intact... with such a flash/ arc to be produced



Breaking current for fuses can be seen from the characteristic curves. I just looked up a GEC 100A fuse and it will take around 150A continuously and takes about 20 seconds to trip with 300A going through it.

The current that will flow will be dependant upon the fault level of the supply , the supply voltage and the distance between the conductors.

A lot of arc faults will start of as a single phase to earth fault or a phase to phase fault. As the air ionises is expands rapidly and will engulf all 3 phases and will become a 3 phase fault. As the expansion continues the ionised air gets around the live side of fuses and then the upstream circuit feeding your 100A fuses can start to feed the fault. If the upstream protection is larger than your 100A fuse then the arc can go on for longer and your 100A fuse doesn't operate as the fault is being fed by the upstream supply.

At LV some arc faults will blow themselves out as there isn't always the energy and voltage level to maintain the arc which again may happen before the fuse blows.

Generally you wouldn't specify the arc energy level for circuits fed by a 100A fuse as it will be quite low. I have had a quick look at some surveys I have done and the closest I get is a 200A fuse which produced 0.8 cal/cm^2.

I also found a 315A fuse with a 10kA fault level that had an arcing fault of 4.9kA and an energy level of 6 cal/cm^2.

There are many factors involved in the calculations so really it has to be done on a case by case study to get the best answers but hopefully the above answers your question.

Kind regards

Donald Lane
 20 August 2012 10:29 AM
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VIRCABLE

Posts: 73
Joined: 02 December 2007

When testing live equipment, particularly at DBs, what are people's opinions on the minimum PPE that should be wore?
 21 August 2012 08:34 AM
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kj scott

Posts: 2144
Joined: 02 April 2006

A commercial site that looks at the issues surrounding arc flash.
Here

-------------------------
http://www.niceic.biz
 21 August 2012 09:20 AM
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Avatar for potential.
potential

Posts: 1287
Joined: 01 February 2007

Originally posted by: AJJewsbury

OP said "hager 3 phase board with 100amp 4 pole 30ma rcd main incomer.. " (my emphasis)
- Andy.


As a consequence the theory that somehow a circuit exists between two phases even when the test button is open is ruled out.

The test button circuit in a 4 pole RCD is routed between a single phase and the neutral.
IET » Wiring and the regulations » A warning to us all

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