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Topic Title: static discharge
Topic Summary: Extinguishing an electrical fire and getting a "shock"
Created On: 06 March 2013 03:19 PM
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 06 March 2013 03:19 PM
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davethomas

Posts: 136
Joined: 09 January 2004


One of my firefighters today was tackling a fire on a 3ph incoming supply in a basement switchroom to a large building. He said he felt a tingling in his hands whilst holding the CO2 extinguisher about a metre away and directing it towards the source of fire. The heat in the enclose was intense (buzzbars glowing), the current drain was apparently huge as the incoming cables were hot 4 metres away where they exited a supply cabinet. Smoke logging was heavy. He had full BA and PPE on. Discharge though carbon particles in the smoke due to ionisation- is this opossible? or his imagination? Any thoughts please.

Dave Thomas, fire officer
 06 March 2013 08:00 PM
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stableford

Posts: 64
Joined: 04 April 2006

As most boilers use an electrode in the flame path to ignite the gas then detect the small current passed through for flame detection, I would think it would be possible to get a voltage through the smoke path. Resistance would be relatively high- I would think, so the supply voltage would be a key factor in limiting current.
I think this would be something that fire departments would already have information on via their union technical reps.
I think a few people here would be interested if you do find more info, me for one, as I have worked on designing a CO2 flooding system for an 600V 4000A electrical room.
 06 March 2013 08:36 PM
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cookers

Posts: 203
Joined: 10 February 2012

A google search produced.

There have been several instances reported of static shocks from CO2 extinguishers in the fire service and other industries. The friction of one material being rapidly passed over another insulated material typically generates static electricity. While static electricity can occur over a wide range of atmospheric conditions, dry and cool environments are the most conducive and susceptible for experiencing the buildup of static electricity. Carbon dioxide extinguishers generate static electricity with the friction created as the high pressure liquid agent quickly passes up the siphon tube, through the valve and out the discharge hose assembly where it releases as a cold gas or snow. Because the rubber hose and insulated nozzle horn are non-conductive material surfaces able to accumulate the buildup of static electricity, these extinguisher components incorporate conductive wires or similar materials to help dissipate most of the static that is generated.
The ANSI/UL-154 Carbon Dioxide Extinguisher Design Standard, as well as the 2002 edition of NFPA-10, has specific references addressing these issues. Extinguisher service recommendations require annual continuity testing and labeling of these extinguisher components to ensure they remain capable of dissipating static buildup. With these components, the fire extinguisher operator carrying and supporting the extinguisher during discharge may experience a static shock, if the static buildup instead grounds itself through the hand and body of the operator. Setting the fire extinguisher cylinder directly onto the ground during discharge can help to reduce or eliminate the buildup of static electricity.

Engineers from [fire extinguisher manufacturers] stated that it is impossible for CO2 to conduct electricity through the gas. This would suggest the electrical panel was not the cause of the shock to the firefighter. The static shock produced will vary with atmospheric conditions from a mild shock similar to a blanket in the winter to a larger shock causing injury.

It is believed that the perfect conditions were encountered by the firefighter operating the extinguisher.
Enclosed Area: Basement room
Dusty Air: Large amount of dry chemical particulates on both floor and firefighter
Damp: Basement and or sweat on firefighter's skin
Cool: Basement or lower level of structure



Lessons Learned

Operators carrying and supporting the extinguisher during discharge may experience a static shock, if the static buildup instead grounds itself through the hand and body of the operator. Setting the fire extinguisher cylinder directly onto the ground during discharge can help to reduce or eliminate the buildup of static electricity.

Caution of other hazards including:
CO2 discharge may cause frost bite; operator must wear the appropriate PPE.
A loud noise will often be heard as the chemical is discharged.

Equipment maintenance should include:
Daily inspection of the hose and cone for damage should be done.
Look for proper service tags.
When sending an extinguisher in for service, the entire unit must go, including the hose and cone. This is done so the complete unit can be tested.
 07 March 2013 11:54 AM
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davethomas

Posts: 136
Joined: 09 January 2004

Your comments are interesting- I admit I was not aware that CO2 extinguishers had components to dissipate static. As an electrician with a background in physics, I have some understanding of this and thought it unlikely that 400v could have caused a shock over such a distance, but wondered if the smoke- (which we now call flammable fire gases, because it contains high levels of unburnt carbon) could have contributed?
Couple of other points - the firefighter felt a tingling whilst he was using the extinguisher, not a sudden discharge you would normally expect with static. He is perfectly fine by-the-way, but also puzzled by what has happened
It was a large basement switchroom in a modern air conditioned building- certainly not dusty or damp in any way- the only significant constituents of the gases around the area would have been, air, smoke, and the CO2.
I have never experienced or heard of this before and have seen (and carried out) exitinguishing of electrical fires using CO2 on many occasions. The only things that appear to be different in this case are the level and density of smoke and sheer amount of energy present at the conductors involved. It is easy to jump to conclusions and assume that because electricty was present, it must be the source: trouble is, I have no evidence which disproves this.

Incidentally, it was not possible place the extiguisher on the floor: we only use small extinguishers and the equipment was at chest height, it would have been held in both hands.
 07 March 2013 09:37 PM
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dlane

Posts: 685
Joined: 28 September 2007

Hi Dave,

Glad to here your firefighter is ok. My thoughts;

To break down air you generally need higher voltages or damp conditions. I would have expected your firefighter to have had a greater clearance from the conductors than the conductors had from themselves or any earthed enclosure around them, so if the smoke/air did become ionised and conductive I would have expected the current to flow between conductors or conductors to earth. If the enclosure was GRP of some form or another then that would need to be taken into account.

My research is in arc blasts and in this case when the air becomes ionised it heats up and glows so you will see the discharge in the air. WIth 400V system you also need to initiate the arc with an initial direct contact between conductors, higher voltages can initiate arcs without direct contact.

WIth regard to static, check the specification of the boots worn, my understanding is that some firefighters footwear have antistatic soles to prevent static discharge from building up but have enough resistance to provide protection against a shock. Any large amounts of static would be dissipated slowly through the soles of the boots. Also, what gloves, tunic and trousers were being worn, Nomex III is nonconductive but Nomex IIIA is antistatic which again prevents static build up.

Could this be muscle reaction or pins and needles if the firefighter had been in the same position holding the extinguisher for a while? We did have a case of someone who was sure they had a shock off an angle grinder that was eventually put down to a muscle reaction due to the amount of time spent holding the grinder as no faults could be found.

Kind regards

Donald Lane.
IET » Energy » static discharge

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