A guide for preventing and extinguishing electrical fires

How to put out electrical fires & 6 other fires that are not compatible with water？

It is vital to stay calm, act swiftly, and use the correct extinguisher when confronted with a laboratory fire that is small enough to be contained.

Take the time to review reagent SDS information prior to commencing reactions, particularly noting ‘Section 5: Firefighting measures’, which can save valuable time when faced with a fire. Also be sure to know the location of fire exits, fire blankets, alarms, and extinguishers.

While water may seem like an obvious choice, under certain conditions it can intensify flames and escalate danger, which should be avoided in settings where flammable and hazardous reagents are just around the corner.

Fires that should not be put out with water

1. Electrical fires:

Electrical fires, also known as Class C fires under the American classification system, are fires caused by electrical equipment (e.g. old faulty wiring and appliances, spills on electrical appliances, improper use of extension cords, etc.). Stopping these fires with water can result in explosions and electrocutions due to the conductive nature of water.

• Turn off the power source or unplug the appliance if safe to do so. Then water or a foam extinguisher can be used.
• If you are not able to turn off the power source, use a dry chemical or a CO₂ extinguisher. A water mist extinguisher may also be used.¹

2. Substances and mixtures which in contact with water emit flammable gases (Category 1, 2, & 3):

These substances undergo spontaneous chemical reactions with water to produce flammable gases that may facilitate larger fires. Included in this group are alkali metals (e.g. sodium, potassium, etc.), alkaline earth metals (e.g. magnesium, barium, etc.), and compounds such as calcium hydride. Sodium reacts with water for example to release flammable hydrogen gas and heat:

2 Na(s) + 2 H₂O(l)   2 Na⁺(aq) + 2 OH^-(aq) + H₂(g)

• Use dry sand, dry chemical, dry powder, or alcohol-resistant foam to extinguish. Consult the SDS for specifics,

3. Chemicals that produce toxic gas:

Chemicals such as aluminum phosphide, phosphorous trichloride, and dichlorodimethylsilane    react with water (some violently) to produce gases that are toxic to human health. For example, aluminum phosphide reacts with water to produce the highly toxic phosphine gas.

2 AlP(s) + 3 H₂O(l)   Al₂O₃(s) + 2 PH₃(g)

Phosphorous trichloride also reacts with water to afford phosphorous acid and hydrochloric acid, both of which are corrosives and irritants.

PCl₃(l) + 3 H₂O(l) H₃PO₃(aq) + 3 HCl(g)

• Use dry sand, dry chemical, CO₂, or alcohol-resistant foam to extinguish.

4. Peroxides of alkali metals:

Peroxides of alkali metals (e.g. sodium peroxide, potassium peroxide, lithium peroxide, etc.) react violently in contact with water and many other substances.²

• Use dry sand, dry chemical, or in some cases alcohol-resistant foam or a CO₂ extinguisher.

 5. Corrosive materials:

Corrosive materials (e.g. HNO₃, HBr, H₂SO₄, KOH, etc.)  are those that can damage and burn living tissue such as the skin and eyes. Recall that when performing acid/base dilutions, water should never be added to a solution because the heat generated may cause the mixture to splash out, potentially inducing bodily harm. Instead, the acid or base should be slowly added to the water with constant stirring. In a similar manner, fires involving corrosive substances should not be heavily doused with water.

• Use dry sand, dry chemical, alcohol-resistant foam, or a CO₂ extinguisher. In some cases, a water spray can also be used (refer to the SDS).

 6. Flammable liquids:

Fires involving flammable liquids (e.g. acetone, xylene, etc.)  should not be heavily doused with water since water sinks below the flammable substance only serving to facilitate its spread.

• Use dry sand, dry chemical, alcohol-resistant foam, CO₂, and in some cases a water spray extinguisher.

7. Class K commercial kitchen fires with oils and grease:

Adding water to hot oils and grease can result in dangerous crackling and splattering, which only serves to spread the fire.

• Use a wet chemical extinguisher, which cools and creates a barrier between the oxygen and fuel.

What type of fire can be put out with water?

Class A fires involving solid non-metal combustible material including wood, paper, cloth, and many plastics can be safely extinguished with water.

Do not fight a fire if³:

1. You do not have appropriate firefighting equipment. Your extinguisher should be the correct class and size.
2. Toxic gas or excess smoke is produced.
3. You are poorly positioned relative to the fire. Ensure that you are at least six feet away and pointing the extinguisher at the base of the fire. Moreover, you should not be positioned above the flames as some fires may grow vertically. There should also be an unobstructed emergency exit behind your back, so you can easily leave the building if the fire cannot be contained.
4. The fire is too large: large fires can cause physical harm and structural damage to the building, which should be addressed by professionals. Always call for help prior to considering the use of a fire extinguisher.
5. If your first attempt with an extinguisher is unsuccessful, do not endanger yourself further.

Reference:

1. Water Mist | Amerex Fire (amerex-fire.com)
2. Peroxides and how to deal with them 28-3-02.doc (bnl.gov)
3. Fire Extinguishers - Facilities & Services (utoronto.ca)