Similar to other electrical cables, fire-resistant cables can be either single-core or multi-core cables. Insulating materials may include elastic materials (XLPE, EPR, SiR, or LSOH) rather than thermoplastic materials (EVA, PE, or PVC) to meet stringent requirements for high-temperature environments. Some characteristics regarding the construction of fire-resistant cables include:
Conductor
Due to aluminum conductors' inability to withstand high temperatures, fire-resistant cables utilize solid round copper conductors, tightly stranded. Solid round copper conductors offer several advantages over fan-shaped conductors: When combined with mica tape, the integration of mica and conductors is tightly bound, beneficial for even distribution of electrical systems and enhancing the insulation performance of the electrical cables. Furthermore, with a melting temperature of °C, copper is not only an excellent electrical conductor but also ensures heat resistance.
Sheathed fire-resistant
The fire-resistant layer is the first line of defense of a fire-resistant cable and is typically made of materials with high heat resistance, most commonly mica. Outside the conductor core, there are usually 2 or more layers of mica tape wound around, with a cross-over ratio usually not less than 30%. The width, thickness, and coverage ratio of the mica layer must be accurately ensured during the manufacturing process to prevent leakage and breakage.
Insulating sheath
The insulating sheath is the next line of defense after the fire-resistant layer. The insulating sheath is made of cross-linked polyethylene. This material also protects cables from burning and ensures their long-term operation.
Outer sheath
The heat resistance of fire-resistant cables depends on the permissible temperature of the conductor and the outer sheath. The outer sheath of fire-resistant cables is prone to significant oxidation when being exposed to air. The higher the temperature, the more severe the oxidation process becomes. When the temperature exceeds 250°C, intense oxidation occurs, forming a layer of CuO oxide. This oxide can be toxic if inhaled or contacted by humans. Therefore, the outer sheath is often made of LSHF (Low Smoke Halogen Free) materials, which do not emit toxic smoke when burned.
>>Find out more: Typical types of electric cables
The fire properties of cables are of great importance from a fire safety perspective. To ensure that cable performs up to the mark on fire safety standards is critical to both manufacturers and end users. Wire & Cable India magazine wished to inquire into how important the fire safety is to a cable manufacturer. To know how manufacturers ensure the fire safety and what all standard procedures they have to stick to, here is a brief questionnaire with their responses:
Piyush Karia: Cables are used for transmission of electricity. Hence, efficient transmission of electricity is the basic value proposition that we offer to the customer. However, mere transmission is not enough. Safety and especially human safety is of prime importance. Cables, therefore, have to be shock proof as well as short circuit proof.
Fires have been a major cause of losses of both human life as well as property, and hence, safety from fire is one of the major areas of focus for us. 90 percent of the fires are electrical fires, mainly caused by a Defective Electric Circuit (DEC). DEC can be caused by overloading, short circuit, faulty installation, or due to poor quality of products used, which again could be poor quality of raw materials, or poor quality of manufacture, or both. The sufferer is mainly the common man, who has to bear the brunt of mistakes or cost cutting measures of someone else. We are acutely conscious of this fact and we have been the first Company in India who offered FR (Flame Retardant) cables as a standard feature on all our XLPE cables.
Since, cost becomes a major factor in almost all purchase decisions, especially for Industrial products, we constantly look to manufacture products that are less prone to catch and propagate fires without adding to costs. Hence, Research & Develop-ment is a constantly ongoing process for making safe cables without added costs.
There are different types of safety norms used for cables as per applications, all are important from safety, technical & commercial point of view. Few of those are:
We manufacture all of the above mentioned cables.
PK: Fire retarding properties in cable are of great importance. To make it simpler to understand, will take example of &#;Fire triangle&#;. It has got three sides &#;Oxygen (Air), Fuel (Flammable material) & Heat (Source of fire)&#;. If anyone of the component is missing, flame or fire will not propagate itself nor will it spread.
For making the cable safe from fire, we improve the properties of cable jacket and try and ensure self-extinguishing compounds, which even if there is external fire, they will not propagate the flames.
Technically it is narrated as below:
PK: Definitely manufacturers have to comply to all above mentioned tests wherever required, and hence has to have:
a. Knowledge and experience of manufacture of such cables, as well as have the expertise of end usage.
b. Has to have machinery for manufacture of these specialized products. For example, LSZH cables require a different extrusion ratio.
c. Has to have knowledge of formula-tion and engineering of raw materials.
d. Has to have knowledge as well as equipment for performance of specialized tests.
e. Obtain and maintain latest specification of processes and testing parameters. Obtain certification wherever necessary.
f. Constantly upgrade the raw materials and processes to ensure better products at optimum costs.
PK: Basically, PVC (Polyvinyl chloride) is a flame retardant material; it has this plus point over PE (Polyethylene). However, it is applicable for outer jacket only. As mentioned above depending on application one has to choose material for outer sheath.
As far as insulation is concerned, PE or XLPE (cross-linked polyethylene) has got more advantage over PVC. PE is not used in insulation of energy cables, and is only used in lower voltages of telecommunication cables.
XLPE is used in insulation world over, and it does not propagate flame, nor does it emit smoke or toxic gases and world over; XLPE is insulation and PVC or LSZH is used in sheathing, which is a safe proposition from the point of flame resistance.
There are developments going on to improve the property of Polyethylene to make it flame retardant, and we will see better standards implemented, which is now imperative especially after the few large fires where lots of valuable human lives have been lost.
PK: Unfortunately, we are not very serious about safety standards in our country. Costs still have precedence over safety. There is increasing number of cases of fire due to short circuit, and especially in crowded cities, and high rises, lots of lives have been lost. We do hope that these losses do not go in vain, and that awareness is also developed.
There are not very strong checks on quality of products, both at manufacturers&#; end as well as at users end. As stated earlier, installation can also play a big role in avoiding mishaps. While we see an increasing trend in industry, especially the large ones who buy cables directly, to focus on flame retardant products, the other segments need to bring in the focus on this aspect, especially builders and utilities.
Some high rises are now asking for FR/FRLS/ as well as Fire Survival cables and this trend is slowly increasing, which we need to speed up so that customers demand cables with additional fire safety features.
PK: Indian standards for cables e.g. IS (for PVC insulated Cables), IS (for XLPE insulated cables, or IS- 694 for wires and flexibles) covers the requirements for cables for safety against fire.On the cable or wire outer jacket additional properties against fire are mentioned by marking word &#;FR&#; or &#;FRLS&#;. By this user understands the cable type.
Internationally also, similar practices are followed and there are tests to monitor the performance of the cable under fire condition.
a) Smoke emission from cable during fire is monitored as under:
b) Halogen Acid gas emission from cable during fire is monitored.
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c) Flame retardant tests for cables installed in group
Depending on the specifications and end usage chosen, we have capabilities to manufacture and test all these products in house.
Amitava Bose: The genre of fire safe cables includes a wide variety. Universal Cables Limited manufactures all type of fire safe cables e.g., Fire Retardant (FR) cables, Fire Retardant Low Smoke (FRLS) cables, Low Smoke Zero Halogen (LSZH) cables and Fire Survival (FS) cables. Each of these types can be sub-categorized depending on their application and safety requirements.
AB: To answer this question, the effect of fire on cables needs to be explained. Cables are insulated and externally covered with polymeric materials which are organic compounds. The conventional polymers which have been in use for many years are halogenated materials. These halogenated materials contain Chlorine, Bromine & Fluorine. When the cables are exposed to fire, these polymeric materials start decomposing releasing halogen and dense toxic smoke causing asphyxiation and impairing visibility. It is reported that 80 percent deaths which occurs due to fire are not caused by burns; they are actually caused by the inhalation of these toxic gases. We can therefore realize that smoke generated from these conventional polymers under fire is the most important issue that a cable manufacturer has to consider. Considering this issue FR, FRLS, LSZH & FS cables have evolved.
AB: We as a cable manufacturer have to first ascertain the intended application, i.e., whether the cable is for indoor or outdoor application. Generally, for outdoor application where the cables are buried, Low Smoke Zero Halogen properties are not needed as the oxygen level is low and does not support burning. The outdoor cables installed over-ground do not need Low Smoke Zero Halogen properties as the dense toxic smoke has space to escape. For indoor application, Low Smoke Zero Halogen property is a necessity.
Dense toxic smoke released from cables during fire cause problems for evacuating people from the premises; it also causes hindrance for the fire fighters to extinguish the conflagration. In such cases, Low Smoke Zero Halogen cables should be the preferred choice. Also, there are applications, such as, emergency lighting system, elevators, fire alarms, water sprinklers, pumps, exhaust fans, etc. where the cables needs to operate under direct fire to supply power to these emergency equipments. For these applications, Fire Survival cables which can operate for a definite duration under direct fire without disrupting the integrity of the circuit, are recommended. These cables can even be designed to withstand fire, falling debris and water jet all at the same time.
Manufacturers have to select the proper source for such cable compounds and sometimes need to blend these compounds in their in-house facilities to meet the desired requirement.
AB: Yes. PVC has greater flame retardant properties than Polyethylene (PE), however, PVC is a halogenated compound and on decomposition releases chlorine gas which is highly toxic. On the contrary, Polyethylene (PE) does not have chlorine but unless cross-linked has a lower melting temperature. Since the outer covering is more vulnerable to the exposure of fire, cross-linked-PE (XLPE) is used as an insulating material which is encapsulated by outer coverings which are of Fire Retardant Low Smoke (FRLS) and Low Smoke Zero Halogen (LSZH) material.
AB: Safety is an important issue for manufacturers and users where safety against fire is considered to be paramount. Indian manufacturers follow International Standards as the complete specification is yet to be implemented by Bureau of Indian Standard (BIS). Utilities, such as, Metro Railways and Shipping Industries have special Standards on fire safety. Now-a-days builders are gradually shifting to Fire Retardant category of cables to mitigate the risk of fire. Moreover, LSZH cables support the Green movement for environmental safety.
AB: For fire safe cables, a combination of Standards are being followed:
&#; Test for Oxygen Index &#; ASTM D
&#; Test for Temperature Index &#; ASTM D
&#; Test for Smoke Density &#; ASTM D
&#; Test for Acid Gas evolution- IEC
&#; Flammability Tests &#; Category A, B, C as per IEC -3
&#; 3 m Cube Test &#; IEC /BS
&#; Fire Resistance Test &#; BS & BS
Our products meet the required specifications and have been tested in the National Laboratories, e.g., Central Power & Research Institute (CPRI) and Electrical Research & Development Association (ERDA).
Sanjeev Vyas: Cables manufactured by us are with improved fire performances CAT &#; C1 for domestic and industrial application which provides additional safety against fire.
SV: Enhanced fire retardant properties in insulation are of main importance from fire safety. Cables are insulated/jacketed by such blend of insulation which offers more resistant to the fire.
SV: The test method specified in ASTM D-95 (Measuring the minimum oxygen concentration to support candle-like combustion of plastics). At room temperature when the oxygen content in the air exceeds the oxygen index, the material will burn by itself automatically. The higher the oxygen index, the more retardant the cable will be. For example, if the oxygen index of a material is 21 percent, it means that the material will burn by itself even at room temperature because at room temperature the normal oxygen content is 21 percent.
Value of minimum oxygen index as 25 percent reasonably demonstrates fire retardant behavior of cable which required to be keep in mind while selecting insulation material for cables.
SV: No doubt PVC has more flame resistance as compared to PE, but PE has much better electric properties as insulation. In current situation, uses of PE is only recommended in underground cable as main insulation which is further protected /jacketed by PVC to provide reasonable fire protection.
SV: Nowadays, all buyers/utilities are very serious against fire safety in cables and incorporated certain fire test in the specification. Indian Standard has also amended the cable specification with introduction of FR/ FRLS type cable as new varieties considering fire safety.
SV: There are various standards which represent fire properties of cable such as Flammability test as per IEC -1/BS -1 (Flame Test On Single Vertical Insulated Wires/Cables); Oxygen Index Test as per ASTM D; Flame retardant test as per Swidish standard &#; SS ; Flame retardant test as IS: (61 & 62); IEC -3/BS -3 (Flame Test On Bunched Wires/Cables); IEC Fire Resistance Test; BS Fire Resistance Test.
We need to have certification from BIS /Central Power Research Institute as appropriate. Our cables are capable to meet above requirements.
V. K. Gupta: We are manufacturing cables as per customer&#;s requirements and as per national/ international specifications, using insulation and jacket material as required. We are offering cables with fire retardant PVC, fire retardant low smoke PVC, extremely low smoke and halogen free compound, TPU to have fire resistant parameters besides highly abrasion resistant property, PTFE cables for high temperature application having low smoke and zero halogen.
VKG: The fire properties for cables are of great importance. General purpose PVC produces excessive black smoke which decreases visibility and produces highly toxic suffocating gases and irritating drips on account of chlorine present in PVC compound. PVC means poly vinyl chloride and it produces acidic droplets besides smoke.
PE compound is used for insulation to have better insulation resistance parameters, but is highly inflammable and fire does not extinguish rather propagates. Smoke and toxic suffocating gases create lot of problem for the persons entrapped and for the fire fighting personnel. The persons cannot see clearly the exit way and feel suffocation and irritation on skin when comes in contact with droplets. The cables should be free from these hazardous elements, and should not propagate fire. In case of fire in plant or at any other place, the burning elements should not produce black and suffocating smoke which results in to zero visibility and difficulty in breathing. This reminds me of Uphar incident when a lot of persons could not escape because of these two culprit elements.
Now, lots of advancements have taken place and various ingredients are available which if mixed with PVC compound can reduce smoke and toxic gases. We are offering cables with fire retardant compounds which reduce fire propagation and there are fire retardant low smoke compounds which besides reducing fire propagation produces low smoke. With another ingredient EVA, we are able to make cable compound which will produce almost white smoke and with very-very low halogen content toxic gases.
We are also offering next generation cables i.e Fire Survival cables which maintain circuit integrity for minimum 3 hrs up to 950 deg Celsius under fire, water and shock conditions to ensure transmission of signals and communication.
VKG: Almost all standards organizations in different countries have specified very stringent fire test parameters for cables, which need be complied to ensure fire safety. The important standards being followed in our country are ASTM Standards (American Society for Testing and Material), IEC Standards (International Electro technical Commission), NES (Naval Engineering Standards-USA), UL (Underwriters Laboratories), Indian Standards, British Standards, Swedish Standards, etc.
These standards specifies various requirements for smoke density rating, halogen content, temperature index, oxygen index, Corrosivity of combustion gases, pH index, flammability tests under different conditions, circuit integrity test, light transmission under smoke, etc.
The cables being offered by us comply all above test parameters.
VKG: Yes, PVC has some flame retardant properties and does not propagate flame easily as PE does. Some ingredients are now available which if mixed with PE it can reduce its inflammability. PE or XLPE are used because of its inherent better electrical/mechanical parameters over PVC e.g. high volume resistivity, low dielectric constant, high dielectric strength, low power factor. For HT cables XLPE is a must.
VKG: Institutional buyers/builders are now quite aware, serious and sensitive about fire safety and procure cables meeting stringent parameters and tests for fire safety requirements from vendors with proven track record & having all in house testing facility with complete in process quality control process. Manufacturers in organized sectors have responded equally and use virgin raw materials meeting different test requirements with zero tolerance.
VKG: With growing awareness about fire safety, Bureau of Indian Standards have incorporated various tests as per international standards in its different cable specifications and does all required tests as per these standards, and manufactures meeting test parameters are allowed to put ISI seal/emblem on their cable. ERDA, CPRI and various other government test labs also conduct these tests for the benefit of manufacturer and buyer to certify the meeting of requirements. FSAI (Fire & Safety Association of India) also certifies cables meeting fire safety tests and they allow approved vendors to put their mark. Some international agencies like BASEC, UL, CE, etc also certifies the fire safety requirement of cables.
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