LSHF cables are made up of halogen free compounds that are good fire retardants but emit less than 0.5% hydrogen chloride gas and smoke when burnt. In case of fire these cables produce small amounts of light grey smoke and HCL gas which greatly increases the chances of escape from populated areas. There’s no PVC in these cables, hence no harmful fumes or dense black smoke are given off in case of fire.

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• They are ideal for public buildings like airports and railway stations, computer rooms, datacentres, where escape is limited and fire safety is critical.
• They emit <0.5 % of HCL gas thus providing a safer environment in the event of a fire.
• The low smoke generation ensures evacuation routes and signage remain visible during a fire.

PVC vs LSF vs LSZH Conclusion

LSF cables are flexible and low cost alternative to PVC cables but can still produce a dangerous amount of toxic gas and smoke. Whereas LSHF cables are less flexible and a higher cost but with a significant reduction in toxic gas and smoke. So in a high risk populated area where escape is limited LSHF cables are strongly recommended. But in low risk areas where the evacuation is easy and high flexibility is required, PVC could still be a good choice.

Though all of the above perform very differently with regards to the amount of toxic fumes and smoke they produce however they all could, and in many cases do, pass the CPR test to class Eca.

So where does CPR come in I hear you ask!

Most cables designed for permanent installation within domestic, residential and commercial buildings are now subject to the Construction Products Regulation (CPR), covered by BS EN 50575 which became a legal requirement in July 2017. The standard is primarily about the spread of fire and heat release, not the toxicity of gases or smoke given off.

The Euroclasses

The regulation classifies products into one of seven classes, but in most instances, only five classes will apply to cables. Lower classes Fca and Eca undertake a basic vertical flame test to BS EN 60332-1-2. If there is a high level of flammability, it would be classified to Class Fca (testing to Fca can be conducted “in-house”). However, for the cable to meet the requirements of Class Eca, the test has to be conducted by an authorised test house, known as a Notified Body (NB) or Approval Body (AB).

In the test, a single cable of approx. 60 cm is mounted vertically using two clamps, a flame is applied to the bottom end for 60 seconds (or 120 seconds in the case of cable diameters greater than 25 mm). The test is deemed passed if, after the flame has been removed, the burning cable extinguishes itself and the fire damage is less than 425mm. It is irrelevant how long the cable burns before extinguishing itself.

Great! I hear you say! “All I have to do is ensure that the cable I install is at least CPR class Eca compliant and I don’t have to worry about what material it’s made from!”

If only it were that simple!

The 18th edition of BS 7671 recognises that where applicable, cables need to meet CPR requirements and carry a Euroclass for fire performance. BS 7671 does not specify or recommend which Euroclass to use in an environment or application, and notably, nor does it outlaw using cables that emit toxic fumes and smoke in the event of fire. Specifiers and installers must therefore ensure the cables they select are appropriate for the fire risks in the building or application and any contractual terms.

Part B of The Building Regulations does offer additional guidance.

“The primary danger associated with fire in its early stages is not flame but the smoke and noxious gases produced by the fire. They cause most of the casualties and may also obscure the way to escape routes and exits. Measures designed to provide safe means of escape must therefore provide appropriate arrangements to limit the rapid spread of smoke and fumes.”

“Okay, so I simply specify the highest CPR class I can find, just to be safe!”

Unfortunately specifying to what many might consider to be a ‘safe’ level might prove cost prohibitive for the contractor, who may also discover cables to higher classifications scarce or impossible to come by. Test procedures to gain even Cca compliance are costly for manufacturers and can involve lengthy R&D to meet the standard – there are many cable types where it’s simply not cost effective to test to that level.

Another complicating factor is that cables redesigned to pass higher CPR classes may shift in electrical or data performance and physical properties - factors that may effective how they perform or their method of installation.

Lower CPR classes focus on spread of fire, not toxic fume and smoke emissions. Specifiers considering the emissions of cables when burnt need to choose Low Smoke Halogen Free (LSHF) but beware, as we’ve already learned, there are plenty of ways terminology and acronyms can trip you up.

CMP vs LSZH: Fire Rated Ethernet Cabling Explained

Written by Don Schultz, Technical Sales Representative and Fluke Networks Certified Technician

Fire. We could not have survived to this point without it. For thousands of years we have used it to cook, heat our homes, generate energy, provide light, and more. Fire is a powerful ally. When misused, or allowed to get out control, fire is simultaneously a powerful enemy capable of destroying and causing great bodily harm.

Today we will be talking about two competing fire ratings you are likely to see stamped onto an Ethernet cable jacket. This is important, as one is far more effective than the other. Read on to avoid getting burned! 

In today’s commercial buildings, oftentimes the plenum space is used to move air around for circulation. The plenum, or HVAC space, is often found below raised floors or above suspended ceilings. In a tight confined space, fire can propagate quickly. Complicating matters, these confined spaces are used to channel air conditioning and heat. When fire ignites, smoke usually follows. Smoke is typically toxic to living organisms and some kinds of smoke are more toxic than others.

A likely spot to run Ethernet data cabling is the plenum space. Ethernet data cable has flexible cable jacket material made out of plastic; specifically PVC or polyvinyl chloride.  

Smoke emitted by untreated burning PVC is extremely toxic, and the last thing you want your ventilation system bringing into your work space is flames and smoke at a very high speed.

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A plenum is any enclosed area that facilitates environmental air handling like an air conditioning duct or an air routing drop ceiling. It can be any air space between walls, under floors, or dropped ceilings.  

Enter Firing Ratings

Given the safety risks associated with running PVC clad cabling through a building, a safer plastic jacket was necessary.

In 1979, British Research Engineering developed the LSZH cable jacket (Low Smoke Zero Halogen) for use in commercial plenum spaces, submarines, and nuclear power plants. This  jacket type was developed in and used by European countries and further adopted by Asian countries. PVC was combined with additives to make it less...offensive.

Simultaneously, in the late 1970s the search for these same attributes began in the United States at Underwriters Laboratories (UL). The result was the development of the UL910/NFPA 262 test for fire safety in commercial plenum air spaces. In the early 1980s, the plenum rated cable standard (CMP) was developed and incorporated in the NEC or National Electric Code under section 90A.

The aim was to address:

• Toxic smoke due to burning PVC

• Flame retardance, so that the cable itself did not make the fire worse

• Flame resistance, so that the cable was less likely to start burning in the first place

So, we had two competing standards. This normally might not be an issue, but LSZH cable jackets do not entirely satisfy the ultimate goal of a safer cable jacket. The reason has to do with the additives to the PVC plastic not aging well, or performing differently under real world conditions versus laboratory testing.

CMP rated cable jackets use different additives, to the point where less than 50% of the cable jacket by weight is plastic. A large amount of the additive is Teflon. If you have ever wondered why plenum rated cable feels different from riser or plain CM rated cabling, that is why.  

In order for an Ethernet data cable to be acceptable for commercial plenum use in the United States, it must be CMP rated in accordance with UL910/NFPA 262 protocols.

So, which cable jacket comes out on top when it comes to fire safety? Well, BER or British Engineering Research did some tests. All pictures are courtesy of Fire Performance of Data Communication Cables as published by The Society of the Plastics Industry, Inc.

Here is the flame promotion result: 

Here is the smoke volume result: 

As the two illustrations make clear, LSZH is no substitute for plenum rated cable. LSZH may be a safer alternative to outdoor (CMX) cable jackets being used indoors, but right now that is all it has going for it.

Now, we know that LSZH does not live up to expectations in regards to spreading fire or the amount of smoke generated. Just how toxic is the smoke when these cables burn? The toxicity in this case is demonstrated by carbon monoxide found in the smoke: 

Huh.  No win here either.  LSZH is three times more toxic when burning.

Ok, so LSZH must be better at stopping a fire in the first place. Right? Well…

 When CMP burns, this is how hot it gets in an enclosed space like a plenum area.  Approximately 350 degrees C or 662 degrees F.

When LSZH burns, this is how hot it gets. Yes, that is showing an unreal 800 deg C or 1,472 deg F. This is enough to melt many metals, and distort steel building frames.

So, now you know how LSZH performs in comparison to CMP rated Ethernet cable. The main takeaway is that you don’t dare use LSZH as a substitute for CMP and never ever use CMX in large quantities inside a commercial building!

Happy Networking!

trueCABLE presents the information on our website, including the “Cable Academy” blog and live chat support, as a service to our customers and other visitors to our website subject to our website terms and conditions. While the information on this website is about data networking and electrical issues, it is not professional advice and any reliance on such material is at your own risk.

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