When you’re planning a network, you usually think about the equipment you need and how you can connect it. That’s already plenty to consider, but it turns out that your responsibilities run deeper than that. You also have to think about safety.
Sure, you want to avoid tripping hazards, but network designs also have to closely consider fire codes. After all, you’re running cables that carry electricity, and that comes with certain fire risks.
In the world of Ethernet, the fire risks are well understood and carefully controlled by governing agencies. In order to keep up with fire codes, what you really need to understand are Ethernet jacket ratings.
Befre we get into the technical aspects of standards and ratings, let’s take a moment to get everyone on the same page. We’re specifically talking about the sheaths that go around cables, and today, we’re focused on the sheaths for Ethernet cables. As soon as other types of cables enter the discussion, the standards and ratings get more complicated. But when we focus only on Ethernet, we can group everything into four categories of ratings.
By understanding the ratings (as well as who is in charge of them), you can easily assess which Ethernet cables are right for specific jobs, especially if you’re running them through the guts of a building.
Standards for Ethernet Jackets
That brings us to the standards. The ratings in the following sections are controlled by the National Electric Code (NEC). This is a federal regulatory outlook on fire safety, and it sets standards of regulation for a whole lot of things in building code. Generally speaking, the NEC makes up the minimum safety requirements for electrical installations, and it is highly regarded among state and local regulations.
To put it another way, a state or locality might have more stringent rules than those found in the NEC, but the NEC typically outlines the lowest level of acceptable safety.
Considering that, the NEC rates different types of Ethernet jackets, and most states and local governments align with these ratings.
What are the ratings? There are many when you look at all cables, but Ethernet cables fit nicely into four categories: plenum, riser, outdoor, and general use. Each of the categories has official designations, but if you see any of these indicators on cable packaging, then you know what you’re getting.
Plenum is formally referred to as CMP-rated cable. This is the highest rating that the NEC provides for Ethernet cables, and it makes sense when you consider the use and fire risks related to plenum cabling.
The word “plenum” refers to spaces designed specifically for air circulation. Your HVAC systems operate in the plenum. It can include air ducts or other designs, and plenums often occupy space between a drop-down ceiling and the structural top of a floor. The plenum can also be in similar gaps that run through flooring.
Any cable that runs through the plenum could easily carry fire from one room to another, one floor to another, and even one building to another. As a result, plenum fire safety requirements are the most stringent, and CMP-rated Ethernet jackets are the most fire-resistant.
If you want to run Ethernet through the plenum of a building, it must be plenum-rated cabling, or you have to run fire-code-approved conduit to house the cables.
The next classification is CMR. This rating is for cables that run through the riser of a building. A riser is any part of a building design that connects vertically between floors. This can include conduits, pipes, and even the cables themselves. The important point is that the thing in discussion does travel from one floor to another.
Thinking in terms of fire safety, you can see why this is important. Any cables that span multiple floors can potentially carry fire from one floor to another, dramatically increasing total risks.
As such, CMR is the second-most-stringent rating for Ethernet cables. Any Ethernet that does span multiple floors (whether behind walls or not) must be CMR.
Riser jackets are actually the most common for Ethernet since many cables are designed to safely operate in this manner. Still, you need to check the rating before purchasing riser cables.
The outdoor classification (CMX) is a little different. It’s not more or less stringent than other Ethernet jackets. It has a different purpose, and as such, it is made with different concerns in mind.
Generally speaking, outdoor Ethernet jackets are not protected against fire. If for some reason you needed to use outdoor cables in an indoor setting, they would only meet the general-use classification.
The purpose of the CMX rating is to provide known protection against UV rays. Ultraviolet light can easily damage cable jackets and the contents inside. Using Ethernet in outdoor applications that is not UV-resistant creates problems, as your cables will not last long unprotected.
However, this UV protection does not enhance fire protection, so do not use outdoor-rated cables incorrectly.
The last classification is CMG. Some labels will shorten the rating to “CM.” If you’re using audio-visual cables instead of Ethernet, this same jacket rating is often classified as CL.
Regardless, these types of cables have no extra fire protection. The plastic jackets on these cables will burn like any other plastic. As a result, these cables can easily carry fire along their runs.
That’s why general-use Ethernet is only allowed to connect devices within the same room. It cannot run from one room to another.
This makes general-use cables fine for short patch work and even in a number of data-center applications. As soon as you need to connect devices in different rooms (much less different floors), general-use jackets are out.
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