Douglas Krantz - Technical Writer - Describing How It Works

Why Separate Feed and Return Circuits on Class A?

By Douglas Krantz | Life-Safety

Why Separate Feed and Return Circuits on Class A?


Why Separate Feed and Return Circuits on Class A?


Greetings Douglas,

In one of your books, "Make It Work - Conventional Fire Alarms", you make mention that for a class A circuit, that NFPA indicates the distance the feed and return pairs ought to be separated.

My question is, what would be behind NFPA's reasoning, and which chapter/section might I find reference to this in NFPA-72?

Thank You, DC

NFPA Code and the Law

The National Fire Protection Association, Inc. (NFPA) is a non-profit publishing house specializing in producing books showing how to protect from fire. By themselves, the published books are just reference books showing the absolute minimum requirements needed to protect from fire. Protecting people from fire saves lives; protecting people from fire is "Life Safety".

The NFPA, though, wanted their reference books to be read by more than those just visiting libraries. (When it's mostly library visitors reading the reference books, the fire protection books become forgotten like so many other reference books.), so they wrote the requirements in a form that governments could copy/paste directly into their legal code. That is the "Code" part of the NFPA Code. Once governments use the NFPA Code as part of their legal code, lots of people are required to read and follow the NFPA Code.

Class A Separation Shown in the Code

Where the NFPA Code itself specifies System Performance and Integrity for Class A signal pathways, the Code does not allow the feed and return signals to be in the same conduits (with a few exceptions). The Code itself, though, does not specify exact distances that are required for the separation.

NFPA Recommendations

In the back of the NFPA 72 Code book is the Annex or Appendix. In the annex associated with the legal code number, the NFPA shows their official recommendations for the absolute minimum separation for the feed and return signal pathways. To protect from mechanical damage, they show a minimum vertical separation or horizontal separation.

NFPA Comments

The NFPA does have comments that go deeper into their views on separations, but the only place you'll find comments from the writers and editors of the "Code" is in the National Fire Alarm Code Handbook, NFPA 72. The Handbook is available from the NFPA.org website along with the NFPA 72 Codebook. Yes, the Handbook costs more, but I think that the extra cost is darn well worth it.

The commentary text is written to assist users in their understanding and applying the provisions in the NFPA 72 Code. In the commentary, they explain more of why they have the recommend minimum separation. If possible, they also recommend more separation.

Redundant Pathway

In other comments, explanations, and annexes, they refer to the return pathway of Class A as a redundant path. The redundant path is so that if the feed pathway is interrupted, like a wire breaking or coming loose from a connection, the redundant path becomes a Class B pathway to connect the devices beyond the interruption to the panel.

In essence, what had been a supervised Class A pathway is now a pair of non-supervised Class B pathways. Even though the pathway has been interrupted, the devices beyond the interruption are still connected to the panel through the redundant Class A return. Much of the time, when a wire comes loose but before the pathway can be fixed, the redundant pathway allows the system to still detect fires and sound the alarm. (That's life safety.)

If there is no separation, if the feed and return are in the same conduit, and that conduit gets damaged, the Class A pathway is really no better than a Class B pathway; all devices beyond the break will be cut off from the panel.

Behind the Thought

Remember that this is the National Fire Protection Association. Their goal is that, at all times, people and property will be protected from fire. Because this is an imperfect world, that goal will never be reached, but using a properly installed Class A pathways, with proper feed and return separation, is a good step towards reaching that goal.



Douglas Krantz

facpdoug@douglaskrantz.com
Index
Residential
Life Safety
Descriptions
Electronics
Maintenance
Suppression
This website uses cookies. See Privacy for details.
Make It Work Series of Books by Douglas Krantz
Want Regular Updates on Articles Like These?



No Charge - Unsubscribe Anytime