The 3 Things You will learn today are as follows.

1. What are the Wiring Classes (A,B,X) defined in NFPA 72
2. The World's top Fire & Gas Standard.
3. Habits that make you world class.

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The Brain [🧠]

Last week,

We learnt about

Addressable V/S Conventional F&G Communication System.

Nearly 4,000+ People shared that they found it valuable.

So thought to go a bit in-depth

On how to Wire them.

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To Clear the Confusion.

The wiring methods that you will learn today (As per NFPA 72)

• Class A
• Class B
• Class X

Can be used to Implement both Addressable or Conventional Communication System.

I am so excited to share this with you.

Let just begin the fun ....

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#1 -Engineering Learning

Level 1: Class B Wiring

This is the simplest arrangement.

The cable leaves the panel.

Then passes via all devices.

It stops at the last device.

Advantages of Class A

╰┈➤ Less cable.

╰┈➤ Lower installation cost.

╰┈➤ Easier installation and maintenance.

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The Super Danger Risk

If the cable is cut.

Devices beyond the cut are lost.

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Example:

Imagine 10 detectors connected on one circuit.

If the cable breaks after Detector No. 3:

• ✅ Detector 1 works.
• ✅ Detector 2 works.
• ✅ Detector 3 works.
• ❌ Detector 4 to 10 are lost.

The panel reports a fault.

But a portion of your protection is gone.

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Now What to do ?

If you know how we engineers think.

This is non-good.

We think smart ways to avoid fault.

Let's look at what engineers designing the NFPA standard though about this.

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Level 2: Class A Wiring

So they created Class A.

The cable leaves the panel.

Passes through all devices.

Advantages

╰┈➤ Higher reliability.

╰┈➤ Better fault tolerance.

╰┈➤ Survives a single cable break.

Now imagine the cable gets cut.

Result:

• Detection remains active.
• Devices remain operational.
• Panel reports a fault.

The system is damaged.

But it is still working.

BUT ......

A Big BUT ......

A Big Big BUT ......

What if ? (If there's a Short-Circuit)

What if the cable is not cut?

What if the conductors touch each other?

In simple words : called a short circuit.

A short circuit can be more severe than a simple cable break.

To solve this problem.

Engineers created Class X.

These devices automatically separate the faulty section from the healthy section.

Result:

• Survives a cable break.
• Survives a short circuit.
• Most devices continue operating.
• Highest level of survivability possible.

This is why Class X is commonly used in critical facilities and below industries (Not for all but Critical Areas only).

• Refineries.
• LNG Plants.
• Offshore Platforms.
• Petrochemical Complexes.

Anywhere loss of detection cannot be tolerated.

I like to call it:

"The Special Forces version of Fire & Gas wiring.

Maybe the NFPA Engineering team was watching X-men hence they call this wiring Class X - Hope this makes it easier to remember" 😄

PS: Would you not want to select a Cable (For Fire & Gas) that works when there is an actual fire ?

If yes, then this Video here will help you learn the wiring selection.

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The "Most Common" Mistake

This is not correct

You can have:

• Conventional with Class B Wiring
• Conventional with Class A Wiring
• Addressable with Class B Wiring
• Addressable with Class A Wiring
• Addressable with Class X Wiring

These are separate design decisions.

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Which One Should You Select?

Class B

Best when:

• Budget is important.
• The system is relatively small.
• Loss of a few devices during a fault is acceptable.

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Class A

Best when:

• Reliability is important.
• Continuous detection is required.
• A single cable cut should not disable the system.

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Class X

Best when:

➜ Maximum reliability is required.

➜ Harsh industrial environments exist.

➜ Process safety is critical and failure is catastrophic.

➜ Loss of detection is unacceptable and the Plant budget allows it.

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A Simple Rule to Remember

Addressable vs Conventional = How devices talk.

But Wiring class is how devices are connected.

• Class B = A cable break can disable devices.
• Class A = Survives one cable break and still functions.
• Class X = Survives one cable break and one short circuit.

If you remember only these four lines.

You understand 90% of the topic.

PS: Would you not want to select a Cable (For Fire & Gas) that works when there is an actual fire ?

If yes, then this Video here will help you learn the wiring selection.

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#2- Engineering Question

Corrosivity Class

If your outer environmental Corrosivity Class is lets say C5.

If you will select lower class ?

Then Client could question that the same Air (C5 corrosive air) will also come inside your control room.

How can you select low corrosivity class ?

How will you answer this question ?

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#3 -Career Tip​

People still assume PDF is sufficient for Job Openings.

The best Job opportunities are not in newspapers anymore.

This is very under-rated.

But trust me...... You will reap the fruits in future.

The People from Exxon / Shell / Technip etc have always reached out to me on LinkedIn and never via Job Portal.

Your linkedin presence matters.

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#4 - Your Feedback

How was todays email lesson?

Reply "1" - Found Super Valuable

Reply "2" - Somewhat valuable

Reply "3" - Needs Improvement

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Until next week,

Happy Learning and Keep Smiling.

​Asad Shaikh​

Your super passionate Instrumentation Friend!!

See you next Week!

​www.Asad-Shaikh.com​

PS : Here is the video to learn the Wiring Selection (Especially if you want your Fire and Gas Cable to work in Actual Fire)