The fire triangle needs fuel, oxygen and ignition. In a hazardous location, which side of the triangle is the electrician’s job to take away?

C. Ignition — sparks and hot surfaces from the equipment

What does an "explosionproof" (flameproof) enclosure actually promise?

B. If gas gets inside and ignites, the enclosure contains the explosion and cools the escaping gases so they cannot ignite the outside

C. The equipment inside cannot make a spark

D. It is sealed airtight so it can be used underwater

Intrinsic safety protects by…

B. limiting the energy in the circuit so that even a spark or hot surface cannot ignite the atmosphere

A. containing an explosion in a heavy enclosure

C. keeping the enclosure at a positive pressure of clean air

D. filling the enclosure with sand

A device is marked with a temperature code (T-code). What does that code tell you?

B. The maximum surface temperature the equipment can reach, which must be below the ignition temperature of the gas or dust present

A. The ambient temperature it can be installed in

C. The temperature rating of the conductors to use

D. How hot the process fluid is

Why are conduit seals installed at the boundary of an explosionproof system?

B. To stop gas and flame travelling through the conduit from one enclosure or area to another

A. To keep water out of the conduit

C. To hold the conductors in place

Two systems are used to classify hazardous locations in North America: Class/Division and Zones. What is the main idea behind the Division or Zone number?

B. How likely and how often the hazardous atmosphere is present

D. Which utility supplies the site

You are about to open an explosionproof junction box in a classified area to add a conductor. What comes first?

B. Confirm the circuit is de-energized and locked out, and that the area is gas-free or a hot-work permit covers it

A. Loosen the cover bolts slowly

C. Spray the flange with lubricant

D. Nothing — explosionproof boxes are safe to open live

Quatrième année Lesson 7 of 12 · about 14 min

Hazardous locations: the ideas behind the rules

What you’ll learn

Why electrical equipment is the ignition source a hazardous location has to control.
The two classification systems — Class/Division and Zones — and what the grade means.
How each protection method works: containing, limiting energy, keeping gas out, or avoiding sparks.
What a temperature code is for, and why seals, bolts and flame paths matter on every job.

Before this: Commercial wiring methods · Grounding and bonding as a system

A hazardous location is anywhere flammable gas, vapour, dust or fibres may be in the air in enough quantity to burn or explode. The fuel and the air are the process’s; the spark or hot surface is usually ours. Every rule in the hazardous-location section of your code book — CSA C22.1 in Canada, the NEC in the US — is a way of taking the ignition out of the picture. This lesson is the ideas; the book has the requirements and the tables.

The fire triangle. The process supplies the fuel and the air is always there, so hazardous-location equipment is built around one corner: ignition.

How a location is classified

Classification answers two questions: what could be in the air, and how likely it is to be there. North America uses two systems for this.

Class and Division. The class says what kind of material — gases and vapours, combustible dusts, or ignitable fibres. The division says how likely: present in normal operation, or only when something goes wrong (a leak, a broken container). Materials are further sorted into groups by how easily they ignite and how violently they burn.
Zones. The international approach. Gas zones and dust zones are each graded in three steps, from “present continuously or for long periods” down to “only briefly, under abnormal conditions”. Gases are grouped by how easy they are to ignite.

Canada’s code puts Zones first for new work, with Class/Division kept for existing installations; the US code carries both in separate articles. You will meet both on plant drawings. The point for you is the same either way: read the area-classification drawing, then install equipment marked for that classification. Deciding the classification is an engineering job.

The protection methods, by idea

Every certified method does one of four things. Knowing which one tells you what you can break during an install.

Idea	Method	What defeats it on the job
Contain the explosion	Explosionproof / flameproof enclosure	Missing or loose bolts, scratched or painted flame paths, a gasket that was never meant to be there, unsealed conduit
Limit the energy	Intrinsic safety (IS)	Mixing IS and non-IS wiring, the wrong barrier, adding cable or devices the loop calculation did not allow for
Keep the gas out	Purging / pressurization, encapsulation, oil or sand filling	A door left open, a failed purge alarm ignored, a cracked potting
Don’t make a spark	Increased safety, non-incendive and non-sparking equipment	Loose terminations (they heat and arc), the wrong lug or terminal, changed internal parts

Temperature codes

A spark is not the only way to light gas. A surface hot enough will do it with no spark at all — every flammable material has an auto-ignition temperature. So hazardous-location equipment is marked with a temperature code: the hottest its surface can get. The rule of the idea is simple — the equipment’s maximum surface temperature must stay below the ignition temperature of what is in the air. The codes, the ignition temperatures and how dust layers change the picture are in your code book and the product marking.

Seals and flame paths

An explosionproof system is not a box, it is a system of boxes and conduit. Conduit is a pipe that would happily carry gas from a classified area into a control room, or carry an explosion from one enclosure to the next. Seals — fittings poured full of compound around the conductors — block that path. They only work if they are in the right place, dammed properly, and poured full. A flame path is the machined joint between cover and body: long and tight, so hot gas escaping an internal explosion cools before it gets out. A burr or a coat of paint on it turns a certified enclosure into an ordinary one.

Worked: matching a T-code, as an idea

Practice numbers, not code values: the gas on a drawing has an auto-ignition temperature of 280 °C (536 °F).
A light fitting’s marking says its surface can reach at most 200 °C (392 °F).
200 °C is below 280 °C, so on temperature alone the fitting could not ignite that gas: margin 80 °C (144 °F).
A second fitting reaches 300 °C (572 °F) — above 280 °C. It could light the gas with no spark at all. Temperature is a separate check from the class/group or gas group marking; both must suit the area.

Sur le chantier

Tips

Find the area-classification drawing before you order material. No drawing, no guessing — ask.
Treat every cover bolt as part of the certification. Count them in; count them out.
Wipe flame paths clean with a rag, never a file or a wire brush, and never paint them.
Pour seals the day you can inspect them — a seal nobody can see poured is a seal nobody can vouch for.
On IS loops, keep the wiring physically separate and identified, and do not add anything the loop documentation did not allow for.

Common mistakes

Opening an explosionproof box on a live circuit because “it’s explosionproof”.
Leaving out the seal, or installing the fitting and never pouring it.
Swapping a certified fitting’s lamp, gland or cover for a “close enough” part.
Mixing Class/Division-marked and Zone-marked equipment without checking the book allows it.
Forgetting dust: a grain elevator or a flour mill is a hazardous location too, and dust layers make surfaces run hotter.

Check yourself

0 right of 0 answered 7 questions · pick one answer each

1.The fire triangle needs fuel, oxygen and ignition. In a hazardous location, which side of the triangle is the electrician’s job to take away?
2.What does an "explosionproof" (flameproof) enclosure actually promise?
3.Intrinsic safety protects by…
4.A device is marked with a temperature code (T-code). What does that code tell you?
5.Why are conduit seals installed at the boundary of an explosionproof system?
6.Two systems are used to classify hazardous locations in North America: Class/Division and Zones. What is the main idea behind the Division or Zone number?
7.You are about to open an explosionproof junction box in a classified area to add a conductor. What comes first?

Answers

C. Ignition — sparks and hot surfaces from the equipment — The process owner controls the fuel and the air is always there. Electrical equipment is a source of ignition — arcs at contacts, sparks at connections, hot surfaces — so hazardous-location equipment is built to stop it igniting whatever may be in the air.
B. If gas gets inside and ignites, the enclosure contains the explosion and cools the escaping gases so they cannot ignite the outside — An explosionproof box assumes gas WILL get in. It is strong enough to hold an internal explosion, and its flame paths (long, tight joints) cool the escaping gas below ignition. That is why a missing bolt or a scratched flange face defeats it.
B. limiting the energy in the circuit so that even a spark or hot surface cannot ignite the atmosphere — An intrinsically safe circuit is designed (usually with a barrier or isolator in the safe area) so the energy available in the hazardous area is too small to ignite anything, even under fault. It is common for instruments and 4–20 mA loops.
B. The maximum surface temperature the equipment can reach, which must be below the ignition temperature of the gas or dust present — Hot surfaces ignite things too. The T-code says how hot the equipment’s outside can get; the gas or dust present has an auto-ignition temperature, and the equipment must stay below it. Matching the two uses tables in your code book and the product marking.
B. To stop gas and flame travelling through the conduit from one enclosure or area to another — A conduit is a pipe: without a seal, gas can migrate along it into another area, and an explosion inside one enclosure can travel down it. The seal compound blocks that path. Seals are only effective when poured correctly with the right dam and fill.
B. How likely and how often the hazardous atmosphere is present — Both systems grade a location by how likely the flammable atmosphere is to be there — normally present, present in normal operation, or only under abnormal conditions. The Zone system splits that into three grades; the Division system into two. Which system and which grade applies is the classifier’s decision, documented on the area drawings.
B. Confirm the circuit is de-energized and locked out, and that the area is gas-free or a hot-work permit covers it — An open explosionproof box is no longer explosionproof. Work with it open only when the circuit is dead and locked out and the site’s permit system says the atmosphere is safe — then reassemble with every bolt, clean undamaged flame paths, and nothing trapped in the joint.

Questions fréquentes

Who decides whether an area is hazardous?

Not the electrician. The owner, usually through a process or safety engineer, classifies the areas and records them on area-classification drawings. Your job is to install equipment and wiring methods suitable for the classification those drawings show, following your code book’s hazardous-location section. If there is no drawing and you suspect a hazard, stop and ask.

Is Class/Division the same as Zones?

They describe the same danger in two languages. Class/Division grades a location in two steps of likelihood and groups the material by type; the Zone system uses three steps and is the international approach. Canada’s code uses Zones for new work with Class/Division kept for existing installations; the US code allows either, in separate articles. Equipment is marked for the system it was certified to, and the two cannot be mixed casually.

Can I use ordinary equipment if I put it in a steel box?

No. A steel box is not an explosionproof enclosure — it has no tested flame paths and will not hold an internal explosion. Hazardous-location equipment is certified as a whole and marked for the class, group or gas and temperature it suits. Anything else is ordinary equipment, wherever it is mounted.

Voir aussi

Commercial wiring methods — the wiring methods before the special ones
Instrumentation and the 4–20 mA loop — where intrinsic safety lives
Cadenassage, pas à pas
Finding it in the code book — the hazardous-location section is a specific section
Journeyman practice exam

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