NEC
Every operator based in the USA who wants to measure the flow of explosive substances with variable area flowmeters has already dealt with the topic of “explosion protection” and has come across the terms “Ex-proof” and “NEC”.Ex-proof refers to devices that are specially designed to work safely in hazardous environments with explosive gases, vapors, dust or fibers.
NEC steht für National Electrical Code (also known as NFPA 70), the standard in the USA to which appliances are certified.
Two equivalent NEC standards exist side by side.
This is intended to give users the opportunity to use the system that is technically and economically optimal for them.
- NEC 500 (traditional system)
Hazardous areas and products are classified according to “class” and “division”. - NEC 505 (since 1996) and NEC 506 (since 2005)
Based on the IECEx, the hazardous areas are divided into “zones”.
NEC 505 was originally only for flammable gases and vapors and was then extended by NEC 506 for dusts.
Key points
-
Legal basis
NEC 500: Required by law in many US states.
NEC 505/506: In the USA, permitted by law, but optional. -
Scope of application
NEC 500: Widely used in the USA, especially in older systems.
NEC 505/506: For new installations with international orientation (IEC-oriented) -
Standards basis
NEC 500: Class/division system.
NEC 505/506: Zone system, based on IEC 60079-10-1. -
Certification system
NEC 500: Devices require division approval (e.g. UL, FM).
NEC 505/506: Devices certified according to zone principle (e.g. IECEx, ATEX possible) -
Permitted types of protection
NEC 500: US-specific designs for divisions.
NEC 505/506: Exd, Exe, Exi, etc. according to IEC.
When will the explosion occur?
An explosion is a sudden and violent release of energy that can be triggered by various physical or chemical processes.It occurs when a rapid transformation of matter or energy takes place, resulting in a sudden increase in pressure. This usually produces shock waves, heat and sometimes light.
Chemical explosions often result from a rapid reaction of flammable substances with oxygen, such as the detonation of explosives or flammable gases.
An explosion occurs when three conditions are met:
- Flammable substance
A combustible substance must be present in the form of gas, vapor, mist or dust in sufficient concentration. - Oxygen
Oxygen (or air) must be present in sufficient concentration. - Ignition source
An ignition source with sufficient energy must be present.
e.g: Heat, sparks, chemical reaction, friction
If one of the 3 conditions is missing, an explosion will not occur.
Explosion protection zones acc. NEC 500
Potentially explosive atmospheres according to NEC 500, also referred to as “division”, occur
wherever flammable substances in the form of gases, vapors, mists or dusts can occur in hazardous
concentrations. These play a central role when working with flammable substances.
In order to minimize risks for people, systems and the environment, potentially explosive atmospheres
are divided into 2 different “divisions”, each of which requires different safety requirements.
The classification is based on the probability and duration of the occurrence of a hazardous
explosive atmosphere.
This systematic classification is the basis for the selection of suitable protective measures,
such as explosion-proof devices and safety-conscious operating procedures, and ensures
operational safety in potentially hazardous environments.
Ex-area
Definition of the "division"
Division 1
Hazardous substances are normally present
in the atmosphere, either under normal operating conditions or during maintenance work.
This applies, for example, inside pipes, containers and around sampling points (e.g. valves for sampling).
This applies, for example, inside pipes, containers and around sampling points (e.g. valves for sampling).
Division 2
Hazardous substances are only present under abnormal conditions,
e.g. in the event of leaks.
In accordance with NEC 505/506, classification is by "zone" according to
ATEX or IECEx.
Device labeling of variable area flowmeters according to NEC 500
Device marking in accordance with NEC is a crucial prerequisite for the safe use of variable area flowmeters in potentially explosive atmospheres.NEC stands for the certification system commonly used in the USA, which regulates the requirements for devices and protective systems for use in environments with a potential explosion hazard.
The marking provides important information about the suitability of an appliance for certain potentially explosive atmospheres and the permitted use for different explosive substances.
The marking must be permanently attached to the device.
Manufacturers, operators and maintenance personnel must understand the marking to ensure the correct selection, installation and maintenance of variable area flowmeters.
Variable area flowmeters without correct labeling must not be used in potentially explosive atmospheres as they can pose a significant safety risk.
In addition to labeling, regular inspections and maintenance are also required to ensure long-term safety.
Main elements of the device labeling:
- Permitted application range of the variable area flowmeters
The marking specifies the potentially explosive areas for which the variable area flowmeters may be used (“Division”).
Furthermore, the maximum surface temperature of the device is specified ("temperature class"). - Property of the explosive product
The marking specifies the type of substances for which the variable area flowmeters may be used ("Class") and how hazardous the product to be measured may be ("Group").
The system operator must ensure that the ignition temperature of the product is not lower than the maximum surface temperature of the appliance ("temperature class").
The ignition temperature for many products can be found at:
(The colored background is only for the assignment to the following explanation).
Class I
Division 1
Group A,B,C
T4A
Class
I
Gases, vapors
II
Dusts, such as coal dust, metal dust or flour dust
III
Flammable lint or airborne particles such as cotton, paper or wood shavings
Division
1
Hazardous substances are normally present in the atmosphere, either under
normal operating conditions or during maintenance work.
This applies, for example, inside pipes, containers and around sampling points (e.g. valves for sampling)
This applies, for example, inside pipes, containers and around sampling points (e.g. valves for sampling)
2
Hazardous substances are only present under abnormal conditions,
e.g. in the event of leaks.
e.g. in the event of leaks.
Group
A
Atmosphere with extremely flammable gas, vapor,
e.g. acetylene
e.g. acetylene
B
Atmosphere with very flammable gas, vapor,
e.g: Hydrogen, ethylene, acrylonitrile, propylene oxide, ethyl oxide
e.g: Hydrogen, ethylene, acrylonitrile, propylene oxide, ethyl oxide
C
Atmospheres with highly flammable gas, vapor
e.g: Ethylene, cyclopropane, ethyl ether
e.g: Ethylene, cyclopropane, ethyl ether
D
Atmosphere with relatively flammable gas, vapor,
e.g: Acetone, methane, hexane, propane
e.g: Acetone, methane, hexane, propane
E
Atmosphere with conductive dust, metal dust,
e.g.: aluminum, magnesium
e.g.: aluminum, magnesium
F
Atmosphere with coal dust, coke, soot
G
Atmospheres with combustible dust not included in Group E or Group F,
e.g.: non-conductive dust, such as flour, grain or wood dust, chemicals
e.g.: non-conductive dust, such as flour, grain or wood dust, chemicals
No group classification for Class III (combustible lint, paper, cott.on).
Temperature class
T1
<450 °C
T2A
<280 °C
T2B
<260 °C
T2C
<230 °C
T2D
<215 °C
T3
<200 °C
T3A
<180 °C
T3B
<165 °C
T3C
<160 °C
T4
<135 °C
T4A
<120 °C
T5
<100 °C
T6
<85 °C
T90°C
90 °C
Note on T90°C:
For dust (Class II) and lint (Class III), the maximum surface temperature is specified directly in °C.
In the case of a dust layer (deposit), the surface temperature of the devices must remain 75 °C below the ignition temperature of the layer.
In the case of a dust cloud (air mixture), the surface temperature of the appliances must remain 25 °C below the ignition temperature of the cloud.
For dust (Class II) and lint (Class III), the maximum surface temperature is specified directly in °C.
In the case of a dust layer (deposit), the surface temperature of the devices must remain 75 °C below the ignition temperature of the layer.
In the case of a dust cloud (air mixture), the surface temperature of the appliances must remain 25 °C below the ignition temperature of the cloud.
Example for the marking of electrical devices according to NEC 505/NEC 506:
(The colored background is only for the assignment to the following explanation).
NEC 505 (gases, vapors)
Class I
Zone 1
AEx db
IIC
T4
Gb
Zone 21
AEx tb
IIC
T4
Db