Pressure switches that are used for industrial applications involving any kind of liquid, such as water, oil or chemicals - or processes that have a risk of explosion - must meet specific standards. Choosing the wrong type of instrument for these challenging conditions can be dangerous for your operation and your people.
As a territory manager for Ashcroft, customers often ask me to explain the difference between a NEMA 4/4X pressure switch and a NEMA 7/9. The short answer is that one classification is watertight (NEMA 4/4x) and one is explosion-proof (NEMA 7/9). But that response doesn't tell you what you need to know about these instruments before selecting the best switch for your specific needs.
Read this article to learn the differences between these types of switches and how each is designed for a specific purpose. When you're done reading, you will have a clear understanding of the pressure switches, features and ratings to look for when choosing the best instrument for your application. You can also deepen your knowledge by clicking on any of the related links to additional resources.
About NEMA Ratings
The National Electrical Manufacturers Association (NEMA) established standards to define the levels of protection provided by electrical enclosures against environmental conditions. NEMA evaluates electrical equipment like pressure switches to determine whether it can prevent elements such as dust, water, and corrosive agents from compromising the functionality and safety of an application.
NEMA standards are widely recognized in North America as a reference point for manufacturers to design and evaluate their products to withstand environmental challenges.
What is a watertight (NEMA 4/4X) pressure switch?
A watertight pressure switch with a NEMA 4 rating is designed with a sealed enclosure for general-purpose, outdoor use. Its primary function is to protect the instrument against harsh environmental conditions such as windblown dust, rain, splashing water, and water that is directed by a hose.
The NEMA 4X rating provides all the same protections as the NEMA 4 with additional corrosion resistance. All NEMA 4 pressure switches are good choices for use in oil and gas plants, burners and boilers, water/wastewater facilities and other harsh outdoor environments.
An example of a watertight pressure switch is the Ashcroft® B-Series NEMA 4X Pressure Switch, engineered for challenging outdoor applications that require high current switch functionality and cycle life. All B-Series pressure switches are calibrated at 70 °F and have an ambient temperature range of -20 °F to 150 °F (128 °C to 65 °C).
Figure 1. Example of an Ashcroft NEMA 4X Pressure Switch
What is an explosion-proof (NEMA 7/9) pressure switch?
Explosion-proof pressure switches with NEMA 7 and 9 ratings are designed for use in hazardous locations, including any area where flammable gases, vapors, or dust may be present. Explosure-proof switches can contain internal explosions without igniting surrounding flammable substances.
NEMA 7 is intended for indoor hazardous locations classified as Class I, Groups A, B, C, and D, where flammable gases may be present.
Figure 2. NEMA 7 Class and Group
NEMA 9 is designed for Class II, Groups E, F, and G environments, which may have combustible dust. These switches are always equipped with a conduit-type fitting on the electrical side to dissipate heat or ignition sources through threaded sections, enhancing safety.
Figure 3. NEMA 9 Class and Group
NEMA 7/9 pressure switches are good choices for use in refineries, chemical and petrochemical plants, offshore oil rigs and other hazardous processing environments that are considered hazardous locations.
An example of a NEMA 7/9 instrument is the Ashcroft® A-Series Explosion-Proof Switch. Its pressure control design is ideal for applications in harsh or hazardous locations, and the explosion-proof construction provides superior protection. This switch is ultra-compact so it can fit easily into an existing system, and it is also available with a NEMA 4X rating.
Figure 4. Ashcroft® A-Series NEMA 7/9 Pressure Switch
Comparing NEMA 4/4X and NEMA 7/9 ratings
The following charts provide a high-level comparison of the NEMA ratings discussed above.
Figure 5. NEMA 4 vs NEMA 4X
Figure 6. NEMA 7 vs NEMA 9X
* For Class III type ignitable fibers or combustible flyings, see the National Electrical Code, Article 500.
** Due to the characteristics of the gas, vapor, or dust, a product suitable for one Class or Group may not be suitable for another Class or Group unless marked on the product.
Ready to learn more?
Choosing the right pressure switch depends on understanding the environmental conditions and safety requirements of your application. But there are other considerations as well. To gain additional insights, here are a few relevant articles that may interest you.
- Choosing a Pressure Switch: 9 Factors to Consider
- Cost of Mechanical Pressure and Temperature Switches
- When Should You Use an Electronic Pressure Switch?
- What is a Deadband on a Pressure Switch and How Does it Work?
- Choosing the right Microswitch for your Application
You can also contact us directly. Our industry experts can help answer your questions.
