Ten common misconceptions about explosion protection ...

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Introduction

While doing the research for the article series on explosion protection standards, I remembered several questions, doubts, and misconceptions that I usually received when I worked doing Explosion Protection training. The most worrying ones were those corresponding to the proper understanding of how explosion protection methods work, how they are supposed to be installed, and how they should be maintained.

So, I made a list of the main concerns that I usually received in training and started to write how to address them, as a writing exercise.

These lines pretend to answer the most recurrent caveats on explosion protection methods in my experience. Of course, this list is not extensive or complete.

So, if any reader has either additional information, suggestions, or recommendations, I’d gently ask you to post them in the comments.

1.   Explosion-proof enclosures are tough, so they do not need maintenance…

When you carefully watch those massive explosion-proof enclosures (or the similar but slightly different flameproof enclosures), made from cast metal, with wide flame paths, numerous big bolts and thick glass windows that seem to belong to a bulletproof vehicle, you may think that these enclosures offer foolproof protection in the most demanding applications.

Well, yes, sort off.

1.- A typical Explosion Proof enclosure, with sealed conduits

To work properly, Explosion Proof enclosures must feature a precisely machined flame path or way that the eventual flames, heat, and pressure - created by an explosion inside the enclosure- must go through to be respectively quenched, reduced, and dissipated. So that when they reach the enclosure's exterior, they are no longer a potential source of ignition.

2.- Table of MESG values according to IECEx

Therefore, the flame path must be designed, built, and maintained across the life of the enclosure. The dimensions of the flame path must comply with the dimensions specified in the corresponding Maximum Experimental Safe Gap or MESG tables. These tables differ in their values depending on whether they comply with The NEC Code or with the ATEX/IECEx schemes.

3.- The explosion-proof/flameproof protection method

The MESG surfaces must be flat and smoothly machined and any damage or modification to them must be avoided. That includes applying grease or paint. If there’s any doubt about this point, ask the manufacturer.

If there is any damage done that modifies the MESG dimensions, the enclosures cannot warranty to withstand the explosion and the ability of the flame path to properly dissipate the energy generated by the explosion is no longer assured.

4.- damaged flame path

The same care must be used with the bolts that secure the lid with the rest of the enclosure. To maintain the MESG dimensions, all the bolts must be present, and the torque applied to them must be exactly the value specified by the manufacturer.

5.- Explosion proof/flameproof enclosure with missing bolts

And one last detail: due to the MESG gap, Explosion Proof or flameproof enclosures are not watertight, so it is not unusual to find the presence of water inside the enclosure due to condensation. Therefore, drainage orifices must be available in the enclosure.

6.- Condensation water accumulated in a field mounted box

7.- Rust damaged field mounted box

In conclusion, Explosion-proof/flameproof enclosures may look tough but are extremely sensitive to environmental, mechanical, and accidental damage.

The consequences of these requirements are that Explosion Proof or flameproof enclosures must be verified at least once a year (more is better) and that the maintenance people shall be professionally trained in the working principles of this explosion protection to avoid bad working practices and mistakes.

 2.   Enhanced safety is conceptually complex

Again. It is more complex than using an explosion-proof enclosure for sure, but for a process engineer the concepts involved should be easy to understand:

8- The basics of the increased safety explosion protection method

Let us suppose we must install some electrical or electronic components in a classified area, but we do not want a huge, heavy, expensive and maintenance requiring explosion-proof or flameproof enclosure.

1.      The first thing that we will need is an enclosure that can ensure the environmental protection of our components against dust and water, so we will select an IP54 enclosure.

2.      This enclosure will have to protect the components mounted inside from a mechanical impact, so our IP 54 enclosure can be made with stainless steel if we have concerns about the environment or with other materials such as aluminum or reinforced fiberglass.

3.      We will use components that cannot generate sparks, known as non-arcing equipment.

4.      We use adequate connectors or cable glands for external connections and use protected or armored cables only.

5.      We take particular care of using components that feature big external terminals for connections, so the chances for a short are so low as to be negligible. For those components that carry high voltages or currents we use a cover for protection, and we label it with a big DO NOT TOUCH THIS COVER UNDER OPERATION.

6.      The internal connections feature stress relief points, so they are not under mechanical stress.

7.      Any bare conductive parts are separated between each other according to specific distances and voltages as specified in the standards.

8.      We do the same thing with the creepage distances.

9.      We perform an analysis of the maximum temperature that any component mounted inside the enclosure may reach in the event of a failure, and make sure that this temperature is compatible with the required Temperature Group of the application.

10.  We perform an analysis of the maximum temperature that the surface of our enclosure can reach in the event of a failure of any one of the components mounted inside and make sure that this temperature cannot affect the thermal stability of the remaining components.

11.  We make sure that this maximum temperature of the surface cannot become a source of ignition.

12.  Finally, after all this work is done, we certify the design of the system.

9.- An increased safety field box

If we followed the steps correctly, we have created our first increased safety or Ex e enclosure.

Of course, the necessary concepts for this design are more complex than the brute force approach of an Explosion Proof enclosure, but there are quite a few advantages using Ex e:

Notable weight savings since the Ex e protection method is not required to dissipate the flames, heat, and pressure generated by an explosion because there is no way for a spark to be generated.

These facts enable the use of much lighter enclosures, and this is especially notable as the enclosure size grows.

10.- An increased safety junction box made from GRP (Glass-fiber Reinforced Plastic)

But although quite a few types of components and devices can be installed in hazardous areas using the Ex e protection method, there are exceptions, such as high voltage devices (more than 11 kV rms) or equipment that cannot be designed in a way that may enable then to work as a non-arcing device. In those cases, other protection methods should be considered.

Ex e enclosures and their contents are considered as an entire system. Therefore, since each Ex e certification corresponds to a specific system, you are not allowed to modify that system either by adding additional components or taking any of them out. That would be a system modification and would require recertification under the new system configuration.

11.- A fully equipped increased safety junction box

All this may sound strange when compared with traditional explosion-proof or flameproof enclosures but is similar in conception to the “non-incendive” protection method mentioned in the NEC Code. The main difference between the non-incendive method and the increased safety method is that the latter cannot become a source of ignition even if there is a fault, therefore enabling its use up to Zone 1. Non-incendive systems do not support failures so they are allowed up to Division 2.

If you are looking for more details, kindly visit explosion proof panels.

It is a matter of risk distribution, you may compare Explosion Proof to being protected by a medieval castle and Increased Safety to a modern schema of several layers of defense, also known as defense in depth.

3.   Verification of Intrinsic safety is complex

Well, again, sort off…

It is complex in the sense that proper training is required to use this protection method properly, but we are talking about process engineers that should have at least a basic understanding of electricity, electronics, and explosions’ physical and chemical phenomena.

The idea of intrinsic safety is that if you know the minimum amount of energy (MIE) required for the ignition of an explosive atmosphere, therefore you can design your circuits to work with lower levels of energy stored in them than the corresponding to the MIE. In this way, since you never reach the MIE, an explosion cannot happen. Even in the event of a fault.

12.- Scheme of an intrinsically safe circuit according to IEC/EN/UL 60079-1

To limit the amount of available energy in the circuit, an energy limiting device must be used.

13.- An intrinsically safe system and the equations used for its verification

So, you end up with an intrinsically safe system composed of an energy limiting device (barrier or isolator) also known as “associated apparatus,” an energy-limited device also known as “intrinsically safe apparatus,” and the connecting cables between them.

You are required to verify that your system complies with the Intrinsic safety verification equations:

Uo <= Ui

Io <= Ii

Po <= Pi

In which Uo, Io and Po are the maximum values of voltage, current and power that the associated apparatus can deliver to the system, even in the event of component faults, and Ui, Ii and Pi are the maximum values that can be available in the system while keeping the accumulated energy below the atmosphere’s ignition level.

Lo >= Lc + Li

Co >= Cc + Ci

And the second group, which accounts for the energy stored in the systems equivalent capacitance and inductance. Every circuit can be represented by an equivalent capacitance and inductance, and the connecting cable also works as a distributed inductance and capacitance. The inductance and capacitance of the associated apparatus must be lower than the equivalent inductance and capacitance of the intrinsically safe device.

This means that the “barrier”, or associated apparatus, will never be able to deliver more energy than the required to avoid the system becoming an ignition source, nor the intrinsically safe device be able to store energy enough to become an ignition source, even in the event of failure. Therefore, it is intrinsically safe.

All the necessary values to perform this verification are available in the devices’ documentation and certifications.

Although in most cases this verification is trivial, in the sense that the results imply great safety margins, there are some cases where they are not, like when using intrinsically safe solenoid valves or actuators.

But as you can see, the verification of intrinsic safety is not rocket science, it is just arithmetic’s. Of course, sometimes these verification calculations do become complex, and in those cases, it is usually better to call a specialist. And you will understand what he will charge you for.

4.   Intrinsic safety is expensive

Well, it depends on what are you comparing it with…

A correctly done cabinet for intrinsically safe interface modules of course is more expensive than a marshalling cabinet, but in most cases, you will not need an additional marshalling cabinet, your IS cabinet will work as one.

14.- Your intrinsic safety interfaces cabinet might work as a marshalling cabinet

Intrinsically safe (IS) interfaces are more expensive than DIN rail marshalling terminal blocks, but they offer the choice of a simplified wiring infrastructure to reach the field.

And this fact means significant installation savings.

Especially when compared to either NEC 500 like Explosion Proof enclosures and sealed conduits or Ex d flameproof and enhanced safety or Ex e cable runs, intrinsic safety allows you to lay your cables without the need of any kind of protection requirements. It is not necessary to provide anything more than mechanical protection for signal integrity and separation from non-IS cables to avoid EMC coupling issues.

15.-An explosion-proof installation, featuring sealed conduits

You can use standard separate cable trays for your IS wiring, or you can use existing cable trays provided with a metallic separation between your IS cables and your non-IS cables. You must ground the cable trays, though.

The savings on conduits, sealing compounds, installation and maintenance are large enough, but you get an additional advantage: you can perform live work without a clean area permit on your intrinsically safe devices in the field, even in Zone 0. In fact, you can short your intrinsically safe wires without further consequences, again even in Zone 0.

16.-An intrinsically safe wiring installation

Additional benefits: it is easier to trace a cable issue in a standard cable try rather than inside a conduit. And you can forget any worries about water condensation and corrosion inside the conduits.

17.- An explosion-proof conduit and sealant wiring installation

Even when compared with Ex d, Zone 1 cable runs are more expensive than IS cable runs by a wide margin.

Last advantage: since you can do live work in the field, the need to perform unexpected maintenance production shutdowns is dramatically reduced.

5.   Everything you connect to an Intrinsically safe interface must be certified as intrinsically safe

Everything connected to an intrinsically safe interface, or “associated apparatus,” must comply with the intrinsic safety verification using the corresponding entity parameters, which would be a more precise description of intrinsic safety.

The key to the relevant exception is the following: intrinsic safety consists of the limitation of stored energy in the system by controlling the available voltage, current and power values and additionally the energy stored in the equivalent capacitances and inductances of the system.

But in that definition, the term resistance is not included, because resistances do not accumulate electrical energy. They just dissipate heat.

This means that if the device connected to the intrinsically safe interface can be represented by an equivalent resistance and it is not capable of storing energy or generating more than 1.2 V, 0,1 A, 20 µ J or 25 mW, then it falls into the definition of “simple apparatus”.

Need an example?

Here are some:

• Thermocouples, generate voltages but well within the simple apparatus definition
• RTD (Resistance Temperature Detectors)
• Load cells, if they are employed in single series or in parallel (usually up to 6 or 7, but you must do the verification calculations).
• Mechanical switches. Volt-free items e.g., push buttons, magnetic contacts, mechanical contacts, mechanical switches like pressure switches or flow switches.
• Single LEDs and diodes, including zener diodes but not LED clusters
• Connectors
• Dry contacts
• Terminals

You can use any of these devices in an intrinsically safe circuit without the need for certification for the simple apparatus because they do not store energy. You do not require an intrinsic safety certificate for simple apparatus, but the enclosures that contain them must have a label with a legend that informs that it "Contains Intrinsic Safety Circuits".

See you next week. 

Mirko Torrez Contreras is a Process Automation consultant and trainer. He has added a new hobby to his already broad range of interest in the last weeks: inline skating. As in any other activity, it requires a balanced mix of training and practice to achieve any results. Likewise, the comprehension of explosion protection methods requires a balanced mix of training and practice in their selection, implementation, and maintenance to ensure they serve their ultimate purpose: the avoidance of explosions in hazardous areas.

Phoenix Contact sponsors this article. The opinions exposed in this article are strictly personal. All the information required for and employed in this article is of public knowledge.

Relating to using barriers, you must realize that people often misunderstand how, when, and why they are used.

For example, say your area is classified as Class I, Div 1 and you want to install a transmitter. Both of these are acceptable:
(1) Install a Div 1 rated transmitter, using applicable wiring methods for a Div 1 area per NEC 500. No barrier is required.

(2) Install an intrinsically safe transmitter, install a properly rated barrier that meets specs of the manufacturer's official Control Drawing, and follow any other requirements of that Control drawing. You can then generally use ordinary wiring methods in accordance with NEC 504. I say generally, because you still have to install required boundary seals and follow other certain provisions as detailed in 504.

The point is that just because any transmitter or other device is in a hazardous area doesn't mean it needs a safety barrier.

The use of a safety barrier, however, is part of a strict overall design that allows instrinsically safe devices to be installed without the expensive Div 1 wiring methods because the energy that can be communicated to the hazardous area is specifically limited by the barrier so that ignition cannot occur.

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