Intrinsic Safety Question - Sensors

Hello,

I have a client who has recently decided to retrofit an existing machine to ensure intrinsic safety. I have very limited experience with IS, so I wanted to run a question by you good folks.

The only current-carrying devices in the hazardous area are some SMC auto switches to indicate cylinder extension and retraction.

The client looked at the data sheet for the sensors and found the max current carried by the sensors to be 40 ma @ 24VDC. Looking at the ignition curves, he says this is well below the voltage+current that would cause an ignition.

I have recommended isolation barriers and NAMUR sensors, but the client thinks they are unnecessary because of the ignition curves. I explained that under normal operation, his thinking makes sense, but if the sensor fails, you may have a lot more than 40 mA entering the hazardous area.

Is my thinking correct, and is there anything more I should explain to him?

Thanks for your help!

A couple of things:

The sensors should not be the defining factor in an intrinsically safe installation, but the barrier.

The other very important thing to remember is that the sensor carries 40mA in normal working conditions... but how much does it take when it fails? How will it fail normally?

Can it continue working with one fail inside the electronics? All of these are taken into consideration when awarding an intrinsically safe rating to a device.

Another thing to consider is that you are assuming the cable will not be damaged... but what if the cable is damaged? how do you guarantee that you won't have more than 40mA?? This is why the barriers are used.

Additionally, you also need to know how hot the sensor will get as spark is not the only way to cause a problem.

What is the ignition curve for? The vapour that will exist in the area?

If you aren't experienced or qualified in ATEX stuff, don't put your name to anything and most certainly don't follow the blind as the legal and personal implications aren't worth it.

Also, most people jump into putting ATEX or IS equipment in a system and never consider two things.

It is often easier and cheaper to move the ignition sources away from the hazardous area.

For there to be an explosion you need the right amount of vapour (dust is a completely different beast) and oxygen. Flooding an enclosure with clean air can guarantee that you never reach the perfect, or any mixture for an explosion to happen (see high voltage ATEX motors, for example).

Great post! Many thanks!

Everything in the circuit must be designed and approved for IS use and generally the barrier sets the limitations. The idea is if any part of the IS circuit is damaged, there isn't enough stored energy to create an ignition source. So if you are using improper cabling or sensors with a lots of capacitance or inductance, there could be enough energy stored to cause issues if the cable is cut or shorted. The barrier only limits the supply side.


If you look at a lot of the control drawings for the barriers, it usually designates the allowed impedance, voltage/current limits involved. Also to meet certification the end devices and cabling must be approved for IS use.



For some passive devices classified as simple apparatus "resisters, dry contacts, non-energy storing components" they don't need to be certified, but the control drawings will usually define the requirements.


When you do your IS calculations, you need to remember your cabling is part of the equation too.

Before choosing what to use, you also need a hazardous area classification done... which in my experience requires experienced people and not the boss who checked the ignition curves.

Most importantly in the IS side is also how and how good the grounding of the instrumentation is, which you probably haven't checked.

Also, any ATEX installation will have requirements on the materials used for cable management which can be a bit of a pain.

One important clause in the legislation (in Europe at least) is that you need to guarantee that the IS system is protected, so even though it won't cause an explosion, you still need to ensure you use proper material (either IP protected for single circuit or minimum Exe for more than one circuit).

the whole idea behind intrinsic safety is that if a sensor in the hazardous area shorts and the wires are exposed to the environment that there is not enough energy in the are powerful enough to create a fire.

Sorry, but your client is wrong. 40ma @ 24 vdc WILL have enough energy in the arc to create a fire.
I dealt with class 1, div 1 and class 2 div 1,2 for 8 years at one plant.

See nfpa 70e section 500-??

if your client won't listen, I would walk away and refuse to do the job.
it doesn't matter if he is the one paying the bill, YOU are the one responsible
and assume all liability.

there are lots of rules to follow.
hazardous area enclosures must be rated for the area and are to be purged / pressurized - NFPA496
Rigid conduit required and seal offs.
lots of other stuff.

do NOT rely on your client to tell you what to do, design your system correctly and tell your client what the cost will be, no shortcuts.

if you need help with intrensic stuff, please seek help.
I'm not trying to be mean, I just want you to make a safe control system so that everyone goes home to work another day safely.

james

James Mcquade said:

hazardous area enclosures must be rated for the area and are to be purged / pressurized - NFPA496

Click to expand...

This is a pretty big sweeping statement... and is wrong in Europe at least. Protection type d is impossible to be pressurised by design and yet can be used in zone 1.

But like you say, there are lots of rules and checks that need doing and if one doesn't know these, then it's best to either walk away or learn.

In Europe you can get COMPEX 12 certification that helps, or at least sheds some light, on the intricacies of hazardous area. I know a acquaintance of mine that is teaching this stuff in Houston, so surely it is possible to have some sort of certification/learning in the States too.

cardosocea,

my mistake, I was in a rush.
Unless I missed something, in the USA, any enclosure with electrical controls must meet the requirements for the hazardous classification.

Nema 7,9 enclosures are exempt. they are massive enclosures with lots of bolts on the door. you can buy NEMA 4 enclosures or one that is NEMA 4/12/13that will meet the requirement of purging / pressurization, but you need to contact the manufacturer or look at the enclosure specifications before you purchase them.. I have bought a lot and pressurized / purged them. you still have to use myers hubs, rigid conduit, seal offs.

enclosures in the hazardous area with intrinsic barriers do not (unless something changed recently) have to be purged / pressurized. they must have myers hubs, rigid conduit, and seal offs.

its up to the individual / company doing the design work to design a system with the components that will meet the requirements set forth in NEC70 national electric code, NEC70E - arc flash, NFPA 496 - purging / pressurization of electrical enclosures, NFPA 79

if I am incorrect, someone please point it out.

james

You are right, but to meet the requirements it can either be pressurized (Ex p), explosion proof (Ex d), increased safety (Ex e), non sparking(Ex n), encapsulation (Ex m), oil filling (Ex o) and powder filling (Ex q).


Pressurization is one way of avoiding explosion, but there are many and one can use them depending on the area for the equipment to be installed.

cardosocea,

I am only familiar with the following enclosure types.
explosion proof
pressurized
purged
intrinsically safe jb's with terminal blocks with approved construction method.

in the states, we must go by the codes I mentioned.
that doesn't mean your methods are not in the code book, I just may not be aware of it.

james