Placing a Motor in an Explosion Proof / Intrinsically Safe Location


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My company is bidding on a project for which we would have to build a web handling machine that would have several motors and drives. A few of the motors would be located in a hazardous location - Class 1, Division 1, Groups C & D. Any components in this location have to be explosion proof or intrinsically safe.

I am able to locate my control panel (in which my drives would be located) outside of this hazardous location. At a minimum, I will need to run motor power and feedback (encoder) wires from each motor to its corresponding drive. I do not have much experience when it comes to explosion proof / intrinsically safe construction. It seems to me that I will most likely need some sort of physical and/or electrical barrier between the hazardous and non-hazardous location.

I was wondering if anyone here could give me a general idea of what the recommended / accepted practices are for this type of scenario. If you can tell me where I should look to read up on this in more detail, I would greatly appreciate it.

For my control purposes, I can use simple DC motors / drive boards / spreed controls, AC vector motors / drives, or even servos. My performance requirements will be easy to satisfy either way. My main concern is how to handle the hazardous location requirements.

Thanks in advance,
Hello Paul,

It seems you refer to US NFPA NEC standards.
I am largely more familiar with ATEX (European) and IEC standards, so i will not be able to provide a definitive and complete answer, but surely can try to help.

For the motor, you would need an explosion proof certified machine.
(with European standard ATEX it could be for instance an Ex d or an Ex de).

I am not sure of the existing protection concepts in US, but the motor needs to be Division 1 and group C&D certified and also to have the right Temperature class (maximum motor surface operating temperature to be certified below gas auto-ignition temperature).
It seems to be a specific feature of explosion proof motors to be run by drives (inverters), so i would pay attention to this point.

Regarding the control loops, unless you can use fiber optics, it may require to be fitted with intrinsically safe barriers (Ex ia or ib with ATEX).

The barriers need to be carefully selected to limit any possible spark on these loops below the gas group ignition energy.

Generally have in mind that any material installed in an hazardous area, or electrically connected to this area, needs to be certified with and explosion protection technique, suitable to the applicable division (Zone with ATEX), gas group and temperature class.

Hope this helps you to start,
Be very careful when you decide on the hazardous area rating, most seem to go overboard and end up overdoing the hardware. There's a big difference between Class 1 Div 1 and class 1 Div 2:

Div I (ignitable mixture intermittently or normally)

Div 2 (ignitable mixture not normally present) the motors can be normal Nema 4 in this area.

Class 1 Div 2 would be a normal chemical plant processing Hydrocarbons

I suggest you have the client specify the Method of Protection

Describe the environment
Hazardous Area Class: Class 1 Div 2 Group B & D
Describe the certification requirements
Certification: FM Approved for Class 1 Div 2 Groups B & D
Temperature Class?

One of the requirements is Non Sparking so you probably won't have a DC motor with brushes.

If you are running the wires in conduit you will need to provide seals to stop any explosive gas migrating up the conduit to the safe area.

I doubt you will need Intrinsic Safety for the encoder EX d Ex e or EX nA

Bob Peterson

this is not really a DIY type of project. You might want to get some experienced help.

Having said that, in general, the motor will probably need to be listed for the area classification and listed as XP.

You can probably get an appropriate motor for the area classification and just buy it with an XP encoder on it.

You will need to use a wiring method from chapter 3 of the NEC that is allowed in your particular classified area.

You will probably need some seals. The code is somewhat confusing about just where they are and are not required, so it is probably best to get someone who is familiar with the requirements to lay this out for you. Personally, I would install the seals at the factory but not pour them until the machine is installed. They are a real pain to undo once the compound is poured and you never know what you might need to do in the field as far as rewiring.

But before I would do anything, I would find out just how whoever classified the area came up with the idea that it was Division 1. A lot of inexperienced or less knowledgeable people over classify areas and it is quite possible that a more experienced/knowledgeable person would call it Division 2. This is pretty common. Trust me on this one - you cannot properly classify an area based on what is found in the NEC.

By the way, it is not correct to describe an area as explosion proof or intrinsically safe. Those are protection techniques, not area classifications.



Thank you Alex, Roy, and Bob for your input. I feel like I am starting to get a general idea of what is involved/required. I think think the key for me now is to get my (potential) customer to clearly define the boundaries of the classified area.

As for whether it really is Class 1, my gut feeling is that it might be. Our process requires us to pass a moving web through a wet process (formation baths, rinse tanks) that we will partially enclose and/or cover and exhaust portions of where it is feasible, but I think it will be unavoidable that vapors from our process will be present.

Thank you all for your help.

Bob Peterson

One other thing. In general, Division 1 areas are surrounded by division two areas. So even if there is a division 1 area involved, it is quite possible that you might be able to design your equipment so that your Motors are in the division 2 area.
> One other thing. In general, Division 1 areas are surrounded by
> division two areas. So even if there is a division 1 area involved,
> it is quite possible that you might be able to design your
> equipment so that your Motors are in the division 2 area.

Bob makes a good point, the motors can be outside the fume hood.
A regular TEFC motor can be installed in Division 2 but it should have a plastic (non sparking) fan, I think aluminum is probably ok.

Some would argue the motor should be explosion proof but NEC 501-8 is fairly clear on what type of motors can be used.

You often find that the client has a site standard, we usually supply Exp. proof motors for Class I Div 2 even though it's not required by code.
A little late to this party... it is all good information.

I just wanted to add my opinion that an intrinsically safe system is mostly about being able to use Article 725 wiring methods for sensing and control, i.e. no HL-rated cabling and fittings or raceway w/seals required. For other than <i>simple apparatus*</i>, all associated equipment must be intrinsically-safe rated. I believe it is rare that there is no Class 1, not-intrinsically-safe-rated equivalent. Evaluate advantages and disadvantages of both methods for each project before concluding which to use.

*NEC definition: An electrical component or combination of components of simple construction with well-defined electrical parameters that does not generate more than 1.5 volts, 100 mA, and 25 mW, or a passive component that does not dissipate more than 1.3 watts and is compatible with the intrinsic safety of the circuit in which it is used.

Informational Note: The following apparatus are examples of simple apparatus:

(1) Passive components; for example, switches, junction boxes, resistance temperature devices, and simple semiconductor devices such as LEDs

(2) Sources of stored energy consisting of single components in simple circuits with well-defined parameters; for example, capacitors or inductors, whose values are considered when determining the overall safety of the system

(3) Sources of generated energy; for example, thermocouples and photocells, that do not generate more than 1.5 volts, 100 mA, and 25 mW